vol. no.3 plant physiology. 6 no.3 plant physiology july, 1931 absorption of mineral elements...

Vol. 6 No. 3

PLANT PHYSIOLOGYJULY, 1931

ABSORPTION OF MINERAL ELEMENTS BY PLANTS INRELATION TO SOIL PROBLEMS'

D. R. HOAGLAND

The discussion which I wish to present falls in the general field of re-search which can be designated by the phrase "Soil Conditions and PlantGrowth," to quote the title of a well known monograph by E. J. RUSSELL.An attempt will be made on this occasion to give some consideration to bothsoils and plants, with particular reference to researches of the past fifteenvears. I am aware that some of our most important knowledge of soils hascome from those who have dealt with the soil as a chemical and physicalsystem without much reference to plants, and-that much of what we knowconcerning plant physiology has come from investigators not greatly inter-ested in soil conditions. Nevertheless, there does remain an indispensableplace for investigations in which specifically planned efforts are made torelate soil conditions to plant growth, the ultimate purpose of all the re-searches from an agricultural point of view. In California the desirabilityof conducting such researches was stressed early by C. B. LiPMAN, and vari-ous circumstances, partly accidental, have made it possible to give long con-tinued attention to this type of investigation. With your permissionI shall refer frequently to experiments with which I have had some per-sonal familiarity including, of course, the work of various colleagues andassociates. I assume that you are not now desirous of a review of litera-ture or an extensive citation of the names of the many investigators overthe world who have contributed to our understanding of soil and plantrelations.

If we are to review the subject of the absorption of mineral elements byhigheir plants, it is well first to inquire which chemical elements must be.absorbed in order to insure the continued growth of such plants. Thepioneer researches of MAZE' very definitely suggested that the older list of

1 STEPHEN HALES address, Cleveland, 1930.3X3

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essential elements was incomplete. Unfortunately these researches did nothave much effect in stimulating other investigators until within a very re-cent period. The interest in this question is now aroused and in variouslaboratories in this country and in Europe very painstaking experimentshave been in progress for some time. Eventually there may be many ele-ments to add to the older list of ten. Several can be added now with muchassurance, not necessarily for all plants, but for many types of agriculturalplants at least. The importance, and in fact necessity, of considering anew and extended list of essential elements is convincingly illustrated bycertain experiments on tomato plants, in connection with the utilization ofboron. A few milligrams of this element make the difference between ahealthy and a diseased plant. To prove this it may not be necessary evento purify the salts used in preparing the culture solutions. The amount ofboron required, although very small, is still appreciable and the amountpresent as an impurity in a culture solution prepared in the routine waymay be entirely inadequate for some plants. Of great interest, likewise, isthe general distribution of copper in plants and the influence of this ele-ment in correcting certain diseases of fruit trees and other plants. Whilesuch observations cannot prove that copper is an essential element, they dooffer an incentive toward additional research. We cannot go farther inour present discussion of such elements for almost nothing is known abouttheir absorption from soil solutions or from artificial culture solutions. Itwould be difficult indeed to make some of the requisite experiments, but thetask must be undertaken sooner or later, if the understanding of soil andplant relations is ever to become reasonably adequate.

The next phase of our discussion in logical order is concerned with thegeneral nature of the processes by which plants absorb essential or non-essential chemical elements from culture media. A clear approach to thisquestion is frequently impeded by the indiscriminate use of certain com-mon terms such as permeability, osmosis, diffusion, antagonism, DONNANequilibrium, etc. At the risk of repetition of previous discussion I intendto review very briefly certain of the conditions of plant growth which de-mand consideration in any survey of the process of absorption of mineralelements by plant cells. As you know, during recent years several inves-tigators have given much attention to the behavior of certain types of algalcells which attain relatively enormous size, and which may be manipulatedas separate units. With cells of this type it is possible to avoid many ofthe difficulties and ambiguities inherent in the examination of the micro-scopic cells and complicated .tissues of higher plants. The investigationson the cells of the marine form, Valonia, yielded data of great significancewith respect to the relations existing between a marine organism and its

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solution environment. Our own special interest has been directed to ex-perimentation on a fresh water alga, Nitella clavata. These plants live andgrow in a medium which may be reasonably compared in a general waywith many soil solutions. Also it is feasible to subject these cells to theinfluence of artificial culture solutions and to make experiments on the in-take of solutes under controlled conditions. For present purposes such acell may be regarded as essentially constituted of a cell wall, an exceed-ingly thin layer of protoplasm, and a large vacuole filled with sap primarilyinorganic in nature. It is found that this sap has a total electrolyte con-centration approximately twenty-five times greater than that of the waterwhich bathes the cells, and furthermore, each of the principal ions (bothanions and cations) is present in the sap in a concentration many-fold thatof the culture medium. Evidently, during the growth of the cell thereoccurs an accumulation of such elements as potassium and chlorine againstconcentration gradients. For certain elements the same statement can bemade for Valonia, and very recently a member of the Characeae living inbrackish water has been studied in Finland, and similar conclusionsreached.

Experiments on Nitella cells showed that the process of concentratingelectrolytes in the cell sap could be conveniently studied by the useof bromide solutions, since bromine was not naturally present in the cellsap in determinable amount, and was non-toxic in the concentrations used.To make a long story short, it was found that bromine could be accumu-lated in the sap against concentration gradients, but only providedthe right metabolic conditions were maintained. Important among theseconditions was illumination. Any large accumulation of bromine was de-pendent on the influence of light of suitable intensity and duration, al-though under certain circumstances an after effect of illumination could bedemonstrated. The temperature coefficient of the process was a high one,and little or no concentrating action was observed at temperaturesapproaching a5 C. The time factor was important and to obtain markedaccumulation required a period of several days. Experiments of veryshort duration would be inadequate to reach an understanding of the proc-esses involved.

Later F. C. STEWARD investigated the accumulation of bromine andpotassium by potato tissues. In this case, of course, stored carbohydrateswere available and photosynthesis was not directly involved. The potatotissues also were found to possess the potentiality for concentrating in thesap the elements mentioned above. However, it was discovered that theconcentrating process could not proceed unless the tissues were in a healthystate and suitably aerated. When a mixture of oxygen and nitrogen with

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a very low proportion of oxygen was employed the concentration of bro-mine in the expressed sap exceeded the concentration in the outside solu-tion very little, if at all. When the proportion of oxygen was increased tothe proper point, and a suitably rapid flow of gas maintained, then bromine(and also potassium) was concentrated in the tissues in a most definite way.Other investigations now in progress on excised root systems of bar-ley plants yield evidence of the same trend. It can be shown, in addition,that the metabolic condition prevailing in the plant previous to excisionof the roots determines in part the degree to which accumulation of min-eral elements can take place subsequent to excision. In other words, theabsorbing power of the root system cannot be evaluated without a knowl-edge of the particular metabolic state in which it has been placed.

I have sketched in barest outline a few of the considerations which leadme to emphasize the point of view that the removal of electrolytes from aculture medium and their storage in the plant cell is a process in which themetabolic activities of the cell are inevitably concerned. Energy exchangesmust be involved, the ultimate source of the energy being sunlight. Nowthis is by no means the view always taken by soil chemists and plant physi-ologists. Sometimes there seems to exist a fear that the adoption of sucha point of view will lead to an undesirable "vitalistic"' theory. In otherinstances, efforts are made to bring the explanation of the absorption ofmineral elements by plant cells into line with the properties of some typeof artificial cell. Some investigators emphasize the DONNAN equilibrium,so important in certain systems, but it is difficult to understand how thisconcept can possibly fit the observations which have just been cited. It isessential to keep in mind that even the simplest plant cell is an extraordi-narily complex system in which energy exchanges are continuously occur-ring, and that these cell activities have an intimate relation to the absorp-tion of mineral elements, as well as to other cell processes.The mechanism involved in accumulation is not now understood, although

several ingenious theories have been proposed which take into accountmany, but probably not all the facts. Whatever the mechanism of the cellmay be, I think we can agree that what may be, called the collodion bagconcept of an absorbing plant cell is hopelessly inadequate. In makingthis statement, I trust it will not be thought that I am lacking in apprecia-tion of the work of several very able investigators, who are elucidating theproperties of simple membranes.

Perhaps these remarks will serve to make it clear that the accumulationof electrolytes by plant cells is not a question of simple permeability. Itis possible to construct an artificial cell which is very permeable to potas-sium, but which cannot accumulate that element. The term "permeabil-

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itv" has come to mean too much. Also, it is obviously not correct to statethat a solute enters a plant cell until a state of diffusion equilibrium isreached. In living cells dynamic equilibria are involved. Still less can itbe said that the entry of solutes is a process of osmosis. A more generalaspect of the whole question can well be summarized by a quotation from alecture on "Physical Chemistry in the Service of Biology " given last yearby F. G. DONTNAN. "Life, however, is not merely an affair of equilibriumand micro-structure, of oriented molecules and ions and delicately balancedmicelles. Within and around this framework there exists a continual fluxof physical and chemical action. The protoplasmic system operates byutilizing the free energy of its environment. It is a free energy converter,utilizing the free energy of oxygen plus convertible organic substances, ofcomplex organic substances alone, and of sunlight. It is a physico-chemical machine, a dynamical or kinetic system. It is a truism of biologyto say that the living organism cannot be considered apart from its envi-ronment. But this environment must possess some quality of non-equi-librium. It is the unbalanced environment which is the source of life andaction, whilst the living organism is but the energy transformer, thoughone of peculiar and still mysterious character."

The view that the metabolic activities of the cell are of paramount im-portance in determining the accumulation of mineral elements by plantcells is not simply a scientific generalization. It has much practical sig-nificance to the investigator of soil problems. It means that special em-phasis must be given to those soil and climatic conditions which favor rootgrowth and activities. Soil aeration becomes of great concern, as does alsothe environment around the top of the plant, which may modify the sup-ply of metabolites to the root system. The power of the soil to supplymineral elements to root cells will influence the synthesis of organic sub-stances, which in turn will influence the continuing ability of the root cellsto absorb mineral elements. Thus, especially with long-lived plants, eithera normal or a vicious cycle may be established.

Let us assume now that we have the proper metabolic conditions for theaccumulation of mineral elements by the plant cell. We are still faced bythe great problem of the relation of the composition of the culture solutionto the actually occurring accumulation. What are the effects of concen-tration and of ionic relationsa Perhaps the first thought w-ill be that wehave at hand a great fund of information upon which to draw, in the enor-mouslv extensive literature on antagonism and permeability. This field ofstudy has long been a major interest of plant and animal physiologists, anda few- basic principles are agreed on. But closer examination discloses thatthe work on antagonism has nearly always dealt with very different prob-

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lems and conditions than those in which we are now interested. You willrecall that our objective is the understanding of plant growth under soilconditions, with special reference to crop plants. We are concerned withthe gradual absorption and accumulation of certain chemical elements dur-ing the growth cycle of the plant. Experiments on antagonism and per-meability are generally conducted with solutions of much greater concen-tration than those we are primarily discussing, and the time periodsemployed are often very short. Frequently the purpose is to utilize a de-cidedly toxic solution and then to determine how such toxity may be over-come by suitable balancing of elements. Many investigators have employedthe technique of plasmolysis which is obviously unsuited to the study ofthe relatively slow normal processes of absorption. Furthermore, in themajority of cases the antagonism demonstrated is concerned simply withcations. On the whole, it would seem that we must expect only very lim-ited assistance from the investigations on antagonism in connection withthe special problems now before us.

We do, however, have! to take careful thought of the influence of oneion on the accumulation of another, whether or not the term "antag-onism" is used. There is definite and extensive experimental evidence forthe statement that importance attaches to each general type of ionic rela-tion; namely, anion-anion, cation-cation, and cation-anion. It will beworth while to cite some specific illustrations. Referring again to the ex-periments on Nitella cells, we find that the interrelations of anions are note-worthy. Bromine ions and chlorine ions have reciprocal relations to eachother and may undergo extensive interchange between the cell and themedium. One may retard the accumulation of the other. It is also pos-sible to provide illustrations of direct agricultural significance. Underappropriate conditions of solution culture technique, in experiments withsuch plants as wheat or barley, it may be shown that there exists a recipro-cal relation between nitrate and phosphate. By decreasing the concentra-tion of phosphate in the culture medium at certain stages of growth, theplant may be caused to accumulate more nitrogen. Conversely, by decreas-ing the concentration of nitrate, plants sometimes may be caused to accu-mulate a very high content of phosphate. Interrelations between cationsand anions are less easily demonstrated with complex solutions, but withsimpler solutions it is feasible to obtain clear indications that the entry ofcations may be facilitated by the presence of rapidly absorbable anions,such as nitrate. All these relations have interest to soil investigators, butsuch relations will of course not necessarily find expression under all soilor solution culture conditions. If marked deficiencies of several elementsexist, other and more complicated situations arise.

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.The interrelations of cations are emphasized in the results of manytypes of plant experiments, including not only those in which artificial cul-ture methods are employed, but also practical field experiments. If I mayrefer to our own investigations, merely for purposes of illustration, the in-terrelation between calcium, magnesium and potassium has been evidencedin a most consistent manner in many types of plant experiments we havehad occasion to carry on during the past several years. As the potassiumcontent of the culture medium is increased there is a very general and con-sistent tendency for the amounts of calcium and magnesium accumulatedby plants to be decreased. Occasionally, under solution culture conditions,there is practically an equivalent substitution of bases. I should like toemphasize that these interrelations between bases are effective in verydilute solutions. A difference of a part or two of potassium in a millionparts of solution may be of great consequence. The literature affordsnumerous indications that the interrelations of bases are also effectiveunder many soil conditions, although the interpretation placed on resultsis not always the same as the one now advanced. German, English andFrench investigations could be cited, as well as investigations reported inthis country.

It is well now to remark that caution is one of the safest attributes ofthe investigator of soil and plant relations. If I should make any broadgeneralization from experiments such as those which have just been cited,'immediately I should be confronted with evidence obtained under otherexperimental conditions, and especially under diverse soil conditions, whichcould not be explained by anything contained in the preceding discussion.Under such circumstances all we can do is to take refuge in that word soprecious to the biologist, "factors." The factors involved in plant growthunder soil conditions are so entangled together, cause and effect so inter-woven, that prediction is perilous. Not only the'soil and the plant, butalso the climate, must contribute factors to the extraordinarily involvedequation, the solution of which would be necessary in order to obtain anadequate and precise explanation of the behavior of a given plant on agiven soil at a given time. This does not mean that there should be a ces-sation of effort directed toward the elucidation of general principles, but itdoes mean that in any instance the operation of one set of processes maybe masked by the operation of another set of processes.

If we could assume some principle of simple limiting factors our pathwould be smoother, but often such an assumption is not permissible. Con-sider, for example, the interrelation of light and potassium. Fertilizationwith this element may be much more effective in some seasons thanin others. One writer in England has suggested that potash fertilization

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may be the best insurance against a sunless summer. You will appreciatethat we do not have sunless summers in California, but the general ques-tion has had some interest there, and opportunity has been afforded to ob-serve the growth of plants under different conditions of light and of potas-sium supply. We may easily arrange an environment such that a largeincrease in plant growth may be obtained by doing either one of two things,increasing the light, or increasing concentration of potassium in the culturesolution. This has been shown in greenhouse experiments, and recently acolleague has demonstrated the existence of these interrelations in the courseof an investigation on the growth of plants under controlled conditions,with the use of artificial light. In one sense two limiting factors may oper-ate at one and the same time, or perhaps more accurately, the efficiency ofone factor is modified by the intensity of another factor. It will be immedi-ately apparent to you how the problems of plant nutrition are complicatedby considerations of this sort.

If you are willing to grant that various interrelations of ions do andnecessarily must influence their absorption by plants, we cannot avoid thequery whether there are certain special ratios between the various ions, bestsuited to the growth of a particular kind of plant. The attempts to dis-cover "best solutions" have had value in stimulating research during thepast fifteen years in the field of plant nutrition, even though the originalobjectives have suffered much alteration as the years have passed. Vitallyimportant to such researches is the appraisal of plant variability and thestatistical significance of any particular yield of plant material.

Probably we are now in a position to say with a good deal of assurancethat there is no "best" solution for the growth of a particular plant, in sofar as yield or the usual criteria of quality are concerned, and within thelimits of error of any practical interest. Instead there are in general manysolutions equally effective for plant production, varying to a greater orlesser extent in composition depending on the kind of plant, and on theclimatic conditions. This is not at all the same as saying that differentsolutions will ever produce identical plants with reference to all details oforganic and inorganic composition, or of metabolism, for this would prob-ably not occur. I am referring to yield as indicated by weight and by thecommonly observed general characteristics of the plant. There may be a"luxury" consumption of certain elements from many types of solutions,but this is a different matter.

These statements are of interest to soil investigators as well as to plantphysiologists. With our present knowledge of soil solutions, it would beimpossible to account for actually observed conditions in the field, if plantsdid not possess great powers of adaptation to solution conditions. Illustra-

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tions of this fact have arisen in recent experiments with a group of Cali-fornia soils. In the course of these studies-over a three year period--tomato plants and barley plants have been grown in soils deficient in avail-able potassium, fertilized in various ways. Within the limits of rathersmall experimental error for such experiments it was possible to obtainplants of the same appearance, rate of growth, and final weight, under theinfluence of soil solutions of highly divergent composition in respect to theircontent of potassium, calcium, magnesium, and other elements. On theother hand, the same group of experiments furnishes equally strong evi-dence that when concentrations of potassium in the soil solution are main-tained sufficiently low an unbalanced condition in the plant ensues, accom-panied by inhibition of growth or the appearance of disease. The range ofadaptation is often a broad one but not indefinitely so.

Experiments of this character show that the ratios between certain min-eral elements present in the plant, for example, calcium, magnesium andpotassium, are profoundly modified when a marked disturbance of nutri-tion is manifest. Yet it would be difficult to prove that these modificationsare the primary cause of the metabolic disturbance. Perhaps in the casementioned we should emphasize rather the balance between potassium andcertain organic constituents. If we ask why such a balance should be im-portant, we find no answer to the question, since the mechanism by whichpotassium operates is still essentially unknown.

It is a natural step now to direct the discussion more definitely to therelations pertaining to concentration. The problems involved are difficultand elusive. The question to be answered is: What magnitudes of concen-tration of an element are adequate to maintain a suitable rate of deliveryto the plant, if a given concentration of the element is maintained fairlyconstant in the culture solution surrounding the plant roots? For reasonswhich have already been given, no general answer could be expected tosuch a very broad question, but perhaps we can make some useful observa-tions. First of all, it is essential to note that the work of several investi-gators is in general agreement on the point that extremely dilute solutions,at least of potassium and phosphate, may produce highly satisfactorygrowth of common agricultural plants. But it is obvious that critical zonesof concentration exist. It needs no argument to show that concentrationsmay become so low that a plant may be unable to absorb in a unit of timean adequate supply of an essential element. It is also scarcely necessaryto remind you that climatic conditions will alter the rate of supplyrequired. Of great importance, likewise, is the kind of plant and the stageof its growth. The influence of one ion on the absorption of another hasalready been mentioned.

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There is much investigation yet to be done on the relation of concentra-tion to absorption and to plant growth. The objective should be the studyof what has been called "supplying power," at different levels. Unfortu-nately the technique of such studies is laborious and expensive. Flowingculture solutions may be needed, or else some other procedure almostequally troublesome. The type of plant, the stage of growth, and the cli-matic environment all complicate the' study of this question.

Any discussion of the absorption of mineral elements by plants mustinclude some statement with reference to the relation between the intakeof water and' the intake of mineral elements. This is an old question, uponwhich different views have been expressed from time to time. A very sim-ple experiment may be cited to illustrate one important phase of the rela-tionship just mentioned. Young barley plants were transferred to a care-fully measured volume of culture solution of known composition. Thearrangements were such that water could escape only through the plants.After a period of about two days the residual culture solution was analyzed.'It was found that the plants had effectively diluted the solution with re-spect to phosphate, potassium and nitrate, while sulphate was increased inconcentration. In this particular experiment calcium happened to remainat almost its original concentration. Thus it is possible for ions to be ab-sorbed more rapidly or less rapidly than water, or at the same rate. Some-times attempts are made to compute the concentration of a soil solutionfrom a knowledge of the total water transpired and of the amount of anelement taken into the plant. It is clear that such computations are notwell founded, since plants can bring about the dilution or concentration ofa culture solution under some circumstances. Obviously all the conditionsof metabolism within the plant, and the conditions of the environment sur-rounding the plant, will determine in any given instance the particular re-lation between absorption of water and that of any given ion.

Simple absorption experiments also throw light on other aspects of theselective action of plants. Some elements essential to the plant may beabsorbed with great rapidity, but other elements not essential, except pos-sibly in minute quantities, may likewise be removed from solution withgreat ease by plants. The predominance of potassium over calciumabsorption is easily demonstrated with some types of plants, but with othertypes calcium may be absorbed as readily as potassium. Comparativerates of anion absorption vary with different plants and with differentenvironments. All facts considered, there is no straightforward way ofapplying the physical chemical properties of ions in any entirely generalor consistent manner. We do not know how to evaluate these propertiesin their relation to different protoplasmic systems, and to the varying

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activities of such systems, although we may surmise the existence of logicalrelationships.

In these days it would be very unorthodox to discuss soil and plantrelations without referring to hydrogen ion concentration, but within aspace of fifteen years the literature has grown so vast in its extent that onehesitates to mention the question at all. Only one or two observationsparticularly pertinent to our present purpose will be ventured. I recallsome early experiments with barley seedlings in California. With con-siderable surprise it was noted that the plants made better development ina definitely acid solution than in a slightly alkaline one. The impressiongained from the literature of that time was to the effect that most agricul-tural plants found their most favorable environment in a slightly alkalinemedium and were injured by acidity. It was a great gain to utilize phys-ical chemical concepts in making measurements of the actual concentrationor activity of hydrogen ions present in a culture solution. Yet I am unableto share the enthusiasm of those investigators who, by inference at least,assume the hydrogen ion concentration of the medium to be the almostexclusive determinant of plant distribution or adaptation. Save in themost extreme cases, it seems that hydrogen ion concentration should alwaysbe evaluated in relation to the concentration of other ions. For example,in recent years many investigators have called attention to the great impor-tance of calcium ion concentration in this connection.

Concerning the relation of hydrogen ion concentration to absorption ofother ions, we seldom have available experiments of an unambiguous nature.We cannot work over any large range of hydrogen ion concentrations andmaintain all other properties of the solution the same, such as iron, phos-phate and bicarbonate concentrations. There are some definite suggestionsof the influence of hydrogen ion concentration on absorption of other ions,but they are not so clear cut and consistent as to warrant further discus-sion at this time.

Let us now speak a little more specifically of soil conditions. In 1911CAMERON published a volume with the title "The Soil Solution." Certainvery radical ideas, which had occasioned extensive controversy, were sum-marized in this volume. It is now well agreed that some of the conceptionsof soil equilibria advanced were erroneous, and led to incorrect deductions.There is no longer any reason to doubt that the soil solutions of differentsoils may be of very different composition, or that the soil solution of anyone soil may vary in composition as a result of seasonal changes, cropgrowth, or the application of fertilizers. Nevertheless, the controversyaroused probably had a beneficial result to soil science. A dynamic pointof view of the soil was stressed, and many investigators were induced to

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think of the soil as something very different from a storehouse of nutrientelements, needing to be inventoried, or as a more or less inert medium towhich plant foods were to be added. A long period of intensive researchon soil solutions has followed, in which various laboratories have partici-pated, especially during recent years. I wish to touch on several of theleading features of these researches in their relation to what we havealready been discussing.

Most of the early work dealing with soil solutions was conducted withthe use of water extracts of one type or another. It was appreciated thatsuch extracts could reflect only imperfectly the "soil solution," that is, thesolution in the soil as it existed at some moisture content suitable for plantgrowth. Some studies were made in which soil solutions were separatedfrom the soil by very high pressure, but these were limited in extent. Anumber of years ago a method was developed in Wisconsin, and later inCalifornia, by means of which the soil solution was displaced from moistsoils by a supernatant column of water or of alcohol. Subsequently muchintensive effort has been devoted to this method of obtaining soil solutions.It has been shown that according to certain chemical criteria solutions dis-placed from soils in this way may often be considered to be practicallyidentical with the solution phase of the soil, as it exists at the moisture con-tent at which the displacement was made. The arguments advanced neednot be discussed on this occasion.

If a solution can be displaced from a soil with the characteristics justnoted, have we then available an effective means of determining the com-position of the plant's culture medium in the soil? This question has beenpresented in a very definite way in connection with researches of recentyears on the phosphate nutrition of plants. It was suggested first by ex-periments made in Alabama that certain soils give displaced solutions withcontents of phosphate entirely inadequate to account for the actual cropgrowth known to occur in these soils. The situation can be illustratedreadily by an account of some later California experiments. Occasionarose to investigate a particular soil in which the phosphate present was

extremely unavailable to many plants. The addition of suitable' amountsof soluble phosphate enormously improved the physiological state of thissoil, so that good growth of crops could be obtained. Yet solutions dis-placed from the fertilized soil did not give significantly higher coneentra-tions of phosphate than those from the unfertilized soil. In all cases thevalues were extremely low. It was difficult to escape the conviction thatthe plant growth actually occurring in the fertilized soil could not havetaken place if the displaced solution represented the real culture mediumof the plant with respect to phosphate.

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Are we to assume, therefore, that phosphate is not in fact absorbed fromthe soil solution, but rather that it is assimilated in some direct way fromcolloidal matter of the soil? This idea has been suggested, but it is not aninescapable deduction from the known facts. Certain essential considera-tions have not always received due emphasis, perhaps because of their veryobviousness. Soils are heterogeneous and plants can deal with such sys-tems in a heterogeneous manner, effecting a sort of physiological integra-tion of many localized soil solutions. Laboratory procedures, on the con-trary, deal with a mass of soil through methods which yield only compositesolutions. Furthermore, in the laboratory, reactions such as those of fixa-tion, may be facilitated by the very operations which the chemistmust make, and which may have no counterpart in the soil as the plantgrows in it, at least not in respect to rate. We must conclude then thatthe soil solution, no matter how prepared, cannot be assumed to be neces-sarily identical with the physiologically effective soil solution. If it is soassumed, then in certain circumstances an entirely erroneous idea may begained of the solutions existing at the closely contacting surfaces of rootmembranes and soil particles. Butl it does not follow that a general soilsolution theory of absorption is erroneous, merely because our methods ofstudy are imperfect.

If no more were said on this point a wrong impression would be left.It has seemed necessary to draw your attention to some of the difficultieswhich may be met with in soil solution studies. As a matter of fact,- diffi-culties of the same degree as those applying to phosphate have not as yetarisen in the study of concentrations of nitrate, calcium, magnesium, sul-phate, nor even of potassium. With these ions the reflections of the physi-ologically effective solutions, although far from perfect, have been very ser-viceable. In general the magnitudes of concentration in the displaced soilsolution have not been found to be radically different from those whichare adequate for plant growth in a flowing culture solution. Even forphosphate this statement is true of many western soils. Notwithstandingthe inherent difficulty of interpreting soil solution data, and the inconsis-tencies often arising, this method of investigating soils still remains one ofthe most important of tools, which can be employed with profit for a longtime yet to come.

As soil solution researches have been continued, it has become more andmore apparent that soil solutions should be studied in conjunction with thesolid phase of the soil, for the nature of the solid components, determinesthe power of the soil to keep the soil solution renewed as withdrawals byplant roots proceed. I shall mention one or two examples. The phenome-non of base exchange is now in the forefront of the discussions of soil chem-

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ists. Much physiological interest attaches to the potassium combined inthe base exchange complex. The experimental evidence shows that thepotassium present in this form is in general available, when present abovecertain minimum amounts, varying with the soil. It can easily be assumed,and there are many results of experiments in favor of this assumption, thathydrogen ions produced by the activities of root cells easily displace a cer-tain proportion of the potassium contained in the base exchange compound.The rapid intake of potassium by the plant continuously disturbs thechemical equilibrium and in a selective manner, which is not imitated inthe laboratory. The potassium in the base exchange complex is, however,not always indispensable. The soil solution potassium may be maintainedin some soils at adequate levels of concentration through the dissolving ofother types of minerals. In fact, it is reasonable to postulate that thereis involved the resultant equilibria between all of the various typesof potassium compounds and the soil solution. These are ideas which can-not be expanded now, but the illustration may serve to emphasize the in-timacy of the relation which should exist between studies on the solid andsolution phases of the soil. If time were available much could be said alsoof the clarification which has come from base exchange studies with refer-ence to acid and "alkali" soils, and the supplying power of the soil for cal-cium under these special soil conditions.

The relation between the solid and solution phases of the soil has a verypractical agricultural aspect in the study of availability and the fixation ofpotassium and phosphate by the solid phase. Although among the earliestobserved of soil phenomena it is doubtful whether the full import of suchfixation is generally appreciated. It must be emphasized that fertilizersreact chemically with soils. As hag already been said, it is not merely aquestion of adding so much "plant food" to an inert medium. For exam-ple, we should inquire, in the case of deep-rooted plants, whether by fer-tilization we really change the soil solution as we desire in all the zones ofroot absorption. Even with shallow rooted plants the understanding ofthe degrees of fixation of phosphate and potassium with reference to avail-ability to different types of plants is still entirely inadequate. We have totake account not only of the base exchange reactions, but also of other typesof fixation. And the old questions concerning the importance of acid ex-cretions by roots and of extent of absorbing root surface are not yet dis-missed. It is clear that in these matters soil chemistry and plant physi-ology cannot be divorced, and that the practical art of soil managementmust have as its scientific basis a definite concept of the chemical and physi-ological interrelation between soil and plant. There are many times whenthe study of the plant itself will best forward an understanding of soil con-

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ditions. It is true that most of the data of past years on inorganic plantcomposition have not carried us far, but experiments under suitably con-trolled conditions still offer much of promise.

I presume that before closing I am entitled to make a few general ob-servations. I have spoken of past researches. What are the prospects forthe future? We may expect that researches of the type already describedwill continue and will be greatly expanded. But we must acknowledgethat the real reason we are so much interested in the mineral nutrition ofplants is because of its relation to the synthesis of organic substances. Wemay hope and anticipate that the organic metabolism of the plant will ex-cite more and more interest. There are indications in recent research thatthis movement is already well under way. Much will be learned from thefurther application of existing methods of research, but far more than thisis required. The knowledge of the chemical nature of most plant constitu-ents is entirely inadequate and consequently the methods used in followingmetabolic changes highly imperfect and incomplete. The plant physiolo-gist will need to call to his service more men who are trained primarily aschemists or physicists. A few of the men now entering on their researchactivities will combine an excellent degree of training in both modern chem-istry or physics as well as in biology, and from these much may be hopedin the future. Ordinarily it will be impossible to make an adequate ad-vance in the phases of plant physiology of which I am speaking, withoutcooperative effort, not of the formal and official type, but rather of thatinformal type of cooperation which results from a common interest in ageneral objective by men of different specialization.

In addition to the intensive study of plant tissues by the methods of theorganic chemist and the physical chemist, we may foresee an era in whichgreat efforts will be made to grow plants under reproducible conditions,which cannot be done without control of light, temperature and humidity,as well as of the culture medium. As you know, such experiments havealready been begun in Europe and in America. The difficulties of growingplants of any desired type satisfactorily under real control are very great,and not yet solved, but definite progress has been recorded with a few typesof plants. It becomes possible to do exactly the same thing twice and thoseof you who are engaged in researches on the nutrition of plants will grantthe cogency of this remark. A quantitative plant physiology in relationto mineral nutrition will develop gradually. From this and from thespecialized chemical and physical research which I have just mentioned, wemay hope for at least partial answers to numerous questions which nowperplex us. I do not say complete answers, because as long as an investi-gator deals with living organisms he cannot hope ordinarily to achieve the

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kind of solution of a problem which is often arrived at by the chemist orphysicist, dealing with specially selected and simplified systems.

There is no compulsion on me to refer to the practical significance ofresearch on soil and plant relations in an address of this character, but wecannot help recalling that practical interest will in a large measure dictatethe support of research. Thus far there has been no adequate apprecia-tion of the ultimate economic significance of plant physiological investiga-tion in relation to soil problems. Its ramifications in the field of generalagriculture are obvious, but of much wider scope than is always realized.Such research should be the vital foundation of the great fertilizer indus-try. Great sums are spent by this industry in practical tests and in pub-licity; relatively little on research dealing with basic principles. Agricul-tural experiment stations are lending important aid, but they have mani-fold demands on their resources. Yet I venture to predict that thesephysiological problems are going to receive ever increasing attention andsupport. If some speaker before this society in a not distant future shallelect to discuss the same field of investigation, I am confident that his re-port will be far more convincing and adequate than the one that I havebeen able to present this evening.

LABORATORY OF PLANT NUTRITION,THE UNIVERSITY OF CALIFORNIA,

BERKELEY, CALIFORNIA.

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vol. no.3 plant physiology. 6 no.3 plant physiology july, 1931 absorption of mineral elements...

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