Manual 21047551

Manual 21047551

CONTENTS

PAGE

Foreword 319

Introduction ... 321

Soil Samples 331

Description of Soils 331

Alkali Analyses 332

Phosphate Content of Good and Poor Soils 332

Culture Experiments 336

Effect of Sub-Division on Phosphate Solubility 340

Rate of Solution 342

Additional Soil Samples 344

Description of Soils 344

Phosphate Content of Soils 346

Salts as Clarifying Agents. 348

Effect of Sodium Chloride and Sodium Sulphate upon the Solubility of Phosphate Rock '.... 349

Progessively Varying Soil:Water Ratios 351

Successive Extractions 351

Electrodialysis 355

Summary — 357

Conclusions 358

Bibliography 359

ILLUSTRATIONS

Fig. 1.— Response of tomatoes to phosphate in soil No. 1 323

Fig. 2.— Response of tomatoes to phosphate in soil No. 3 323

Fig. 3.— Response of tomatoes to phosphate in soil No. 11 324

Fig. 4.— Comparison of plant growth in soil from good and poor areas of a single field , 325

Fig. 5.— Response of tomato plants to phosphate in unproductive soil 326

Fig. 6.— Conditions in a puddled soil which promote phosphate deficiency 328

Fig. 7.— Conditions in an aerated soil which promote phosphate availability 329

Fig. 8.— Relative water-soluble phosphate in good and poor soils 334

Fig. 9.— Response of millet to phosphate in soil No. 1 336

Fig. 10.— Response of millet to phosphate in soil No. 3 337

Fig. 11.— Response of millet to phosphate in soil No. 2 338

Fig. 12.— Manner in which channeling interferes with leaching of soils 345

FOREWORD

This is the first of a series of technical bulletins which will deal with the chemical composition, mineralogy, solubility, and availability of the naturally-occuring phosphates as found in the calcareous soils of the arid

Southwest. While Arizona soils have been largely used in these studies, soils of similar character are found in all of the irrigated valleys of west

Texas, New Mexico, Utah, Nevada, and southern California, and the conditions noted in Arizona are no more prevalent than in these other states.

Past history records few instances where irrigated agriculture as it is carried on today has proved permanent over large areas. A better knowledge of arid soils must be gained if this goal is to be attained. The work reported in these bulletins is an attempt to attack certain of the more fundamental problems which underlie permanent soil productivity in these areas of little rainfall. Arizona is young. Less than 1 percent of her soils have felt the plow. Agricultural expansion is sure to come, and quickly. Water resources are being developed. New dams are being built. New lands are being cleared and planted. Dependable information must keep pace with this expansion. Large areas of farming lands in the East are abandoned due, at least in part, to a lack of usable information. We must " know the reasons why."

These bulletins deal with the physical, chemical, and biological factors which affect the availability of phosphates in highly alkaline soils. The first is largely a chemical study of the effects of size of particle, successive teachings, common ions, and carbon dioxide on soil-phosphate solubility, as well as work with electro-dialysis and other extraction methods for measuring such solubility. Subsequent bulletins will take up the important correlation of phosphate availability with permeability and carbon-dioxide production; the effect of alkali salts on phosphate solubility ; the molecular structure of the soil-phosphate minerals; the carbon-dioxide roxygen balance in the soil solution as influencing phosphate availability and root growth; the use of algse as an indicator of phosphate need; the influence of carbon dioxide on the use by plants of the phosphates of iron, aluminum, and calcium in calcareous soils; the fixation of phosphates by soils and the rates of diffusion; the effects of low phosphate concentrations on soil bacteriological processes; and finally, the use of phosphate fertilizers. In addition to the above, a theoretical discussion is being prepared which pertains to the chemical structure of the soil phosphates and to their solubilities and degrees of ionization, together with the factors which influence them.

PAUL S. BURGESS,

Head of the Department of

Agricultural Chemistry.

PHOSPHATE SOLUBILITY STUDIES ON SOME

UNPRODUCTIVE CALCAREOUS SOILS

By

W. T. MCGIXORGF, and I. F. BRKAZIULT*

INTRODUCTION

During the past few years the attention of the members of the Arizona

Agricultural Experiment Station, and the Extension Service, has been called repeatedly to certain deleterious soil conditions which exist in the irrigated valleys of this State. Many farmers, at different times, have reported observations upon their ranches, which have been made year after year, and, while several crops have been involved, and while different methods of farming have been practiced, there has been a striking similarity in these observations. The evidence warrants the assumption that some fundamental cause exists, which is applicable to all conditions.

Investigational work, which will be herein reported, was therefore begun as a major project.

In the opinion of both the farmers, and County Agents of the Extension Service, who have been frequently visiting these ranches, the soils in question have been, for the past several years, slowly losing" their original productivity. In some cases the evidence is quite conclusive, while in others, the absence of quantitative field data makes conclusions less convincing. There is no proof that the trouble is very general, and neither can it be said to be very extensive. It may occur in large fields, in small areas, or even as barren, or " slick spots," in otherwise good fields. However, although these poor, or barren, spots may be relatively small, they offer a serious problem to many farmers of the State, where under irrigation, land values and overhead charges are necessarily high, and where good yields must be obtained annually in order to justify cropping. A few barren spots in an otherwise good field, may mean the difference between profit and loss.

The literature on agriculture in the older farming sections of the United

States relates many instances of the abandonment of farms because of reduced fertility of the soils. Soil research has shown that in most such cases the causes of this decreased fertility were avoidable, and would probably not have occurred had the farmers been familiar with the fundamental nature and properties of the soils. A knowledge of the latter

322 TECHNICAL BULLETIN No. 35 is therefore a prerequisite in any program of permanent fertility, especially in irrigation agriculture. Methods of cultivation, fertilization, and irrigation are not standardized practices for all conditions, and must be modified to suit local peculiarities of climate or other growth factors which are often due to local environment. While it is certain that (Arizona soils have not been cropped for a period sufficiently long to show signs of wearing out, as is the case with many Eastern soils, it is only by close study of the fundamental properties of our soils that we will be able to recognize such evidence should it appear. Fundamental soil investigations are the key to the maintenance of soil fertility.

When arid soils are reclaimed from the desert, put under irrigation, and planted to some crop, alfalfa for example, good yields may be obtained for several years, after which a gradual decline may set in. The plants make a stunted growth, they show a tendency to mature early, and after 5 or 6 years of continuous cropping the growth is often not sufficient to justify a cutting. When such soils are rotated to another crop, cotton for example, the loss in fertility usually is shown in the rotation. In certain cases these soils are barren of desert vegetation even before they are reclaimed. It behooves us therefore to study the fundamental properties of these soils with a view toward identifying the factors involved.

According to the observation of farmers and extension workers, the trouble is not confined to any particular soil type, but it is most prevalent upon the heavy, or clay soils. Such soils are usually dispersed, or puddled, either upon the surface or in layers of varying thicknesses in the subsoil. Many times a succession of dispersed layers occurs with good soil between the layers. In either of these cases the soils do not take water readily, and when once wet they retain the excess moisture for long periods of time. The soils are always calcareous, and have an alkaline reaction. They are low in organic matter. At least such is the general character of the soils where observations have been made.

One of the fundamental facts which we constantly observe in arid soils and which will be considered later in the discussion of puddled soils, is that an alkaline reaction, or the presence of relatively high concentrations of hydroxyl ions in solution, is necessary before puddling can take place.

Wlien the excess of hydroxyl ions is removed from the soil either by leaching, by neutralization, or otherwise, flocculation will take place. The difference in reaction between a puddled and a flocculated soil is often represented by less than 0.5 pH. At a reaction of about pH 8.3, or about the alkalinity which will give a pink color to phenolphthalein, there are sufficient hydroxyl ions in solution to react with the colloids and cause dispersion. This alkalinity cannot exist in the presence of free carbon

PHOSPHATE SOLUBILITY STUDIES

325

Fig. 4.—•Comparing plant growth in soil from poor (left) and good (right) areas of a single field. Soil from good area was more permeable to water. Neither pot received any treatment other than watering.

thorough leaching, by incorporating manure, organic matter or even sand, by deep cultivation, or by other means of aerating them, without the addition of any corrective. These reclamation processes have become systematized. When near a sandy river bed it is often economical to haul out large quantities of sand to be plowed into the slick spots. However, the usual method of reclamation is to throw up high borders around the affected spots, and to apply large amounts of irrigation water. The penetration may be very slow at first, and the soils may show a tendency to

" freeze up " and become more impermeable. If the applications of water are continued, the soil will often gradually loosen up, and finally it will take water fairly well. Apparently this system of reclamation is more effective when done during the winter than during the summer.

After leaching, the reclamation is assisted further by allowing a growth of some hardy plant, such as Bermuda grass, to grow upon the soil to be plowed in later.

The successful reclamation by such methods makes it evident that some factor, or factors, which are associated with water movement, or permeability, are closely connected with the degrees of fertility of the soil.

Many investigators have therefore assumed that the conditions are entirely a matter of water penetration, or physical structure of the soil.

326

TECHNICAL BULLETIN No. 35

Fig- 5—Tomato plants in unproductive soil from Salt River Valley. The two plants were transplanted to the untreated pot on the left. They both grew poorly, and looked alike. The plant on the right was then bent over, and a part of the stem covered with the soil of the other pot, which had been; fertilized with superphosphate at the rate of one ton per acre. The plant took root in this soil and grew vigorously.

It has been the experience of the authors, however, that when samples of these soils are collected, -placed in pots, fertilized with soluble phosphates, and planted to a sensitive crop, such as lettuce, tomatoes, or millet, increased yields are always obtained. This is illustrated in figures 1, 2,

3, 4, and 5. Seedlings of tomatoes often die when transplanted to such soils, but when their roots are first dusted with a little phosphate, or when rolled in a soil which contains soluble phosphate, they grow vigorously.

PHOSPHATE SOLUBILITY STUDIES 327

Dying plants may be revived and made to grow by watering such soil cultures with dilute phosphate solutions. Such work has been done repeatedly in pot cultures, and it has been demonstrated by C r i d e r ( l ) and others in the field. This reclamation may be accomplished without leaching, or without the addition of any organic matter, or any other correctant. Blue-green slime, or algae will not grow upon such poor soils when they are kept wet. However, if a soluble phosphate, even as little as two parts per million, is added to the soil, a growth of algse will soon form. Many of these soils do not contain enough soluble phosphates, therefore, to support the growth of such small organisms as algae.

Such are the conditions that have been observed. On the one hand, the soils may be reclaimed by thorough leaching without the addition of any corrective, and on the other, they may be reclaimed equally as effectively by the addition of a soluble phosphate without leaching.

Since the time of Liebig, the pioneer soil chemist, soil scientists have attempted to evaluate the plant food content and requirements of soils by means of laboratory methods. In calcareous types which includes practically all the cultivated soils of this State, the chemist is greatly handicapped by the interference of calcium carbonate with the solubility of phosphate in any solvent which may be employed in extracting the soil.

Any chemical method of soil extraction should be supported by culture experiments upon the soils themselves and empirical relationships established, for after all, the behavior of the plant is the final answer, and phosphate requirements of different crops sometimes vary enormously.

W h e r e possible, field experiments should be available from which soils may be obtained for solubility studies. I n the absence of extensive field experiments it has been necessary for us to classify the soils, good or poor, depending upon the condition of the crop at the time the soils were sampled. It cannot be assumed therefore that in all cases the samples designated as poor represent phosphate deficiencies but we have ample reason to believe that most of them do.

W h e n digested with strong acid, these soils show an abundance of phosphorus, but the same soils yield little or no phosphorus when extracted with pure water. T h e soil solutions, when displaced with oil, alcohol, or water seldom show more than a trace of phosphorus. These observations have been made repeatedly during our investigations, and therefore our efforts have thus been guided to phosphate availability as one of major proportions. It appears that there is a close relationship between the availability of soil phosphates and the penetration of water, as well as soil oration, and the incorporation of organic matter, which furnishes food for bacterial activity, causing subsequent carbon-dioxide production. It is not believed that this explanation will apply to all cases

328

TECHNICAL BULLETIN No. 35

SoU

Texture

Puddled

Poor

Aeration

Mo Root

Growth

Undesirable

Organisms

Impermeable to w a t e r

Carbon

di-oxioie

Phosphate

Deficiency

Fig. 6.—Conditions in a puddled soil which promote phosphate deficiency. Each succeeding condition is dependent upon each preceding condition and is governed thereby.

of infertility, but unquestionably, in a great many cases which have come under our observation, the loss in fertility may be explained by the scarcity of soluble phosphorus in the soil solution. Our data show that a close correlation exists between puddled soils, poor aeration, impermeability, undesirable organisms, absence of free carbon dioxide, and phosphate deficiency. This correlation, or interdependence, is shown graphically in

PHOSPHATE SOLUBILITY STUDIES

Soil

Texture

Rocculatton

Aeration

329

Normal

Root Growth

Bacteria

ar\d Fungi

Permeability to Water

Soluble

Phosphates

pig. 7,— Conditions in an aerated soil which promote phosphate availability. Each succeeding" condition is dependent upon each preceding condition and is governed thereby.

330 TECHNICAL BULLETIN No. 35 figure 6. In the same way an inter-relation exists between a flocculated soil, aeration, permeability, organic matter, bacteria and fungi, presence of carbon dioxide, and phosphate availability. This inter-relation is illustrated graphically in figure 7.

For the past several months much time has been devoted to a study of the above mentioned inter-relationships. Soil and plant relations and soil micro-organisms have been studied, and such a quantity of data has been accumulated that it has been found necessary to report it in a series of bulletins. The present publication will deal with the solubility of the phosphates in good and poor soils, under different conditions and with different methods of extraction. The relative rates of solution, which inlicate the

" staying power " of the soil, will be discussed also.

Under practically every system of agriculture, crop production, or the amount of crop which can be produced from a given acreage, may be influenced by many factors, but usually a single limiting factor is sufficient to control production. This factor may vary with the season or with the locality, but it is nearly always clearly indicated. In the semi-arid regions of the Great Plains for example, the limiting factor is water. Alfalfa is a crop w

T hich requires a large amount of water, so, no matter how fertile a soil may be, the yield of alfalfa cannot exceed that which is determined by the water requirement of the plant. Obviously in this region it is not advisable to devote much time or money to fertilizer experiments or to any other soil research which is not closely connected with water conservation. When arid lands are brought under irrigation, water no longer remains the limiting factor, but other and equally important factors may appear. These may be the scarcity of plant food in the soil, the lack of adequate drainage, plant diseases, insect pests, etc. Which of these becomes the limiting factor will depend upon local conditions.

Irrigation agriculture, as now practiced in Arizona, was begun only a few decades ago, and therefore it is unreasonable to assume that any of our soils have become worn out, or exhausted of plant food elements, as they are in some other sections which have been under cultivation for longer periods. However, there is strong evidence that the limiting factor in many of our soils is the scarcity of soluble phosphates. At the same time there is apparently an abundance of total, or insoluble phosphate in the soil. Evidently some specific cause or causes must be operating to keep the phosphate out of solution, and thus to reduce fertility.

On the basis of these observations and theories, a representative group of fields was selected, and samples of soil taken from them for study.

It is evident that any method of estimating the phosphate requirements of a soil should be developed and correlated with extensive field tests, in order to prove the relationship between the method and crop perform-

PHOSPHATE SOLUBILITY STUDIES 331 ance. Not having facilities to do this, our investigations for the present are being confined to solubility studies and small scale pot experiments.

S O I L S A M P L E S

These soils were selected from different parts of the State, where distinct differences in fertility were noticed in the same soil type, in the same or adjoining fields. When possible, selections were made according to degree of permeability and areas of high alkalinity were avoided.

Often a poor spot in a good field may be explained by the presence of alkali, or by too much or too little water, or some other equally evident reason. In the poor soils selected, no cause of infertility was evident, except possibly poor penetration of water. The good and poor soils which were compared, were under good management, and often each had the same cultural treatment, yet one would produce good crops of alfalfa or cotton, while the other was less productive.

DESCRIPTION OF SOILS

The following is a brief description of the soils used:

No. 1. A poor clay-loam soil from a ranch near Scottsdale. This soil had already shown indications of improvement in fertility, when leached, before planting to cotton or alfalfa. The soil had a moisture equivalent of 22.8.

No. 2. A good clay-loam soil from a ranch near Peoria. This soil had a moisture equivalent of 18.8.

No. 3. A poor clay-loam soil taken just across the road from No. 2, and to all appearances representing the same soil conditions. This had a moisture equivalent of 15.9.

No. 4. A good clay-loam soil from a ranch near Mesa. This field, just 2 weeks before sampling, had received 300 pounds of superphosphate per acre. Penetration was good.

No. 5. A good clay-loam soil, adjacent to No. 4, which had received no phosphate fertilizer.

No. 6. A good clay-loam soil from a ranch near Mesa, from a good spot in an alfalfa field, where penetration was fair. 0 to 8 inches.

No. 7. Sa-me soil, 8 to 18 inches.

No. 8. Soil from poor spot in the same field as sample No. 6, where penetration was poor. 0 to 8 inches.

No. 9. Same soil as No. 8, 8 to 18 inches.

No. 10. A poor black alkali soil from the old University Farm near

Tucson This is included in this series in order to illustrate the effect of black alkali upon phosphate solubility.

332

TECHNICAL BULLETIN No. 35

No. 11. A poor soil from a ranch near Yuma. This soil contained a relatively large amount of white alkali, and had a moisture equivalent of

23.1.

ALKALI ANALYSES

As a matter of interest, the alkali analyses, that is, the soluble salt contents of these soils are given in table I.

TABLE I.*—ALKALI ANALYSES OF SOILS USED. RESULTS IN

PARTS PER MILLION DRY SOIL.

Sample

No.

l g

2-G

3 P

4 g

5g

60

70

8 P

9 P

10

U p

Total soluble salts

Sodium

Na

Calcium

Ca

Magne sium

Mg

601

484

462

677

586

529

796

595

678

3166

8930

129

95

91

181

181

147

230

169

205

1013

2578

0

30

345

0

0

0

0

30

30

15

30

15

15

22

0

Trace

7

7

7

7

8

90

Chlorides

Cl

110

100

90

100

100

70

120

90

100

360

1950

Sulphates

so.,

0

625

3650

0

0

0

•0

0

0

0

0

Carbon- Bicarates bonates

COa HCO

3

0 317

0 244

0 244

0 366

0 305

0 305

0 439

0 329

0 366

228 902

0 317

* Capital letters represent good (G) and poor (P) samples from same field. Small letters represent good (g) and poor (p) fields from which only one sample was taken. These designations will be used throughout the bulletin.

There does not appear to be any outstanding relation between the soluble salt content of these soils and their fertility, as determined by field observations. The pot cultures shown in figures 1, 2, and 3 also prove that salt concentration is not a direct factor as the soils used in these pots were not leached.

PHOSPHATE CONTENT OF GOOD AND POOR SOILS

There have been many methods suggested for the determination of available phosphorous in the soil. A few of these were compared with the good and poor soils just described. These data are shown in table II.

As stated already while the calcareous soils which have been studied are often very low in water-soluble phosphorus, they are rarely deficient in the insoluble form. The total phosphorus was determined by analyses of extracts obtained by digesting the soils in aqua-regia, column 3. The citric-acid-soluble was determined by leaching 10 grams of soil with 1percent citric acid, until the leachate maintained a reaction of pH 4.0,

(column 4 ) .

l p *

2G

3 P

4 g

5 g

6 G

7Q

8 P

9 P

10 l i p

PHOSPHATE SOLUBILITY STUDIES

333

TABLE IT.—REACTION AND PHOSPHATE CONTENT OF SOILS.

Soil

No.

Reaction p H

Total

P O t percent

Soluble in 1% citric acid percent

Soluble in

1%K

2

CO

3 percent

Soluble in

1 :5 water extract ppm. ext.

Pounds P.

per acre

Truog method

8.3

8.0

7.8

7.7

8.0

7.7

8.1

8.1

8.2

9.4

7.8

0.147

0.287

0.246

0.471

0.451

0.287

0.205

0.184

0.143

0.307

0.180

0.081

0.203

0.154

0.185

0.179

0.077

0.091

0.092

0.057

0.199

0.046

0.0044

O.0200

0.0106

0.0286

0.0156

0.0282

0.CO28

Trace

0.0061 I 8.0

0.0024 . Trace

Trace

1.2

0.4

6.2

0.6

0.6

0.4

Trace

104

522

522

391

417

639

456

717

391

391

52

In determining the solubility of phosphorus in potassium carbonate solutions, a method suggested by Das(2), which consists in shaking the soil with a 1-percent solution of potassium carbonate, was used (column

In column 6 is shown the solubility of phosphorus in water, obtained by shaking 100 grams of soil with 500 cc. of carbon-dioxide-free water.

In column 7 is shown the determination of available phosphorus by a method proposed recently by Truog(9). This consists in extracting the soil with .002-normal sulphuric acid, buffered with 3 grams of ammonium sulphate per liter. The dissolved phosphate was determined colorimetrically by Deniges' method. The method of Truog was included because of the extent of its adoption by many soil investigators. The soil reactions are given in column 2.

Regarding the data in table II it will be noted that all soils are well supplied with total phosphate, varying from 2,800 to 9,000 pounds per acre

(2,000,000 lbs. soil). The solubility in citric acid is also very high, varying from .046 to .203 percent or 1,380 to 6,090 pounds per acre. However, it is significant that numbers 1, 9, and 11 are among the lowest in citric-acid-soluble phosphate. The use of 1-percent potassium carbonate solution as a solvent for available phosphate has been recommended, for calcareous soils, by Das (2'), who has had some success with its use on the soils of India. This method shows a rather good agreement with the known availability of phosphate in the set of soils examined. The poorest soils, numbers 1, 8, and 11 which represent extreme cases of phosphate deficiency gave remarkably low values. The method warrants further

PHOSPHATE SOLUBILITY STUDIES 335 in calcareous soils has not attained sufficient reliability to be used in routine determinations. In view of the fact that there is a close agreement between the solubility of phosphate in distilled water and fertility, as well as with the performance of these same soils in pot experiments, some modification of this method appeared to be most desirable for studying the problem at hand.

The high solubility of phosphate in citric acid shows that phosphate will readily enter the soil solution provided sufficiently low pH (high acidity) is attainable. The only reagent with which the plant is equipped to create a lower pH is carbon dioxide. But opposed to this attainment are two factors: (1 ) the buffering property of the soil, caused largely by calcium carbonate, and (2) the property of soluble salts of reducing the phosphate concentration of the soil solution. All of which illustrates the need for fundamental solubility studies, using water as the solvent, or vehicle.

All analyses of soil solutions, that is as near as any one has yet approached the true soil solution, have shown the presence of only very small amounts of phosphate. Rarely does it exceed 1 part of inorganic phosphate per million. This fact also holds true for the determinations we have made on Arizona soils. From such a low concentration the plant must supply its phosphate requirement which is very high as compared to the very soluble food materials such as sulphate, potash, lime, etc. In view of the fact that crops demand the largest part of their phosphate requirements during the early stages of growth one must admit a high state of efficiency in the mechanism of phosphate absorption even where soluble phosphate is abundant in the soil.

This further assists one in visualizing the difficulty met in attempting to determine the available phosphate supply and estimate its rate of solubility, especially in a calcareous soil. Dilute acids must necessarily react with calcium carbonate in preference to phosphate, so this precludes their use. Alkalis or water, CO

2

-saturated, or CO

2

-free, are the only solvents, which are at all suitable, and of these water appears preferable.

But whether used as the displaced soil solution, or as an extracting or leaching liquid, it may have its limitations. Since equilibrium relations maintain a low concentration of phosphate in the soil solution one is not always primarily interested in the actual amount of phosphate in the soil solution or in a water extract. However, one is interested in the ability of the soil to replace phosphate in the soil solution as rapidly as it is withdrawn by the plant, in other words, its velocity of solution. Of equal importance are the environmental factors which offer resistance to normal root respiration and development, notably the mechanical condition of the soil. A logical knowledge of the properties of soil phos-

PHOSPHATE SOLUBILITY STUDIES 339 solubility. T h e dry weight of plants and phosphate absorbed per 100 plants is shown as follows :

Untreated .

Fertilized

Dry weight grams

410

8.05

Phosphate grams P

0.0099

0.0252

It is significant in this part of the experiment that even with the better type of soil the millet plant shows response to phosphate both in the dry weight and in the amount of phosphate absorbed.

Comparing the grams phosphorus per 100 plants in the three different soils, untreated, the weight of phosphorus absorbed by the plant correlates well with the classifications made according to the condition of the crops in the field at time of sampling.

The only field experiment available was located on soils 4 and 5, where

300 pounds per acre superphosphate had been added after plowing and before seeding to alfalfa. The soil was disked after fertilization. Samples 4 and 5 were taken about one week after fertilization. The fertilized and unfertilized plots were again sampled six months later and samples of alfalfa from the same plots were also obtained for analysis.

Soil samples. Soil samples were taken to a depth of 4 feet, each foot being kept separate, and analyzed separately by shaking with CO

2

-free, and CO

2

-saturated water in the proportion of 1 part soil to 5 parts water.

The results are given in parts per million of water extract.

Untertilized plot:

First foot

Second foot

Third foot

Fourth foot

Fertilized plot:

First foot

Second foot

Third foot

Fourth foot

P.p.m. PO4 P.p.m. PO

4

CO-free CO-saturated water extract water extract

0.8

0.2

0.0

0 0

1.4

0.3

Trace

Trace

1.2

0.6

0.0

0.0

3.4

0.3

Trace

Trace

The effect of fertilization upon water-soluble phosphate is still shown after a period of 6 months. The low r

solubility of phosphate in the third and fourth depths is due to the presence of a calcareous hardpan, or caliche.

340

TECHNICAL BULLETIN No. 35

Alfalfa analyses. Ash and phosphate determinations were made upon the samples of alfalfa with the following results :

Alfalfa, unfertlized..

Alfalfa, fertilized

Ash

Percent dry matter

10.28

10.27

Phosphate (POO

Percent dry matter

Phosphate (PO*)

Percent ash

0.983

1.112

9.5

10.8

This soil had been classified as a good soil but it will be noted that the alfalfa plants absorbed slightly more phosphate on the fertilized plots both as percent dry matter and percent ash, There was little or no difference in the size of the plants growing in the two plots but unfortunately the crop had to be cut early because of an attack by aphids.

We believe that the culture experiments show beyond question that a phosphate deficiency is a major factor in the variable fertility and cropproducing power of the soils represented by our samples.

E F F E C T OF STATE OF SUBDIVISION U P O N

SOLUBILITY O F SOIL P H O S P H A T E

In contemplating our investigations the question of fineness of division of the soil particles and solubility of phosphate arose. That is to say, might it not be possible for solubility of phosphates to be limited by protective films which prevent the soil solution and the active agents of solution from coming in contact with particles of calcium phosphate. It is known, for example, that in manganiferous soils, the manganese is present as a film around the soil particles and the soil particles are thereby protected to a certain extent from the soil solution. Fineness of division has also been shown by Kelley, Dore, and Brown, to influence the quantitative base exchange capacity of soil colloids (5 ).

In order to determine this, two soils, 1 and 2, a poor and a good soil respectively, were ground in the ball mill as follows:

Soil number I. This soil was ground for 73 hours and at the end of this time the solubility of phosphate in CO

2

-f ree, and CO

2

-saturated water, was determined. The results are given in table III.

It will be noted that the effect of grinding in the case of this soil was to decrease the solubility of phosphate. While the solubility in this soil is very low, the molybdic-blue color wias distinctly less for the extracts of the ground soils, and these differences are signified by the plus signs.

On shaking the ground soils with water they flocculated very rapidly,

PHOSPHATE SOLUBILITY STUDIES

341 in contradistinction to the unground soils. This was due to the conversion of calcium carbonate into a more finely divided form, more active in reducing the solubility of phosphate. This will be discussed later. This fact is also manifested in the reaction of the extracts.

TABLE III.—EFFECT OF GRINDING ON PHOSPHATE SOLUBILITY I.

Phosphate p.p.m. PO4 in ext.

Ground soil, CO

3

-free water

Unground soil, CO

2

-free water

Ground soil, CO

2

-saturated water

Unground soil, CO

2

~saturated water.

Trace+

Trace+ +

Trace+

0.2

Soil number 2. This soil, which is fairly well supplied with watersoluble phosphate, was ground in a like manner for 72 hours, and the solubility of phosphate determined as with the preceding sample. The results are given in the following table: '

TABLE IV.— EFFECT OF GRINDING ON PHOSPHATE SOLUBILITY II.

Reaction pH PO4 p.p.m. ext.

Ground soil, CO

2

-free water

Unground soil, CO

2

-free water

Ground soil, CO

2

-saturated water

Unground soil, CO

2

-saturated water.

Here again, grinding the soil has reduced the solubility of phosphate.

The remaining portion of the soil was then returned to the ball mill, and ground for 50 hours, making 122 hours in all. The soil was then extracted with water as above with the results given in table V.

TABLE V.— EFFECT OF GRINDING ON PHOSPHATE SOLUBILITY III.

p H

P.p.m. in extract

PO

4

Ca

Ground soil, CO^-free water

Unground soil, CO

2

-free water

Ground soil, CO

2

-saturated water

Unground soii, CO

3

-saturated water.

?.$

7.8

6.0

6.0

0.5

1.2

0.4

1.6

33

33

270

303

342

TBCHXICAl BULLETIN No. 35

l p

2G

3 P

4 g

5g

6G

7 G

8 P

9 P

10 l i p

Grinding has further reduced the solubility of the phosphates, and this shows quite conclusively that both fineness of division, and soluble calcium, will reduce phosphate solubility, and probably availability. On this basis, it was thought best to conduct all our solubility studies upon unground 'samples, so as to represent field conditions as nearly as possible.

RATE OP SOLUTION

The next experiment was to determine the rate of solution by means of continuous leaching. Three hundred grams of soil, including each of the samples being studied, were placed in percolators and leached with seven 500-cc. portions of distilled water. The first 500 cc. of leachate was rather completely analyzed while only water-soluble phosphate, total solids, hydroxyl ion and bicarbonate were determined in the rest. These results are given in tables VI, VII, and VIII. Hydroxyl and bicarbonate determinations on the successive teachings are omitted for brevity.

Bicarbonates showed a steady decrease in all cases while only samples 1,

9. and 10 showed traces of hydroxvl ions after the second leaching.

TABLE VI.—ANALYSIS OF FIRST 500

c

.c. LEACHATE —RESULTS

GIVEN IN PARTS PER MILLION OF LEACHATE.

Soil No.

PO.i

O H

H C O

3

Solids Ca

Mg

K Na

T r

1.6

0.6

6.0

1.3

0.6

0.6

0.3

0.3

6.0

T r

T r

T r

T r

0

0

0

0

T r

14

T r

242

254

194

314

340

326

364

290

352

824

278

307

423

306

498

465

423

557

366

423

2175

2470

37

58

41

50

65

40

36

36

125

267

18

14

17

18

17

28

13

20

300

21

28

60

17

7

17

11

22

58

53

104

107

15

130

124

116

180

103

717

1976

The data in these tables are of decided interest. The successive leachings demonstrate a fact of essential value in estimating phosphate requirements of soils, and probably explain a great deal regarding the better crop performance of the soils which we have designated as good types.

As first stated, in attempting to determine the phosphate requirements of a soil one is interested not only in the total phosphate content of the soil, but also in the ability of the soil to maintain an adequate supply of phosphate in a water-soluble form in the soil solution. All the good soils, 2, 4,

5, 6, and 7, show the property of maintaining a good supply of phosphate

PHOSPHATE SOLUBILITY STUDIES

343

I p

2 G

3 P

4 g

Sg

6G

7G

8 P

9 P

10 l i p

TABLE VII.— PHOSPHATE CONTENT OF SUCCESSIVE LEACHINGS

IN PARTS PER MILLION OF LEACHATE.

Soil No.

1 2

3

4

5 6

7

Tr

1.6

0.6

6.0

1.3

0.6

0.6

0.3

0.4

6.0

Tr

0

1.0

Tr

12.0

0.9

0.6

1.0

Tr

Tr

4.4

Tr

Tr

0.9

Tr

9.6

1.6

0.9

1.0

0.4

Tr

2.4

0

0.8

0.9

0.5

T r

0

0.8

0

Tr

1.0

Tr

6.0

Tr

1.2

Tr

4.5

0.3

0.3

0.2

0

Tr

0

0

1.2

0

3.2

0.6

0.4

Tr

0

0

0.8

0,6

0

T r

0

1.2

0

1.5

0

TABLE VIII.—TOTAL SOLIDS CONTENT OF SUCCESSIVE LEACHINGS

IN PARTS PER MILLION OF LEACHATE.

Soil No.

1

2

3 4

5 6

7

I p

2G

3 P

4 g

Sg

60

70

8 P

10

9 P

U p

367

423

306

498

465

423

557

366

423

2175

2470

205

206

102

224

184

234

144

245

133

618

566

108

131

102

112

102

117

106

92

66

327

196

95

92

83

137

50

302

113

74

95

69

83

107

106

42

106

66

60

67

54

73

92

107

107

49

90

52

57

39

88

83

47

57

39

88

90

90

49

90

79 for the solvent action of water. All the poor soils either show only faint traces from the very beginning, or else the initial solubility rapidly diminishes to nothing. The other determinations which were made upon the leachates did not show any relation to the known performance of the soils.

The practicability of such a method as a routine procedure is not very promising for the types of soils under investigation, as leaching a column of soil is too slow a process. Almost two months was consumed in obtaining the seven 500-cc. leachings from these soils. Then again in any method of displacement the first leachings are always discolored, and it is impossible to obtain an accurate colorimetric determination of phosphate by the Deniges method. Even with the addition of Bismark brown to the standard, we failed to obtain accurate results. There is left then

344 TBCHX1CAI BULLETIN No. 35 only extraction methods for studying phosphates as, except for black alkali soils, the extracts are quite free from color.

Before reaching this conclusion several modifications of the Burdpressure-tube method of obtaining the soil solution were tried. The soils were packed in the pressure-tubes and leached, under pressure, with CO

2

free water. Successive leachings by this method up to the fourth 500 cc.

continued to dissolve color which eliminated this method. Carbondioxide-saturated water was then substituted with equally unsuccessful results. The leaching proceeded with extreme slowness in spite of tne pressure of 60 pounds applied, and then again the solvent effect of CO

2

saturated water under these conditions was less than that obtained by the method of shaking. Attempts to modify the method by packing the soil in the tubes both while air dry, and at optimum moisture content, and saturating the soil-water mixture with carbon dioxide, by passing a stream of CO

2

gas through the whole, before placing the soil in the tubes, failed to improve the defects of the method. The greatest drawback is that the leaching proceeds too slowly, but even with the slow leaching the low solubility of phosphate indicates that there must be considerable channelling, as illustrated in figure 12. The addition of small amounts of sodium chloride was also tried, but this too failed to increase the velocity of leaching, and it also reduced the solubility of phosphate. In view of this the use of salts as coagulants for the clay and organic matter is also out of the question, on account of the influence upon solubility of phosphate.

In view of the above, methods of leaching were abandoned for extraction methods. It is significant that, for calcareous soils, those showing an initially high concentration of phosphate by extraction with water, also continued to maintain high solubility in successive leachings.

ADDITIONAL SOIL SAMPLES

Up to this point in our study of phosphates the solubility agreed so closely with known availability, permeability, and growing conditions observed in the fields that we decided to procure soil samples from additional fields before proceeding further. A description of these follows:

DESCRIPTIONS OF SOILS

Nos. 12 and 13. Poor and good areas respectively from an alfalfa ranch at Litchfield. A clay-loam type.

No. 14. A poor field from a ranch near Litchfield.

No. 15. A poor field from a ranch near Litchfield.

No. 16. A good field from a ranch near Litchfield. This soil, unlike the rest, is a silty sand containing very little clay.

346

TECHNICAL BULLETIN No. 35

Nos. 29, 30, and 3 1 . Silty clay loam from the Bard, California, field plots of the office of W e s t e r n Irrigation Agriculture. T h i s soil gives a definite response to phosphate when cropped to alfalfa. N o . 29 represents the check plots, no phosphate, 30 received an application of phosphorus 3 years ago, and 31 was fertilized one month before sampling.

Nos. 32 and 33. Sandy loam from Newlands Experimental F a r m ,

Fallon, Nevtada. N o . 32 represents the fertilized and 33 the unfertilized plot. Th'fs soil does not respond to phosphate.

T h e alkali analyses ( 1 : 5 water extract) of these soils are given in table I X .

TABLE IX.—ALKALI ANALYSES —PARTS PER MILLION 1 DRY SOIL.

Soil

No.

Total solids

Sodium

Na

Calcium

Ca

Magnesium

Mg

Chlorine

Cl

Sulphate

so,

Car- 1 bonate

Bicarbonate

CO

3

HCO3

Nitrate

NO

3

Reaction pH

615

500

542

786

445

389

377

726

698

889

726

1031

1389

1574

716

4676

2614

21 P

22 G

23 P

24 G

25 P

26 G

12 P

13 G

14p

15 p

16g.

17 P

18 G

19 P

20 G

27 P

28 G

1609

909

15

15

75

45

30

45

30

0

0

15

45

45

45

15

30

15

15

264

202

301

464

403

183

147

87

112

222

92

67

45

200

201

8

0

0

8

8

8

0

0

8

8

0

8

8

0

8

0

8

140

60

130

10O

80

30

20

110

60

50

80

120

150

690

80

1930

880

0

0

600

330

0

0

0

0

0

0

0

0

0

0

0

0

0

15

15

15

20

15

15

15

20

15

10

15

10

15

20

15

20

20

268

280

232

427

220

224

224

336

378

402

524

488

488

451

415

437

422

12

36

84

252

0

0

0

36

0

0

0

12

Tr

0

0

0

0

7.85

7.70

7.50

8.20

7.95

7.50

7.85

8.10

8.55

8.10

8.65

8.55

9.20

8.20

8.10

7.95

8.35

PHOSPHATE CONTENT OF SOILS

The phosphate content of these soils as determined by five different methods is given in table X.

The data given in the preceding table are more or less in agreement with those already presented. That is, all the soils show a good supply of phosphate of good solubility in dilute acids. With few exceptions the solubility in 1-percent potassium carbonate is very low.

With these additional soils the solubility studies were continued.

In studying the phosphate content of any soil extract three things must

12 P

13 G

14 p

15p

16 g

1 7 P

18 G

19 P

20 G

21 P

22 G

23 P

24 G

25 P

26 G

27 P

28 G

29 p

30

31

32

33 g

PHOSPHATE SOLUBILITY STUDIES

326

284

52

52

130

340

247

808

1017

847

S3 4-

130

260

117

415

326

652

926

326

232

260

209

0.0028

0.0073

0.0065

0.0037

0.0032

0.0020

0.0032

0.0049

0.0032

0.0049

0.0032

0.0168

0.0192

0.0225

0.0044 .

0.0061

0.0013

0.0020

0.0033

0.0023

0.0015

1.0

5.0

1.0

1.6

0.6

2.0

1.0

Tr

0.4

0.6

0.3

0.3

1.6

0.4

2.0

0.8

1.0

1.0

0.6

0.3

0.8

0.5

347

TABLE X.—ANALYSES OF SOILS BY DIFFERENT METHODS.

Soil

No.

Total phosphate

Percent PO

4

Soluble in

t% citric acid

Percent PO

4

Truog method

Pounds P per acre

Soluble in

1 % K

2

C O

3

P e r c e n t P O t

1 :5 w a t e r extract

P.p.m. PO

4

0.461

0.410

0.236

0.348

0.574

0.584

0.584

0.195

0.184

0.235

0.195

0.255

0.378

0.502

0.328

0.554

0.328

0.276

0.164

0.292

0.205

0.339

0.354

0.246

0.142

0.178

0.141

0.193

0.225

0.055

0.051

0.086

0.088

0.116

0.267

0.297

0.389

0.369

0.079

0.104

0.193

0.169

0.094

0.287

be recognized in interpreting the results. These are the inorganic phosphate actually present in solution, the organic phosphate in solution, and the phosphate adsorbed by the finely divided and dispersed colloids. The first is of primary importance as it is, so far as we know, the only form taken up by the plant. It has been observed that if a soil extract, or solution, is evaporated to dry ness with magnesium nitrate and ignited, and then dissolved in acid this solution will always show larger amounts of phosphate than the original solution before evaporation and ignition.

This also holds true for the soils used in our investigations, in spite of the fact that in some cases not enough organic matter was present in the extract to show color during ignition. The phosphate brought into solution following ignition has been referred to as organic phosphate. Pierre and Parker (8) have shown that this is not absorbed as such by the plant.

Our observations lead us to question whether this phosphate is actually present in organic forms. At all events, however, it does not interfere with the colorimetric determination of phosphate in the extract, so will riot be further discussed at this point.

348 TECHNICAL BULLETIN No. 35

SALTS AS CLARIFYING AGENTS

Often in working with water extracts or leachates of soils finely dispersed colloids are present to such an extent as to introduce an error. In some cases the extracts may not be too cloudy for colorimetric comparison but it has been found that cloudy solutions always show higher concentrations of phosphate than do clear solutions. This appears to be due to phosphate adsorbed on the surface of the colloids. For quantitative work, therefore, all extracts must be as free from colloids as possible.

Filters of the Pasteur type absorb phosphate so they cannot be used for filtering extracts. In lieu of such a procedure some investigators have suggested the use of salts as clarifying agents. Wrangell(lO) extracted soils with 0.0125-percent potassium chloride, removing the soil by centrifuging and determining the phosphate in the supernatant liquid. In these investigations the soluble phosphate determined by this method agreed very closely w

T ith the Neubauer method for determining phosphate deficiencies in soils.

In our work somewhat similar experiments have been conducted but with little success. In most of our soils potassium chloride of the above concentration does not flocculate the colloids. After centrifuging, some of the supernatant extracts may even contain large amounts of reddish* colloidal clay. On the other hand, larger amounts of potassium chloride reduce the solubility of phosphate. This effect is demonstrated in the following experiment. The salt was first dissolved in the water, and the salt solution added to the soil just before shaking. Five parts water to one of soil were the proportions used, and they were shaken for one hour in an end-over-end shaking machine. The results are given in the following table and show a marked reduction in phosphate solubility where potassium chloride was added.

TABLE XL—EFFECT OF POTASSIUM CHLORIDE UPON THE SOLU-

BILITY OF SOIL PHOSPHATE.

PO4 in solution

1. Extract made with distilled water 1.2 p.p.m.

2. Extract made with distilled water — KCL 1 gm. per liter 0.3 p.p.m.

3. Extract made with distilled water — KCL 2 gm. per liter 0.2 p.p.m.

4. Extract made with distilled water — KCL 10 gm. per liter Trace

5. Extract made with distilled water — KCL 20 gm. per liter Trace

6. Extract made with distilled water — KCL 50 gm. per liter Trace

This phenomenon is true also with pulverized phosphate rock, as shown by the following experiment.

One gram phosphate rock was shaken with 500 cc. of water and allowed

PHOSPHATE SOLUBILITY STUDIES

349 to stand over night. The solutions were then filtered and analyzed for phosphorus.

TABLE XII.—EFFECT OF POTASSIUM CHLORIDE AND CALCIUM

CARBONATE UPON THE SOLUBILITY OF ROCK PHOSPHATE.

Solution

1. Phosphate rock, distilled water

2. Phosphate rock, saturated solution of CaCO

3

, no solid phase

3. Phosphate rock, 1% KCL

4. Phosphate rock, 1% KCL, 1 gm. CaCOa....

5. Phosphate rock, 1% KCL, in saturated solution of CaCO'a, no solid phase

P.p.m. PO

4

in I P.p.m. Ca in solution solution

5.6

22

3.4

1.2

Tr

5.3

3.7

15.0

0.6

18.0

Potassium chloride alone materially decreased the solubility of phosphorus in the rock, but potassium chloride and calcium carbonate prevented solution almost completely. When the potassium chloride was made up with a saturated solution of calcium carbonate with no solid phase present, No. 5, the solubility of the phosphorus was much less than calcium carbonate solution alone, No. 2. There was a reaction between the potassium chloride and the calcium carbonate, sufficient to dissolve 18 parts per million of calcium.

E F F E C T OF SODIUM CHLORIDE AND SODIUM S U L P H A T E

U P O N T H E SOLUBILITY OF P H O S P H A T E ROCK

Sodium chloride and sodium sulphate also decrease the solubility of rock phosphate, but not in the same degree, as is shown in table XIII.

TABLE XIII.—EFFECT OF SODIUM CHLORIDE AND SODIUM SUL-

PHATE ON THE SOLUBILITY OF ROCK PHOSPHATE.

Solution pH

P.p.m. PO4 in solution

1. Rock phosphate, distilled nvater

2. Rock phosphate, 1% NaCl

3. Rock phosphate, 1% NaCl-CaCO

3

solid phase

4. Rock phosphate, 1% NaCl saturated solution CaCOa..

5. Rock phosphate, 1% NaaSCX

6. Rock phosphate, 1% Na,SO

4

solid CaCO*

7. Rock phosphate, 1% NasSd saturated solution CaCO»

72

7A

8.2

7.6

7.2

8.6

7.8

6.0

0.9

0.3

0.7

1.6

0.5

1.1

It is evident here again that the use of salts should be avoided if possible.

The necessity of re-solution data was also recognized by Wrangell( 10), in which two successive extractions were made upon the soil, using 1 gram soil and 100 cc. of 0.0125-percent potassium chloride. After shak-

350

TECHNICAL BULLETIN No. 35

ing, these were centrifuged and phosphate determined in the supernatant extract. A second extraction was made upon this same soil, and the data a

2 substituted in the equation x = , in which " a " represents the a — b phosphate removed by the first extraction and " b " that removed by the second. In much of Wrangell's work it is evident that the information sought is essentially what we were seeking in our solubility studies. But we have no evidence as yet that re-solution velocity can be calculated from successive extracts. The variations are not exactly progressive in many cases, regardless of whether water is used as a solvent or as a vehicle for some other active agent.

The dissolution effects of the monobasic salts may be physical or chemical, but is probably largely the former. If a salt is added to a cloudy soil extract (filtered) there is no aid to filtration, or clarification, except on long standing. The salt must be added to the soil-water mixture preceding filtration of the whole. This is illustrated by the following experiment in which three soils were shaken with distilled water, 100 grams of soil 500 cc. water, as follows :

1. Control, 100 grams of soil, 500 cc. of water

2. 100 grams of soil, 500 cc. of water, 1 gram NaCl

3. Same as 2 except that NaCl added to filtrate from 1

The phosphate content of these three solutions in three successive filtrations was determined with the following results:

Solution a b c

1

Control

P.p.m. PO,

1.0

1.0

3.8

2

NaCl to soil

P.p.m. PO4

0.6

0.8

2.0

3

NaCl to cloudy filtrate

P.p.m. PO4

1.0

1.1

- 4.0

These data show quite conclusively that the phosphate is carried down with the soil mass during the rlocculation of the colloid. The principal significance of these data is that they demonstrate the influence which soluble salts in alkali soils have upon dissolution of phosphate especially in poorly drained areas and therefore the concentration and re-solution of phosphate in the soil solution.

In studying the solubility of phosphate in water extracts of soils two procedures are important.

1. Progressively varying the soil:water ratios for extraction.

2. Subjecting a single soil sample to successive extractions.

PO4 in CO-.-free water

PO

4

in CO-saturated water

Ca in COv-saturated water

TABLE XIV.—SHOWING EFFECT OF VARYING SOIL: WATER RATIO ON PHOSPHATE SOLUBILITY IN CO.-FREE AND CO2-SATURATED WATE

Soil water ratio

1-b 2-G 3-P

4-g

5-g

6-G 8-P

11-p

12-P

13-G I4~p

15-p

16-g 17-P

18-G 19-P

20-G 21-P 22-G

1:5

1:10

1 :25

1:50

1:100

1 :5

1:10

1:25

1:50

1:100

11:5

U :10

1 :25

71:50

( 1 :100

"f*

+

0.5

0.6

0.6

0.7

0.7

1.2

1.0

0.9

0.6

2.2

2.0

1.7

1.8

1.6

0.4

0.5

0.8

1.8

2.0

1.8

0.4

0.3

J

2.8

3.6

2.5

1.6

0.9

7.1

5.5

4.2

3.3

2.0

276

269

276

254

187

2.5

2 2

2*.l

1.6

1.4

1.0

0.8

1.1

0.5

0.3

210

157

97

75

60

1.4

1.9

2.8

3.2

2.8

1.0

1.0

0.7

0.5

0.4

75

82

135

45

38

1.2

0.6

1.8

25

2.3

+

-j-

-j-

-j-

+

0.4

0.5

0.5

0.5

0.5

470

396

336

291

239

164

120

75

60

52

0.7

0.4

0.3

-|-

+

0.9

1.6

3.8

6.2

6.0

1.0

1.2

1.5

2.5

4.5

239

187

112

75

67

0.9

0.7

0.5

0.4

0.3

217

165

97

75

45

1.0

0.6

0.4

0.3

+

1.2

2.0

3.3

6.0

6.2

270

225

150

120

67

0.9

1.1

2.4

4.0

4.2

0.6

0.5

0.4

0.4

0.3

0.8

1.1

1.0

0.9

0.8

292

299

262

172

135

0.3

0.2

0.2

-|-

+

0.3 0.3

0.2 0.2

0.2

217

165

105

62

52

0.4

0.5

0.9

1.4

1.7

292

285

239

172

112

0.5

0.6

0.5

0.6

0.6

262

270

239

187

135

26

2.3

2.0

1.7

1.6

1.8

1.6

1.2

0.9

0.7

246

246

142

127

97

0.6

0.6

0.7

0.8

0.9

0.5

0.3

0.2

1.5

1.0

0.7

0.6

"'0.5

2.0

2 0

2.8

4.0

4.4

269

246

195

142

105

254

262

209

150

120

3.0

3.0

3.2

3.2

3.2

2.5

1.8

1.3

1.0

0.7

'ING SOIL: WATER RATIO ON PHOSPHATE SOLUBILITY IN CO,-FREE AND CO

2

-SATURATED WATER. RESULTS IN PARTS PER MILLION EXTRACT.

8-P

11-p

12-P

13-G

14-p

15-p

16-g 17-P 18-G 19-P

20-G

21-P 22-G 23-G 24-P 25 P 26-G

27-P 28-G 29-p 30-p 31-p 32-g

33-g

210

157

97

75

60

1.4

1.9

2.8

3.2

2.8

1.0

1.0

0.7

0.5

0.4

+ + +

+ +

+

+

+

1.2

0.6

1.8

25

2.3

75

82

135

45

38

+

+

+

+

0.4

0.5

0.5

0.5

0.5

470

396

336

291

239

0.9

1.6

3.8

6.2

6.0

0.7

0.4

0.3

+

+

164

120

75

60

52

0.9

0.7

0.5

0.4

0.3

1.0

1.2

1.5

2.5

4.5

239

187

112

7S

67

1.0

0.6

0.4

0.3

+

1.2

2.0

3.3

6.0

6.2

217

165

97

75

45

0.9

1.1

2.4

4.0

4.2

0.6

0.5

0.4

0.4

0.3

270

225

150

120

67

0.8

1.1

1.0

0.9

0.8

0.3

0.2

0.2

+

+ -

292

299

262

172

135

0.4

0.5

0.9

1.4

1.7

0.3

0.2

0.2

+

+

217

165

105

62

52

0.5

0.6

0.5

0.6

0.6

0.3

0.2

+

+

+

292

285

239

172

112

26

2.3

2.0

1.7

1.6

262

270

239

187

135

1.8

1.6

1.2

0.9

0.7

0.6

0.6

0.7

0.8

0.9

0.5

0.3

02

+

+

246

246

142

127

97

2.0

2.0

2.8

4.0

4.4

269

246

195

142

105

1.5

1.0

0.7

0.6

0.5

3.0

3.0

3.2

3.2

3.2

25

1.8

1.3

1.0

0.7

254

262

209

150

120

269

284

267

246

187

9.2

8.6

7.1

5.2

4.5

5.0

2.2

2.6

1.8

1.4

232

254

254

369

172

2.4

2.6

2.4

2.5

2.1

1.4

1.1

0.7

0.6

0.4

2.2

2.0

1.3

0.8

0.6

262

262

262

246

187

3.3

3.2

3.1

2.9

2.6

0.7

0.6

0.4

0.3

0.2

1.7

2.0

2.4

2.1

1.9

269

284

284

269

232

2.0

2.2

2.3

2.3

2.1

232

254

270

262

240

3.1

3.2

0.7

0.6

232

232

246

270

209

2.0

1.6

1.4

0.9

0.7

1.5

1.1

1.3

1.2

1.1

+

+

-h

+

+

0.2

0.3

0.3

0.4

0.4

0.5

0.5

0.4

0.4

0.4

0.4

-}-

+

-f-

+

0.6

0.6

0,5

0.4

0.3

1.9

1.8

1.5

1,2

1.0

0.8

0.7

0.5

0.3

0.3

5.0

4.2

3.6

2.8

1.8

0.5

0.5

0.5

0.5

0.4

1.2

1.6

2.9

2.5

2.1

PHOSPHATE SOLUBILITY STUDIES 351

P R O G R E S S I V E L Y V A R Y I N G S O I L : W A T E R R A T I O S

In this experiment separate weighings of soil were placed in bottles with the volumes of water shown and extracted by two methods.

1. Shaken for one hour in an end-over-end shaking machine, filtered, and phosphate determined in the filtrate. 2. Same as 1, except that C O

2

gas was passed through the whole for 15 minutes, filtered, and P O

4

determined in the filtrate. Phosphate was determined in all of the extracts and calcium in most of them. These data are given in table X I V .

T h e water extracts show a steadily decreasing concentration of phosphate in solution with dilution. In most cases the decrease is more rapid in the poor soils but this does not hold true for all samples. In agreement with our other observations the soils of highest initial solubility exhibit best continuous solubility.

T h e solubility in CO

?

-saturated water is of considerable interest. In the first set of soils which are samples of definitely known phosphate availability, every soil of low availability shows an increasing phosphate solubility with increase in soil: water ratio or dilution, while the opposite is true for the good soils and where superphosphate has been added to the soil. T h e same relations hold true to a certain extent in the second set of soils but our information regarding phosphate availability in these is less definite. I n view of the fact that the poor soils possess a higher clay content this may be an adsorption phenomenon. Calcium ion plays an important part as shown by the fact that where P O

4

concentration increases with dilution, calcium concentration shows a decrease, while where Ca concentration remains high up to 1 :100 dilution, P O

4

decreases or shows little change in concentration.

SUCCESSIVE E X T R A C T I O N S

Successive extraction as a means of obtaining solubility data offers some difficulty, chiefly in the matter of obtaining clear extracts. The magnitude of re-solution velocity for phosphate should be experimentally shown by repeated extraction and for this reason much time has been spent in studying this phase of the problem. We have already mentioned the dissolution effects of salts and in spite of the fact that this factor is met by the plant in obtaining its phosphate from the soil under field conditions their value in the laboratory is questionable. Water alone is an impossibility although for a single extraction by repeated filtration through the mat of soil on the filter paper sufficient clear extract for a determination may usually be obtained.

Soil colloids are easily flocculated by carbon dioxide and since this is the agent at the plant's disposal for attacking the insoluble soil phosphates it appeared to warrant most study. All our soils contain calcium car-

352 TECHNICAL BULLETIN No. 35 bonate in varying amounts and with varying rates of solution. That is, in many calcareous soils the calcium carbonate appears to be protected by films deposited upon the surfaces of the soil particles. This materially retards the velocity of solution or re-solution of calcium. An example of this is given in the experiment showing the effect of grinding upon phosphate solubility. In view of this, and in order to have calcium carbonate in an active form present in all cases, it should be added to the soil preceding extractive methods. In this manner a more or less constant ratio between CO

2

:Ca(HCO

3

)

2

:CaCO

3

will be maintained as these are the principal factors governing phosphate solution and re-solution in soils. This conclusion was reached after several attempts to use carbon dioxide alone.

When carbon dioxide was passed through the soil-water mixture to saturation, phosphate solubility was greatly increased even in the poorest soils while if the water is saturated with carbon dioxide before shaking with soil, there is no increase in solubility for the poor soils. One hundred-gram samples of soils 1 and 11, the two poorest soils, were mixed with 500 cc. distilled water and a stream of carbon dioxide passed through the whole for 15 minutes. These were filtered with the aid of suction, the soil returned to the bottle and treated again with carbon dioxide in the same manner. This was repeated to obtain six successive extractions and the phosphate determined by the molybdic-blue method in all. The results are given in the following data as p.p.m. PO

4

in the extract.

5oil

1.

2

1

0.44

0.38

Number or

2 3 extractions

4 5

0.56

0.38

0.60

0.38

0.80

0.38

0.80

0.46

6

0.90

060

These data indicate that when subjected to the successive attack of carbon dioxide, phosphate solubility will be maintained indefinitely or even be increased as the number of extractions increases. This applies to the poor as well as the good soils.

In our next experiment less soil was used, 10 grams per 100 cc. of water, and after the first treatment with carbon dioxide as above, the subsequent extractions were made with CO

2

-ffee water. These results are given in the following data as p.p.m. PO

4

in the extract.

Number of extractions

Soil 1 2 3

1

2

3

... 0.7

2.5

1.0

0.4

1.2

1.0

Trace

0.56

0.32

PHOSPHATE SOLUBILITY STUDIES 353

Extractions had to be discontinued at this point because the soil had become too badly dispersed to permit filtration.

A n attempt to modify this was made by substituting tap water for distilled water after the initial treatment with carbon dioxide. In this experiment 50 grams of soil and 250 cc. of water were used. While this modification did aid filtration to a certain extent the results obtained were very similar to those given in the preceding table.

These data show that unquestionably carbon dioxide is the most satisfactory aid to filtration and to solution studies on soil phosphates.

Extractions with carbon-dioxide solutions can be continued indefinitely without loss in phosphate concentration of the extract. If the initial extraction with carbon dioxide is followed by extraction with CO

2

-free water or tap water, solubility progressively decreases but clay dispersion increases to such an extent as to prevent filtration and therefore interferes with the colorimetric determination of phosphate in the extract.

Calcium carbonate is present in all the soils, but in different physical states which determine its rate of solution. This is probably due either to the size of the calcium carbonate particles or colloidal coatings deposited upon their surfaces. T h e degree of activity correlates with the degree of dispersion. T h e same phenomenon may be true with magnesium carbonate. In view of this it is evident that uniformly consistent solubility data should be obtainable if equally active calcium carbonate is present in all cases. This state was accomplished in the following experiment.

Thirty grams of soil were weighed into a 1-liter bottle, 1 gram calcium carbonate added and then shaken with CO

2

-saturated water. T h e latter was prepared by passing carbon dioxide through distilled water until 100 cc. was equivalent to 80 cc. of 0.1 normal sodium carbonate by titration.

T h e soil-water mixture was then shaken for one hour in an end-over-end shaking machine, filtered on a Buchner funnel w r ith the aid of a suction pump, and phosphate determined in the filtrate by the molybdic-blue method. T h e soii was returned to the bottle, another 100 cc. of C O

2

saturated water added, and the extraction repeated as above. This operation was continued until four successive extractions had been made. T h e data are given in table X V .

In most cases, in fact in all cases where we are positive that phosphate deficiency is a major factor, the rate of re-solution is nil or very low.

T h e r e is a very close agreement between these data and fi-eld performance or response to phosphate and the method has considerable merit as a measure of the re-solution velocity of soil phosphate.

Dirks and Scheffer ( 4 ) , recognizing the relation between carbon dioxide, calcium bicarbonate, and phosphate availability devised a method, based

6G

8 P

10 l i p

12 P l p

2G

3 P

4 g

Sg

13 G

14p

15p

16 G

17P

18 G

19 P

20G

21 P

22 G

23 G

24P

25 P

26 G

27 P

28 G

2 9 p

30 p

31 p

32 g

33 g

354

TECHNICAL BULLETIN No. 35

TABLE XV.—SUCCESSIVE EXTRACTIONS OF PHOSPHATE WITH

CO a

-SATURATED WATER. RESULTS IN PARTS PER MILLION

EXTRACT.

0.44

0.60

0.48

0.30

Trace

0.30

1.86

Trace

1.60

0.90

5.00

0.90

1.60

0.80

0.70

Trace

1.00

0.30

4.00

1.00

0.50

Trace

1.80

Trace

0.40

0.70

Trace

Trace

0.90

0.40

0.40

0.30

1.80

0.20

1.10

0.74

3.80

0.30

1.40

0.70

0.50

0.50

Trace

Trace

1.0

0.60

Trace

Trace

0.70

Trace

3.30

0.70

0.40

Trace

1.30

Trace

0.36

0.40

0.56

0.34

0.30

0.20

3.80

0.70

1.24

0.70

0.60

0.60

Trace

Trace

1.10

0.70

0.60

0.50

0.70

0.60

0.30

Trace

0.30

1.70

Trace

1.50

1.20

Trace

0.70

Trace

3.30

0.80

0.40

Trace

1.20

Trace

0.50

0.76

0.90

0 7 4

0.36

Trace

Trace

1.70

Trace

2.00

1.60

3.30

0.60

1.12

0.70

0.52

Trace

0.70

0.30

3.00

0.80

0.40

Trace

1.40

Trace

0.44

0.48

Trace

Trace

0.90

0.50

0.60

largely on phenomena similar to what we have just discussed and for the soils used yielded results that agreed very closely with the Neubauer method. Their method consists of two distinct steps as follows:

(a) Thirty grams of soil to which 1 gram calcium carbonate is added, are shaken with 75 cc. of CO

2

-saturated water for one hour, filtered, and phosphate determined in the filtrate; (b) a similar extraction is made with 0.0125-percent potassium chloride solution. The results are expressed in test units ( T ) , each unit representing 0.035 milligrams

P

2

O

5

* in 100 grams of soil. The ratio of the (a) value to (b) value

In our table the results are calculated to PO4, as this is used throughout our bulletin.

PHOSPHATE SOLUBILITY STUDIES

355 represents relative availability and was found to decrease with increase in pH of the soil. This method was included in our solubility studies and the results are given in the following table.

TABLE XVI.— DIRKS AND SCHEFFER METHOD FOR DETERMINING

PHOSPHATE AVAILABILITY

Soil l p

2G

3 P

4 g

5g

6 G

8 P l i p

12 P

13 G

14 p

1 5 p

16 g

17P

18 G

19 P

20G

21 G

22P

23 G ppm.

PO<

Trace

1.00

0.20

3.40

0.60

0.54

0.20

Trace

0.30

0.50

0.50

0.30

0.20

0.20

0.20

1.50

0.20

1.10

0:60

4.00

Mg. per

100 gm.

Trace

0.332

0.066

1.113

0.200

0.180

0.066

0.099

0.167

0.167

0.099

0.066

0.066

0.066

0.501

0.066

0.363

0.198

1.333

0.264

0.303

0.158

0.178

0.191

ppm.

Ti PO

4

; Trace

9.5 | 0.80

1.9 i 0.20

31.7

]

2.60

5.7 ; 0.44

5.1 ! 0.52

1.9 ! 0.16

Trace

2.8 1 0.80

4.7 0.50

i

4.7 0.70

2.8 0.56

1.9 | Trace

1.9 0.30

1.9 ; 0.20

j

14.3

1 0.80

1.9 I 0.26

10.3

| 1.50

5.6 i 0.90

38.1

; 2.20

0.265

0.167

0.232

0.186

0.099

0.066

0.264

0.085

0.495

0.297

0.732

Mg. per

100 gm.

Trace

0.265

0.066

0.866

0.146

0.173

0.053

T

2

7.5

2.4

14.0

8.5

20.9

1.4

5.3

2.8

1.9

7.5

1.9

24.7

4.2

4.9

1.5

7.5

4.7

Ratio

T

2

:Tx

0.80

1.00

0.78

0.73

0.96

0.45

24 P

25 P

26 G

27 P

28 G

0.80

0.92

0.48

0.54

0.58

7.5

;

8.6 1.00

4.5 0.60

5.1 0.40

5.4 0.20

1

0.429

0.198

0.132

0.660

12.3

5.7

3.8

18.8

1.10

1.30

0.70

0.34

The T values, both as obtained with calcium bicarbonate and potassium chloride closely correlate with the known availability of phosphate in these soils. On the other hand the ratios do not. Dirks and Scheffer used soils varying in reaction from pH 4.0 to 7.5 and in their soil series the ratio decreased with increase in pH. Availability also increased with increasing pH and diminishing ratio. Our soils are all of closely grouped reactions and above neutrality which may be the reason why our data, when calculated to ratios, show little significance.

2.70

0.50

0.70

1.90

1.30

1.00

0.50

1.30

1.40

1.50

0.55

ELECTRODIALYSIS

The principle of dialysis, using collodion sacks, for studying the phosphate solubility in soils has been used by Pierre and Parker(7). The principle of electrodialysis has been extensively applied in soil investiga-

356 TECHNICAL BULLETIN No. 35 tions notably for studying base exchange phenomena and other properties of soil colloids. It occurred to us that electrodialysis, using a current of low voltage, would be an improvement over the collodion-sack method and yield data of value in evaluating the re-solution velocity of soil phosphates.

We obtained a small rectifier (battery charger) of 2.5-ampere, 2- to 12volt capacity and attached this to a 110-volt A.C. circuit. A dialysis chamber similar to the Mattson cell(6) was used except that the middle dhamber was smaller, 100-cc. capacity, and the two outer chambers larger,

1-liter capacity. A current of air was used in the middle chamber to keep the soil-water mixture in constant ebullition. Ten grams of soil and 100 cc. of water were placed in the middle chamber and one liter of water in each of the outer chambers. Platinum gauze was used for the anode and copper plate for the cathode. Tap water, free from phosphate, was used in the outer chambers in order to maintain a uniform ampereage. During the entire series of analyses the current varied between 0.1 and 0.3

amperes and from 9 to 11 volts. Samples were withdrawn from the anode chamber at hourly intervals and phosphate determined in these by the molybdic-blue method. The data are given in table XVII.

On the whole, the rate of dialysis and total PO

4

concentration in the anode chamber show considerable variation and less correlation with the good and poor classification than the CO

2

-CaCO

3

method of extraction or the dilution experiment. There are clearly evident reasons for this, however. The data obtained on the first 11 soils are extremely consistent with phosphate availability as shown both by the plant cultures and in the solubility data. Reference is made to the fact that for all the poor soils the concentration of PO

4

in the anode chamber reaches a maximum concentration of about two parts per million and does not increase beyond this. The data obtained on the rest of the soil samples, which admittedly have not been under as close observation as the first set, are not so convincing. We are not definitely assured of their state of phosphate deficiency. It is believed that in the electrodialysis of soils it will be difficult to compare the data with solubility as obtained with CO

2

-saturated water because of the different manner in which the physical state of the soil will influence solubility. A highly colloidal type which will not yield its adsorbed phosphate to a water extract will quite readily give up adsorbed phosphate when subjected to electrodialysis. The same line of reasoning

would apply to highly saline types in which salt concentration might interfere with phosphate solubility as measured by water extraction but would have less influence in electrodialysis because the salts would soon be be removed from the middle chamber which contains the soil. It is

PHOSPHATE SOLUBILITY STUDIES 357 believed that these two factors account for the large amounts of phosphate obtained in the dialysis of some of the designated poor soils.

T h e influence of physical composition of the soil is well illustrated in soils 16 and S3. These are sandy types and are quite definitely known to be well supplied with available phosphate, that is, they do not respond to phosphate fertilization, yet they are low in dialysible phosphate and all our extraction studies showed a low solubility. It is evident from this that one must take into consideration the physical composition, in other words the soil environment to which the plant roots are subjected during their foraging activities in interpreting solubility data. T h e effect of physical state as illustrated in figures 6 and 7 and salt concentration will be dealt with in a later bulletin so will not be discussed further at this point.

Suffice it to say that the influence which they exert under field conditions and in solubility studies appears to be lost when the soil is subjected to electrodialysis. I n other words both the soluble and adsorbed phosphate will be removed by dialysis and it is suggested that such an experimental procedure may be utilized in differentiating between adsorbed and soluble pihosphate.

SUMMARY

As stated in the introduction to this bulletin, preliminary observations led us to believe that phosphate availability is either directly or indirectly associated wth the variation in productivity of many calcareous soils of this State. Solubility studies and plant cultures lend confirmation to these original observations and show definite relations to plant response in all cases where phosphate deficiency has been proved. In some cases, where phosphate solubility does not agree with our good or poor classification according to the conditions of the alfalfa in the field at the time the soils were sampled, we have found some correlation with variations in clay content and salt concentrations both of which influence phosphate solubility as well as its assimilation by the plant.

It is significant throughout the investigations that where initial solubility of soil phosphate is high, this high concentration is maintained by successive leachings or extractions and vice versa, w

T here low, the solubility will continue low or nil in successive extractions or leachings. It should be borne in mind that practically all samples were taken from the surface foot of soil and that the dispersed layers, where phosphate availability is most depressed in the soil, are often at lower depths. It should be further mentioned that in practically all cases the poor areas were bare of crops when the soils were sampled while the good areas w r ere bearing good crops and that this might materially alter the relative amount of water-soluble phosphate of the same, soil in adjacent parts of the field.

358 TECHNICAL BULIBTIN No. 35

CONCLUSIONS

1. Many cultivated fields in the calcareous soils of this State contain areas in otherwise productive fields which yield poor crops or are entirely free of vegetation.

2. There is no relation between the alkali content of these poor areas and their fertility.

3. They may be reclaimed either by leaching or by phosphate fertilization.

4. Leaching and good drainage improve phosphate availability.

5. The soils are well supplied with total phosphate but the availability as measured by solubility in water is low.

6. Soils from some areas contain only very faint traces of soluble phosphate, so small that it cannot be determined by colorimetric methods.

7. Grinding calcareous soils reduced the solubility of phosphate in water.

8. Leaching soils with water and determining the phosphate concentration of successive leachings is a good method for determining the re-solution velocity and therefore availability of phosphate, but is too slow an operation for a routine laboratory method.

9. The presence of soluble salts in the soil solution or extract depresses phosphate solubility.

10. In t^e colorimetric determination of phosphate, cloudy soil extracts are difficult to read accurately. Salts cannot therefore be used to aid clarification or filtration without materially reducing solubility.

11. Phosphate solubility in soils is best determined in water extracts by refiltering the extract through the mat of soil until clear.

12. The concentration of phosphate in a water extract decreases with increase in soil :water ratio. If CO

2

-saturated water is used the solubility increases with increase in soil .-water ratio for most of the phospihatedeficient soils and decreases for the soils not deficient in phosphate.

13. Carbon dioxide is the most suitable aid for clarifying soil extracts in solubility studies.

14. On account of the variability in activity of soil calcium carbonate, a small amount of finely divided calcium carbonate should be added to the soils before extracting with CO

2

-saturated water if uniform results are to be obtained.

15. There is a close agreement between the good and poor classification and solubility of phosphate in successive extracts of soils to which calcium carbonate is added when they are extracted with CO

2

-saturated water.

PHOSPHATE SOLUBILITY STUDIES 359

16. The relative solubility of phosphate in good and poor soils may be shown by subjecting the soils to electrodialysis and determining the phosphate concentration of the anode chamber. In interpreting the data, however, allowance must be made for the clay content and soluble salts present in the soil.

17. Practically all the soils showed response to phosphate when planted to tomatoes and millet, short growing crops which react readily to phosphate deficiencies in soils.

18. The plants responding to phosphate fertilization also show an increase in absorption as measured by a quantitative determination of phosphate in the plant.

19. Crops sometimes vary greatly in feeding power and phosphate requirement. There is therefore a need for studying these two properties for alfalfa, cotton, or other important crops of the State before our investigations are complete.

BIBLIOGRAPHY

1. Crider, F. J., 1927 — Effect of phosphorus in the form of acid phosphate upon maturity and yield of lettuce. Ariz. Agri. Exp. Sta. Bui.

121.

2. Das, S., 1930 —An improved method for the determination of available phosphoric acid in soils. Soil Sci. vol. 30, p. 33.

3. Deniges, G., 1920 —• Reaction de coloration extremement sensible des phosphates et de arseniates. Compt. Rend. Acad. Sci. (Paris) vol. 171, p. 802.

4. Dirks, B. and Scheffer, S., 1930—The carbonic acid-bicarbonate and water extracts for determining the phosphoric acid requirements of soils. Landw. Jahrb. vol. 71, p. 74.

5. Kelley, W. P., Dore, W. H., and Brown, S. M., 1931 —The nature of the base exchange material of Bentonite, soils, and zeolites as revealed by chemical investigation and X-ray analysis. Soil Sci.

vol. 31, p. 25.

6. Matson, S., 1926 —• Electrodialysis of the colloidal soil material and the exchangeable bases. Jour. Agr. Res. vol. 33, p. 553.

7. Pierre, W. H. and Parker, F. W., 1927 —The use of collodion sacks in obtaining clear soil extracts for the determination of the watersoluble constituents. Soil Sci. vol. 33, p. 13.

360 TECHNICAL BULLETIN No. 35

8. Pierre, W. II. and Parker, F. W., 1927 —The concentration of organic and inorganic phosphorus in the soil solution and the availability of organic phosphorus to plants. Soil Sci. vol. 34, p. 119.

9. Truog, E., 1930 —The determination of readily available phosphorus in soils. Jour. Amer. Soc. Agron. vol. 22, p. 874.

10. Wrangell, M. von, 1930 —The estimation of plant food requirements of soils. Landw. Jahrb. vol. 71, p. 149.

Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement