Chapter XI.. The Organic Constituents of the Soil Solution.

THE ORGANIC CONSTITUENTS OF THE SOIL SOLUTION.

The organic substances in the soil are tissue remains, to a large extent of plants, and to a less extent of animals; and it is to be expected that there may be found also in the soil the substances which were in the organisms at the time of their death, and degradation and decomposition products derived from these. Moreover, there are to be anticipated numerous products of bacterial origin, secretions of algae, fungi, etc., so that the organic complex in the soil may contain numerous substances of widely different chemical characteristics. Degradation products of proteins, fats, and carbohydrates, as well as decomposition products may be expected in almost any soil. But it does not follow that any particular organic substance (excluding, of course, carbon dioxide or nitrates) is to be found in every soil. No generalization regarding the organic substances in the soil can be made such as that formulated for the inorganic compounds. It is probable that further investigation will show certain organic substances or classes of substances to be common to most soils, but it is reasonably certain that many other organic substances will be found in only a few soils, or occasionally, and these latter will be often a prominent factor characterizing the particular soil in which they may occur.

Although no broad generalization is justified regarding the composition of the soil solution with respect to organic substances dissolved, nevertheless the extension of the methods developed in the study of the inorganic substances dissolved has led to a considerable knowledge of the organic ones.

In view of the facts shown in the preceding chapters, and at the same time recognizing that good and poor soils respectively must show differences in the soil solution if the fundamental thesis is valid as to the relation of soils to crop production, experiments have been made to investigate in a comparative way solutions obtained from good and poor soils of the same type, locality, and physical characteristics. For this purpose two samples of soil were taken from adjacent fields which had been under observation for two years. The soils were of the same type, Cecil clay, and were so similar in their physical characteristics as to be distinguished with difficulty in the laboratory. On one field a good crop of wheat was grown, followed by a good crop of clover and tame grasses. On the other field, the corresponding crops had been quite poor. The field yielding the good crops had been plowed somewhat deeper, and had previously received a moderate application of stable manure. Otherwise, so far as could be learned, the cultural history of the fields had been the same. For convenience, the sample from the first field will be designated “good,” and from the other “poor.”

Aqueous extracts from these soils were prepared, the same proportion of distilled water to soil being taken in each case, and the time of contact being the same. The solutions were freed from suspended matter by being passed through Pasteur-Chamberland bougies under pressure. Young wheat seedlings germinated at the same time, and selected carefully for uniformity of size and apparent vigor, were grown in these solutions for three days. At the expiration of this period the seedlings in the extract from the good soil were about five inches in height, and the roots were clear, clean and turgid. The plants in the poor extract were scarcely three inches in height, and the roots were assuming a slimy, unhealthy appearance and becoming flaccid at the tips. The plants were then all removed, the roots washed carefully in tap water; the plants which had been in the poor solution were placed in the good solution, and those which had been in the good solution were placed in the poor solution. At the end of four days further, the poor plants had surpassed in height the ones which had previously been in the good solution, and the roots had acquired the general characteristics of healthy plants. These which had been originally in the good solution and then transferred to the poor, had made little additional growth, and the roots had become somewhat flaccid.

The success of this and of many of the following experiments was due in large measure to the skill and patience of Mr. James E. Breazeale.

This experiment was repeated several times, not only with the soils cited but with samples from adjacent good and poor spots in fields on several soil types from widely separated areas; for instance, Cecil clay from near Statesville, North Carolina; Sassafras loam from Maryland; Windsor sand from Delaware; and similar results were obtained. In other words, these water cultures produced plants which showed much the same differences, in kind and degree, as had been observed in the field. This was recognized as an important step forward, for it indicated that whatever was making a difference in the crop-producing power of these soils in the field was transmitted to their aqueous extracts, and methods for studying the chemical properties of solutions are far in advance of methods for studying mixtures of solids.

The soil extracts described above were subjected to a careful analysis for their mineral constituents. They were found to be practically identical in this respect. Further, the poor extract contained decidedly more nitrates than the good—from three to four times as much. It follows, therefore, that the difference in the soils which produced a good and a poor crop respectively, was not due to a difference in mineral plant nutrients, or other mineral differences probably, nor to their respective content of nitrates. Consequently, the poor solution was such, not because of the lack of anything, but because of the presence of something inimical or “toxic” to plant growth; and further, this something must be an organic substance or substances more or less soluble in water. This conclusion was confirmed in the following way.

Samples of the poor solution from the soil obtained near Statesville, N. C., were diluted twice, five times, and ten times, and wheat seedlings were grown in these solutions, using a sample of the good solution as a check. It was found after several days growth that the plants in the solution diluted tenfold were about as good, or perhaps slightly better, than those grown in the check solution. In every case diluting the poor solution had improved it for plant growth, and the higher the dilution the greater the improvement, in spite of the consequent dilution of the mineral plant nutrients. The only explanation of these results which has yet suggested itself is that the toxic organic substances present were less effective on dilution until the concentration reached a point where they actually became stimulative, as is common with toxins of every character.

Another set of experiments confirmed the conclusion that the poor solution contained some organic substance inhibitory to plant growth. A number of water cultures was prepared from the aqueous extract of the poor soil, and lime in various forms was added to the cultures. To two of the cultures lime carbonate and lime sulphate respectively were added in excess, so that there was in each case a powdered solid at the bottom of the containing vessel. At the end of two days the wheat seedlings which were growing in the vessels containing the powdered solids had decidedly outstripped those growing in all the others, the tops having the appearance of unusually good and healthy plants. The roots were of a very remarkable character, being exceptionally long, very turgid, clear, clean and translucent.

At once, new experiments were carried out in which there were added to the poor solution, precipitated ferric hydroxide freed from all adhering salts, precipitated alumina, shredded filter-paper, absorbent cotton, or carbon black. In every case the same result was obtained as before, a much improved growth of top and a vastly better root development. Since, by no possibility could these various added substances have increased the concentration with respect to mineral nutrients, another explanation must be sought. Aside from their insolubility, the one property common to these various substances was the large amount of surface they brought into contact with the solution. The one obvious explanation of their effects on the growth of the wheat seedlings, therefore, is that they withdrew or absorbed from the solution some substance or substances deleterious to plant growth. As diluting with respect to mineral nutrients could not possibly be expected to improve the cultural value of the solution, the conclusion seems evident that the effect produced by these various absorbents was due to more or less complete removal from the solution of organic substances inhibitory to plant growth. These experiments were then repeated in a modified form by shaking the poor solution with such absorbents as precipitated ferric oxide or carbon black and filtering before adding the seedling plants. The solutions thus prepared proved very satisfactory nutrient media, although the decided elongation of the roots, always observed when the absorbents were in contact with the solutions, was not so noticeable with these filtered solutions.

The experiments just described were repeated with extracts from a number of soils which were supporting or had recently supported poor crops. The accumulated mass of evidence admits of no doubt that in many cases the apparent lack of fertility of a soil is due to the presence of some organic substance or substances soluble in soil water. This point established, there was studied the effect of fertilizers when added to aqueous extracts from poor soils.

A large amount of experimenting has been done on this subject. It has been found that the common commercial fertilizers, as well as many other substances, when added to the soil extract containing growing plants, sometimes improve the plants, sometimes the contrary. But, in general, those particular substances which improve any given soil for a crop also improve the aqueous extract of the soil for the growth of the same crop plant: i. e., should a soil be known to respond well to the application of superphosphates when planted to wheat, then the probability is great that the aqueous extract of the soil will be improved as a culture medium for the wheat plant by addition of calcium phosphate. Particularly important in this connection are certain experiments with organic fertilizers.

A soil which had been found to be quite unproductive with regard to wheat and ordinary tame grasses yielded, however, a much better growth of plants if pyrogallol or better pyrogallol and lime were added to the soil some days before planting. An aqueous extract of this soil tested with young wheat seedlings produced but a poor growth, as did the soil itself. But with the addition of pyrogallol or pyrogallol and lime to the soil extract, and especially if the extract so treated were allowed to stand for a few days with free access of air, there was obtained a culture medium which yielded remarkably good results with wheat seedlings. Not only was there an excellent and increased development of tops, but the roots of the seedlings grown in the solution treated with pyrogallol were unusually long, turgid, clear and translucent. Here, then, there was obtained an increased amount and improved character of growth by the addition of a substance which contained only carbon, hydrogen and oxygen, and no recognized plant food. Other organic substances, such for instance as tannin, gave similar results.

With the recognition that the presence of organic dissolved substances in the nutrient medium produced effects on a growing plant of as great or even greater magnitude than those produced by inorganic dissolved substances, there was carried out a number of experiments to test more specifically such substances as might reasonably be expected to be present naturally in soils. The results thus obtained suggested experiments with other related substances. The first substance to suggest itself is stable manure. Taking it all in all, this substance is probably the most efficient as well as the most generally used soil amendment in the experience of mankind. The good effects produced by this substance have in the past been generally considered as due to the readily “available” potash, phosphoric acid and nitrogen it contains, but thoughtful experimenters and agriculturists have long doubted that this explanation is sufficient, since, after all, the mineral constituents of stable manure are usually small in amount, and out of all proportion to the effects resulting from its use. That some of the results are due to an improvement in the physical condition of the soil when manure is used has quite rightly been generally assumed; but to its content of nitrogenous components its value has in the main been ascribed.

A well-fermented aqueous extract of stable manure was prepared, and filtered free of suspended solids. Four equal volumes of this solution were taken. Three of these portions were evaporated to dryness in platinum dishes, and the residues incinerated. To the dishes containing: the ash were added respectively nitric acid, sulphuric acid, and hydrochloric acid in slight excess, and the dishes again brought to dryness. Water cultures for wheat seedlings were then prepared. Into one was introduced the given volume of manure extract; into another the ash from an equal volume of the extract which had subsequently been treated with nitric acid; and cultures with the ash which had been treated respectively with sulphuric and hydrochloric acid were similarly prepared. After ten days growth, the plants from the several cultures were compared. The plants from the cultures which contained the sulphates and the chlorides were not materially different from the plants grown in the check culture. The plants from the nitrate culture had larger shoots, but shorter roots than the check plants. But the plants grown in the culture to which the manure extract had been added directly had by far larger and better shoots and the roots were incomparably superior to those grown in any other culture, being larger, thicker, better branched, clear, bright and translucent, and very turgid, very like the roots obtained in cultures to which carbon black or precipitated ferric oxide had been added.

Further studies on the properties of unproductive soils, B. E. Livingston et al., Bull. =36=, 1907, and =48=, 1908, Bureau of Soils, U. S. Dept. Agriculture.

The results of this experiment, which has been repeated a number of times, using manure extracts of various origins, leave no doubt that it is the organic components of the manure which produce the characteristic effects, for the ash culture contained all and even more of the mineral constituents “available” in the original extract, and the nitrate culture excluded any explanation based on the nitrogenous content of the manure. This conclusion was supported by the results of another experiment.

To a manure extract was added alcohol, which precipitated most of the organic dissolved substances but very little of the inorganic ones. The precipitated organic matter was filtered off, dried carefully in a water oven to eliminate the alcohol, and then taken up in sufficient water to equal the original volume of manure extract. The nitrate containing the major part of the salts was boiled vigorously to eliminate the alcohol and water was then added to restore the original concentration. A third solution was prepared by bringing together the organic and inorganic substances which had previously been separated as above described. The three solutions were used as water cultures for wheat seedlings, a solution of the original manure extract being taken for a check culture. The original manure extract and the reconstructed manure extract gave plants of about equal development. The culture containing the organic dissolved substances only, gave plants of nearly, but not quite, equal development to those grown in the check culture. But the plants grown in the solution containing the dissolved minerals only, while fine plants and making what would ordinarily be considered a good development, were decidedly smaller as regards their aerial parts, and the roots were in no wise comparable to the roots of the plants grown in the cultures containing the dissolved organic substances.

This last experiment has been repeated, with dissolved substances prepared from another manure extract, but in this case the organic and inorganic substances were separated by dialysis. This suggested yet another experiment, in which it was sought to hasten the process of dialysis, by introducing electrodes into the manure extract, each electrode being surrounded by some porous membrane, either of parchment paper, or unglazed porcelain. Not only were the mineral constituents of the manure extract readily separated in this way, passing into the electrode chambers, as did also to some slight extent organic compounds, but also about the outer walls of the electrode chambers there was marked segregation and deposition of organic materials. The organic substances deposited at the cathode were found to stimulate greatly the growth of wheat seedlings while those deposited at the anode were found to retard the growth of seedlings. It seems probable, therefore, that stable manure contains organic components which produce as great or greater effects upon growing plants as do the inorganic substances it contains: that on the whole these organic components induce increased plant growth, but some of them, by themselves alone, would retard plant growth.

In a similar way green manures have been examined. If fresh clover, alfalfa, or cowpeas, be macerated and an aqueous extract thus prepared, it will in general be quite toxic to plants such as wheat; and if this extract be allowed to stand and ferment or sour the resulting solution will be totally unfit for the growth of seedling plants. But if the clover, alfalfa, or cowpea vines be allowed to wilt thoroughly before being macerated and extracted, or if they be macerated and incorporated with soil and allowed to remain thus for ten days or a fortnight before being extracted; then, the resulting solution will be quite stimulating to such plants as wheat, corn or the grasses, when added either to water or soil cultures. It would seem, therefore, that the mineral constituents of the legumes commonly employed as green manures are less important than the organic, in affecting the growth of crops subsequently planted, and the inhibitory or toxic action of fresh green manure seems to be recognized in the common practice of waiting some days after turning under a green manure crop before seeding to a new crop.

The wilting of a green manure involves a darkening and some blackening of the mass, with apparently some absorption of oxygen. This fact has suggested a trial of other organic substances which show a decided ability to absorb oxygen. Among such substances, pyrogallol stands preëminent. It has been shown that when pyrogallol, or better pyrogallol and lime, is added to certain soils, naturally low in productive power, and allowed to stand for a few days, these soils are readily brought into good condition and support good crops of wheat, rye, or grasses. Pyrogallol in water cultures is rather toxic to wheat plants, even in quite dilute solutions. But if the aqueous solution of pyrogallol be allowed to stand exposed to the air, and better if the solution be made slightly alkaline as by the addition of lime, oxygen is absorbed, and a dark brown or blackened solution is soon formed, which is stimulating to wheat seedlings. Many experiments have indicated it to be a general rule that soluble organic substances which are toxic to plant growth yield oxidation products which are harmless or positively beneficial.

The suggestion has been made that the well-known infertility of subsoils, when freshly turned up, is caused by the presence of alkaloids of the purine or codeine type, due to the activities of anaerobic bacteria. Water cultures and pot cultures show that while these substances do have a marked effect on plant growth, it is, frequently, quite beneficial; strychnine for example, in certain concentrations, produces a very decided stimulation in the growth of wheat seedlings. It is clear that some other explanation will have to be sought for the lack of fertility of subsoils.

A number of the substances which may be expected for one reason or another to be present in soils, have been investigated as to their effect on plants. In this connection may be cited the work of Livingston and of Dachnowski, who have studied the effect on vegetation of the organic substances dissolved in bog waters. In the following table are given the results obtained by growing wheat seedlings in solutions containing some one of a number of substances which might be expected to occur in a soil or to be derivatives of such substances. It will be observed that in the case of these dissolved organic substances, as has been repeatedly established with the inorganic ones, in concentrations sufficiently dilute not to be toxic, they generally show the opposite effect and appear to be stimulating.

Physiological Properties of Bog Water, by B. E. Livingston, Bot. gaz., =39=, 348-355, (1905).

The toxic property of bog water and bog soil, by Alfred Dachnowski, Bot. gaz., =46=, 130-143, (1908).

TABLE I.—EFFECT OF VARIOUS ORGANIC COMPOUNDS UPON THE GROWTH OF WHEAT PLANTS, WITH ESPECIAL REFERENCE TO THEIR TOXIC PROPERTIES

Certain organic constituents of soils in relation to soil fertility, by Oswald Schreiner and Howard S. Reed, assisted by J. J. Skinner, Bull. No. =47=, Bureau of Soils, U. S. Dept. Agriculture, 1907.

LEGEND: A = Duration of experiment B = Lowest concentration causing death C = Lowest concentration causing injury D = Concentration causing greatest stimulation =======================+====+======+======+======+=================== | | | | | | | | | | Compound | A | B | C | D | Remarks | | | | | -----------------------+----+------+------+------+------------------- |days|p.p.m.|p.p.m.|p.p.m.| | | | | | a Aspartic acid | 10 | 500 | 100| .... |Normal growth in HOOC.CH₂.CH(NH₂).COOH | | | | |concentration | | | | |below 100 p.p.m. -----------------------+----+------+------+------+------------------- b Asparagine | 9 | | | |No injury below NH₂.OC.CH₂.CH(NH₂).COOH| | | | |1,000 p.p.m. -----------------------+----+------+------+------+------------------- c Glycocoll, | 9 | | | |Tops of all plants CH₂(NH₂).COOH | | | | |good. Roots slightly | | | | |injured at higher | | | | |concentrations -----------------------+----+------+------+------+------------------- d Alanine, | 10 | .... | 500 | 25 |Only roots were CH₃.CH(NH₂).COOH | | | | |injured at | | | | |500 p.p.m. -----------------------+----+------+------+------+------------------- e Leucine | 9 | .... | .... | .... |No injurious action CH₃.(CH₂)₃.CH(NH₂).COOH| | | | | -----------------------+----+------+------+------+------------------- f Tyrosine, | 11 | .... | 10 | | OH | | | | | / | | | | | C₆ H₄ | | | | | \ | | | | | CH₂.CH(NH₂).COOH | | | | | -----------------------+----+------+------+------+------------------- g Choline, | 10 | | 500 | 1 |Roots affected more CH₂CH₂OH | | | | | than tops / | | | | | (CH₃)₃N | | | | | \ | | | | | OH | | | | | -----------------------+----+------+------+------+------------------- | | | | | h Neurine, | 9 | 250 | 25 | | CH:CH₂ | | | | | / | | | | | (CH₃)₃N | | | | | \ | | | | | OH | | | | | -----------------------+----+------+------+------+------------------- Neurine (neutralized) | 8 | 250 | 25 | | -----------------------+----+------+------+------+------------------- i Betaine, | 9 | ... | ... | |No injury CH₂.CO | | | | | / / | | | | | (CH₃)₃N / | | | | | \ / | | | | | O | | | | | -----------------------+----+------+------+------+------------------- j Alloxan, | 10 |1,000 | 100 | | NH.CO | | | | | / \ | | | | | CO CO | | | | | \ / | | | | | NH.CO | | | | | -----------------------+----+------+------+------+------------------- k Guanine, | 12 | | | |Insoluble above 40 NH.C.NH.CO.C.NH | | | | |p.p.m. No harmful \\ || \ | | | | |effects. \\ || CH | | | | | \\ || // | | | | | N————C. N | | | | | -----------------------+----+------+------+------+------------------- l Xanthine | | | | |No injurious | | | | |action. CO.NH.CO.C.NH | | | | | \ || \ | | | | | \ || CH | | | | | \ || // | | | | | NH——C—N | | | | | -----------------------+----+------+------+------+------------------- m Guanadine, | 9 | 100 | 1 | | NH₂ | | | | | / | | | | | HN : C | | | | | \ | | | | | NH₂ | | | | | -----------------------+----+------+------+------+------------------- n Skatol, | 9 | 200 | 50 | |Roots injured more C.CH₃ | | | | |than tops / \\ | | | | | C₆H₄ CH | | | | | \ / | | | | | NH | | | | | -----------------------+----+------+------+------+------------------- | | | | | o Pyridine, C₅H₅N | 9 | .... | 50 | .... |In solutions of 50 | | | | |p.p.m. and less | | | | |the root growth | | | | |was normal. -----------------------+----+------+-------+------+------------------ Picoline, C₅H₄N.CH₃ | 7 |1,000 | 500 | 100 | -----------------------+----+------+-------+------+------------------ | | | | | Piperidin | 7 | 250 | 25 | | CH₂ | | | | | H₂C / \ CH₂ | | | | | | | | | | | | | | | | | | | | | | | | | | H₂C \ / CH₂ | | | | | NH | | | | | -----------------------+----+------+-------+------+------------------ Piperidine | 7 | 100 | 25 | 1 | (neutralized) | | | | | -----------------------+----+------+-------+------+------------------ / \ / \ | | | | | | | | | | | | | Quinolin, | | | | 6 | 500 | 5 | | | | | | | | | | \ / \ / | | | | | N | | | | | -----------------------+----+------+-------+------+------------------ p Ricin | 10 | | 40 | |Insoluble above 50 | | | | | p.p.m. -----------------------+----+------+-------+------+------------------ q Mucin | 10 | | 100 | |Not tested in | | | | | concentrations | | | | |higher than | | | | |100 p.p.m. -----------------------+----+------+-------+------+------------------ | | | | | r Pyrocatechin, | 12 | 500 | 25 | 1 | C₆H₄(OH)₂(1,2) | | | | | -----------------------+----+------+------+------+------------------- s Arbutin, C₁₂H₁₆O₇ | 12 | 500 | 25 | 1 | -----------------------+----+------+------+------+------------------- t Phloroglucin, | 13 | 500 | 25 | 1 | C₆H₃(OH)₃(1,3,5) | | | | | -----------------------+----+------+------+------+------------------- u Vanillin, | 9 | 500 | 1 | | CHO | | | | | / | | | | | C₆H₃——O.CH₃ | | | | | \ | | | | | OH | | | | | -----------------------+----+------+------+------+------------------- Vanillic acid, | 7 | 100 | 25 | 5 | COOH | | | | | / | | | | | C₆H₃—O.CH₃ | | | | | \ | | | | | OH | | | | | -----------------------+----+------+------+------+------------------- v Quinic acid, | 10 | 500 | 100 | | C₆H₇(OH)₄.COOH | | | | | -----------------------+----+------+------+------+------------------- O | | | | | / | | | | | | w Quinone, C₆H₄ | | 9 | 100 | 1 | | \ | | | | | | O | | | | | -----------------------+----+------+------+------+-------------------- x Cinnamic acid, | 8 | 100 | 25 | | C₆H₅CH : CH.COOH | | | | | -----------------------+----+------+------+------+------------------- Sodium cinnamate | 12 | ... | 100 | |Roots were | | | | |stimulated | | | | |in lower | | | | |concentrations -----------------------+----+------+------+------+------------------- y Cumarin, | 8 | 100 | 1 | | CH:CH.CO | | | | | / / | | | | | C₈H₄/ / | 8 | 100 | 1 | | \ / | | | | | O | | | | | -----------------------+----+------+------+------+------------------- | | | | |Insoluble above z Daphnetin | 12 | | 50 | |50 p.p.m. Roots | | | | |somewhat injured CH : CH.CO | | | | | / / | | | | | C₆H₂ ——— O | | | | | \\ | | | | | (OH)₂ | | | | | ----------------------+----+------+------+------+------------------- aa Esculin, C₁₅H₁₆O₉ | 13 | 500 | 1 | | ----------------------+----+------+------+------+------------------- bb Piperonal | | | | | (heliotropine)— | | | | | CHO | | | | | / | | | | | C₆H₅——O | | | | | \ \ | | | | | \ \ | | | | | O——CH₂ | 7 | 100 | 1 | ... | ------------------------+----+------+------+------+------------------- cc Borneol, C₁₀H₁₇(OH)| 10 | 100 | 1 | ... | dd Camphor, C₁₀H₁₆O | 8 | 300 | 5 | ... | ee Turpentine, C₁₀H₁₆ | 8 | 500 | 10 | ... | ------------------------+----+------+------+------+-------------------

a. Aspartic acid has been found in young sugar-cane and in seedlings of the bean and pumpkin.

b. Asparagine was first found in asparagus; but has since been shown to be relatively abundant in many species.

c. Glycocoll is one of the simpler and more common degradation products of proteins.

d. Alanine is a common degradation product of proteins and is related chemically to phenylalanine, and to tyrosine, which has been found in many plants.

e. Leucine, an amino-acid of a paraffine series and a decomposition product of proteids, has been found in certain mushrooms, vetches, lupine, gourds, potatoes, corn, etc.

f. Tyrosine is an important decomposition product of proteids, is widely distributed and found in many plants and fungi.

g. Choline is a derivative of certain lecithins and is found in many seeds and growing plants.

h. Neurine is a substance closely related to choline, and probably formed from it.

i. Betaine is closely related to both choline and neurine, and is found in many seeds and plants.

j. Alloxan is closely related chemically to convicine, which latter is found in beets and certain beans.

k. Guanine is a widely distributed nitrogenous body, and has been found in the seeds of vetch, alfalfa, clover, gourds, barley, sugar-beets and sugar-cane.

l. Xanthine, a substance closely related to guanine, has been found in a number of plants.

m. Guanidine, a substance chemically related to guanine, has been found in a number of plants of different species.

n. Skatol is a derivative of proteids and is a common product of the activities of some varieties of bacteria.

o. Pyridine has been shown to exist in soils, as such probably, by Shorey, who obtained it from certain soils in Hawaii.

p. Ricin is found in the castor-oil plant.

q. Mucin has been found in yams.

r. Pyrocatechin has been found in the bark of various trees, the berries of the Virginia creeper, the sap of sugar-beets and in several varieties of willows.

s. Arbutin has been found in many plants, especially in some of the grasses.

t. Phloroglucin is easily derived from a number of plant constituents.

u. Vanillin forms readily from a glucoside, which is very widely distributed in many plants, and by some authorities is supposed to be a product of the decomposition of wood tissues.

v. Quinic acid, which is found with quinine in the cinchona bark, also occurs in beet leaves, certain hays, cranberry leaves, and occasionally in other plants.

w. Quinone has been shown to result from the action of a certain fungus, Streptothrix chromogena, common in soils.

x. Cinnamic acid is found in certain barks, and forms esters which have been found in the leaves of various plants.

y. Cumarin has been found in a large number of plants, including the grasses, beets, sweet clover, etc.

z. Daphnetin occurs in some species of Daphne and is closely related to cumarin.

aa. Esculin, as well as the corresponding esculetin, has been found occasionally in a number of plants.

bb. Heliotropine, or piperonal, has the odor of heliotrope and is found in flowers.

cc. Borneol occurs in needles of different varieties of pine, fir, spruce and hemlock, golden rod and thyme.

dd. Camphor is closely related chemically to borneol and is secreted by a number of plants; it is found in the wood of Cinnamomum, cinnamon root, in the leaves of sassafras, spikenard, rosemary, rosewood, etc.

ee. Turpentine is a constituent of many plants and coniferous trees.

Finally, a number of organic substances has been isolated from soils. Their composition, and in several cases their constitutions have been determined. The effects of these on plants, when they are present in the cultural media have been studied. Thus, Shorey was able to isolate picoline carboxylic acid (C₇H₇NO₂) from certain soils in Hawaii, and this same substance has since been found in several soils of the United States. In aqueous solutions it is quite toxic to wheat seedlings. Since then a number of other definite organic compounds have been isolated from soils belonging to at least eight different classes of organic substances, including:

Hentriacontane, C₃₁H₆₄. Monohydroxystearic acid, CH₃(CH₂)₆CHOH(CH₂)₉COOH. Dihydroxystearic acid, CH₂(CH₂)₇CHOH.CHOH.(CH₂)₇ COOH. Agroceric acid, C₂₁H₄₂O₃. Paraffinic acid, C₂₄H₄₈O₂. Lignoceric acid, C₂₄H₄₈O₂. Phytosterol, C₂₆H₄₄O.H₂O. Pentosan, C₅H₈O₄. Agrosterol, C₂₆H₄₄O.H₂0. Picoline carboxylic acid, C₇H₇O₂N. Histidine, C₆H₉O₂N₃. Arginine, C₆H₁₄O₂,N₄. Cytosine, C₄H₅ON₃.H₂O. Xanthine, C₅H₄O₂N₄. Hypoxanthine, C₅H₄ON₄. Glycerides, resin acids, etc.

Organic nitrogen in Hawaiian soils, by E. C. Shorey, report of Hawaii Experiment Station, 1906, 37-59.

Chemical Nature of Soil Organic Matter, by Oswald Schreiner and Edmund C. Shorey, Bull. 74, Bureau of Soils, U. S. Department of Agriculture, 1910.

Some of these, picoline carboxylic acid, dihydroxystearic acid and the pentosan just cited, are toxic to growing plants; others are not. The origin and mode of production of these substances in the soil is, generally speaking, uncertain and obscure, and is yet one of the important fundamental problems confronting the soil chemist.

It is important to note that the organic substances thus far isolated from soils are of widely varying types, and with very different chemical characteristics. As pointed out above, almost any type of organic substance is likely to be found in soils, and the effects of any of them on growing plants can hardly be predicted from a priori considerations.

It has been found that as a general rule the continued growth of one crop in any soil results in a low crop production. Pot cultures have given even more pronounced results in the same direction. The explanation long accepted is that the soil has, as a result of continued cropping, become deficient in some one or more of the “available” mineral nutrients. Pot experiments, where the garnered crop was returned to the soil and still a diminished yield was obtained, throw doubt on this explanation. Still further doubt results from water-cultures which, by growing a crop in them, become “poor” for subsequent crops, although there is maintained in them an ample supply of mineral plant nutrients, and they are easily renovated by good absorbers. These facts find a more satisfactory explanation as being due to the production in the nutrient medium of deleterious organic substances originating in the growing plant itself. This idea seems to have been advanced first by De Candolle, in 1832, to account for the beneficial results obtained by employing a rotation of crops. It appears to have been held by Liebig at one time, although he subsequently abandoned it in favor of the view that the benefits of a crop rotation are due to the several crops requiring different proportions of mineral nutrients, and that the disturbance of the balance in the soil produced by one crop is not unfavorable to the growth of some other crop. Although lacking direct experimental confirmation, this latter view of Liebig’s has long prevailed among agricultural investigators, partly by reason of his authority, partly by reason of the dominance of the plant-food theory of fertilizers, and partly by reason of the fact that the ideas of De Candolle as originally advanced included certain errors soon detected. The trend of recent investigations has been distinctly in favor of a modified form of the view of De Candolle. It has been recognized that other factors enter into crop rotations, such as the elimination of associated weeds, various kinds of animal, insect and plant parasites, preparation of the soil by a deep-rooted crop for a shallow-rooted following crop, etc. It has come to be recognized that there are natural associations of plants, and natural rotations of vegetation certainly determined by other than plant food factors. Thus, in the eastern United States, wheat is followed by ragweed naturally, while across the fence cocklebur and wild sunflower come in after the corn, the difference in vegetation being as sharply marked after the removal of the crops as when they still occupied the land. Analyses of the ragweed, for instance, although it is a shallower rooted crop than wheat, show that it takes from the soil as much of the mineral nutrients as does the preceding wheat crop. The investigation of Lawes and Gilbert on fairy rings showed that the continual widening of the rings can not be satisfactorily explained by the comparison of the mineral constituents in the soil within and without the rings. Work at Woburn on the effect of grass on apple trees finds no other plausible explanation than that the growing grass produces in the soil organic substances detrimental to young apple trees. A number of similar cases have been recorded.

See in this connection, Further studies on the properties of unproductive soils, by B. E. Livingston, Bull. No. =36=, Bureau of soils, Dept. of Agric., 1907, p. 7-9.

Mr. J. G. Smith has made a comparison between the potash and phosphoric acid content of the wheat and following crop of ragweed grown on a farm in Fairfax Co., Va. His unpublished results, with some others found in the literature, are given in the following table:

======================+======+==========+=========================== |Potash|Phosphoric| Material | K₂O |acid, P₂O₅| Analyst | % | % | ----------------------+------+----------+--------------------------- Wheat | 0.76 | 0.52 |Smith Young ragweed | 1.78 | 0.73 |Smith Ragweed in seed | 1.28 | 0.35 |Smith Ragweed in seed and | | | accompanying plants | 1.18 | 0.39 |Smith Winter wheat in flower| 1.796| 0.51 |Wolff’s tables in Johnson’s | | | “How Crops Grow,” p. 376. Ragweed | 1.79 | 0.41 |DeRoode,in Bull. 19, W. Va. | | | Agr. Exp. Sta., 1891 Ragweed | 1.809| 0.54 |Burney, 2d. Ann. rept. | | | S. C. Stat., 1889, p. 146 ----------------------+------+----------+---------------------------

On the whole, ragweed seems to require and take from the soil about as much mineral matter as does wheat. It is stated by some of the dairy farmers near Washington, who cut the mixture of ragweed, other weeds and grass following wheat, for a hay crop, that the weight of the ragweed crop is generally heavier than that of the wheat crop. Therefore the ragweed actually removes more mineral matter from the field than does the wheat. These facts lend no support to the popular notion that wheat “exhausts” the soil of its “available” mineral plant nutrients. For analyses of a number of common American weeds, see Analyses of the ashes of certain weeds, by Francis P. Dunnington: Am. Chem. Jour., =2=, 24-27, (1880).

Note on the occurrence of “fairy rings,” by J. H. Gilbert: Jour. Linn. Soc, =15=, 17-24, (1875).

Second, third and fifth reports of the Woburn Experimental Fruit Farm, =1900=, =1903=, =1905=.

Finally, although less work has been done in this direction with higher plants than with other organisms, it is now recognized as a general law of all living organisms that they function less readily as the products of their activities accumulate. These products may, however, be inimical, neutral or even stimulating to other organisms.

It may not be amiss to point out here that this general law holds for all dynamic phenomena. In chemistry, for instance, the general law is well recognized that the rate of reaction diminishes with increase in the active mass of the reaction products. It can be shown that the principle applies to plant growth. Young plants will withdraw potassium more rapidly than chlorine from solutions of potassium chloride; that is, the solution soon contains free hydrochloric acid. Conversely the plants cause a solution of sodium nitrate to become alkaline. Therefore, if the above principle holds, then the initial addition of small amounts of hydrochloric acid to a solution of potassium chloride should slow up the absorption of potassium by seedling wheat plants, or the addition of sodium hydroxide the absorption of nitrogen from a solution of sodium nitrate. Mr. J. J. Skinner has tested this idea with the following results, growing carefully selected wheat seedlings, for 3 days in solutions of pure potassium chloride, solutions of potassium chloride containing initially enough excess of hydrochloric acid to be of an N/₅,₀₀₀ concentration with respect to the acid, solutions of sodium nitrate, and solutions of sodium nitrate containing initially an excess of sodium hydroxide.

Solutions of KCl containing 80 p.p.m. K₂O.

1 K₂O absorbed 40.0 p.p.m. 2 K₂O absorbed 40.0 p.p.m. 3 K₂O absorbed 36.3 p.p.m.

Solutions of KCl (80 p.p.m. K₂O) and HCl (N/₅,₀₀₀).

4 K₂O absorbed 26.7 p.p.m. 5 K₂O absorbed 29.5 p.p.m. 6 K₂O absorbed 26.7 p.p.m.

Solutions of NaNO₃ containing 80 p.p.m. NH₃.

7 NH₃ absorbed 30.2 p.p.m. S NH₃ absorbed 30.2 p.p.m. 9 NH₃ absorbed 32.5 p.p.m.

Solutions of NaNO₃ (80 p.p.m. NH₃) and NaOH (N/₅,₀₀₀).

10 NH₃ absorbed 27.8 p.p.m. 11 NH₃ absorbed 34.3 p.p.m. 12 NH₃ absorbed 27.8 p.p.m.

This problem has been investigated critically by direct experimentation, growing wheat, and other seedlings in water and agar cultures. It has been shown that wheat renders the culture media unsuitable for subsequent wheat crops, though it can be reclaimed or renovated by treatment with such absorbents as carbon black, or by other methods. Wheat did about as well when grown in a medium which had previously supported a growth of cowpeas as when planted in a fresh medium; poorer results were obtained after oats; no crop produced such poor results in the succeeding wheat crop as did wheat itself.

Some factors in soil fertility, by Oswald Schreiner and Howard S. Reed, Bull. No. =40=, Bureau of Soils, U. S. Dept. Agriculture, 1907.

Soil fatigue caused by organic compounds, by Oswald Schreiner and M. X. Sullivan: Jour. Biol. Chem., =6=, 39-50, (1909).

It is yet a matter of dispute as to whether the substances thus added to nutrient media are truly excretory products of the plant, sloughed off or otherwise eliminated from the surface of the roots, or further elaborated by bacterial or other agencies before becoming effective. These are important problems for the plant physiologist and the soil chemist alike. It is beyond dispute, however, by reason of a large and increasing weight of evidence, much of it direct experiment, that, as a result of the growing of plants, soils and the soil water do contain organic substances; harmful to the plant or organism eliminating them; harmful, innocuous, or even stimulating to other plants or organisms.

For the elimination from the soil of toxic or inhibitory organic substances, whether excreted by roots or otherwise produced, several methods are more or less effective. When, as is sometimes the case, the substance is volatile, it may be removed by heating, distilling with steam, or passing a current of air through the soil or cultural medium. These methods, while effective in the laboratory and possibly applicable to greenhouse conditions, are naturally inapplicable to field conditions. In this last case the obvious procedure is to increase as much as possible the absorptive powers of the soil; to secure the best possible drainage; and with these, the best possible aeration of the soil.

It has been found that, in general, a cultural medium which has been rendered unfit for the continued growth of a crop, is readily renovated by treatment with oxidizing agents, and is sometimes rendered even better than ever by such treatment, which would suggest that the oxidation products from plant effluvia may be even beneficial to the plant. To this end the growing plant seems itself to be an active agent, apparently attempting automatically to protect itself against the products of its own activities. It has been pointed out by Molisch that root secretions have an oxidizing power, apparently of an enzymotic character. Some doubt of the validity of Molisch’s work has been raised by Czapek, Pfeffer, and others; nevertheless it is now accepted that while intercellular autoxidation or reduction processes may take place in living roots, the higher plants, such as our common crop plants, also show a more or less well-developed extracellular oxidizing power in the neighborhood of the root tips and root hairs. That this oxidizing power displayed by growing roots is enzymotic is indicated by the fact that artificial culture media frequently display it also after plants have been grown in them for a short while.

Über Wurzelausscheidungen und deren Einwirkung auf organische Substanzen, von Hans Molisch. Sitzungsber. Akad. Wiss. Wien, Math. nat. K1., =96=, 84-109 (1888).

The rôle of oxidation in soil fertility, by Oswald Schreiner and Howard S. Reed: Bull. No. =56=, Bureau of Soils, U. S. Dept. Agriculture, 1909.

From considerations as yet highly speculative, a different type of oxidation by roots might be anticipated. It is recognized that in the absorption of mineral nutrients by plants a certain amount of selection enters. For example, a plant with its roots in a solution of potassium chloride, absorbs more potassium than chlorine, relatively, and free hydrochloric acid is left in the solution. Obviously in the absorption, work is done, and a possible explanation is that water is decomposed at the absorbing surface of the root, with the liberation of oxygen. Theoretically, it ought not to be difficult to investigate this by a study of the energy changes during absorption, but growing plants do not lend themselves readily to such experimentation.

It has been shown that the oxidizing action of growing roots is generally promoted by having the cultural medium slightly alkaline or neutral rather than acid. It is also promoted by the addition of various mineral salts, notably by nitrates, phosphates, or lime salts. Potassium salts promote the oxidation but slightly, and in some experiments have even produced a slight decrease. The corresponding sodium and ammonium salts are more favorable than those of potassium. It appears altogether probable, therefore, that the mineral salts in commercial fertilizers may have some importance in this connection.

Whatever may be the role of mineral fertilizers towards organic substances toxic to growing plants, it is certain that they have an importance and one that is probably specific, as indicated by some recent investigations. Culture solutions containing the constituents potassium, nitric acid and phosphoric acid were prepared in such manner that they covered the range of all possible ratios of these constituents in intervals of ten per cent. in each. Into one set of these solutions was introduced dihydroxystearic acid, into another set cumarin, and into a third set, vanillin, and into a fourth set, quinone. The growth of wheat seedlings in these several sets showed indubitably that these several organic substances which are all deterrent to the growth of wheat, were modified in their influence by the presence of the mineral salts; but that nitrates were more efficient than the other minerals in the case of the solutions containing dihydroxystearic acid or vanillin; phosphates were most efficient in the case of the solutions containing cumarin, and potassium most efficient in solutions containing quinone. As the organic substances used in these experiments, either in themselves or as typifying classes of compounds, may be anticipated in soils under natural conditions, it is again apparent that mineral fertilizers have a function in addition to the traditional one of increasing the supply of mineral nutrients.

Action of fertilizing salts on plant enzymes, by M. X. Sullivan, Jour. biol. chem., =6=, (1909), proceed. XLIV.

Private communication by Dr. Oswald Schreiner and Mr. J. J. Skinner.

The fact that the oxidizing power of roots is more marked when grown in aqueous extracts of soils in good tilth than in extracts made from soils in poor tilth, shows that cultural methods are no less important in field practice than are fertilizers in promoting this important activity of plants. There is little reason to doubt that oxidizing agencies other than plant roots (bacterial for instance) are more or less active in every arable soil, and numerous investigations, among which Russell’s researches are conspicuous, leave little doubt that oxidation processes are promoted by good tilth. It is apparent, therefore, that by the activities of the plant itself as well as other agencies, the general tendency in soils is the destruction of or rendering innocuous harmful plant effluvia or other organic substances, and to this end are effective each of the three methods of soil control generally practiced, namely, tillage, crop rotation and fertilizers.

Among the organic components of the soil none have greater importance and interest than those containing nitrogen or as they are frequently called the nitrogen carriers. Conspicuous among these are the nitrates. While it is now generally conceded that ammonia and other nitrogen compounds can be taken up by higher plants and elaborated by them under special conditions, it nevertheless remains true that plants draw their needed supplies of nitrogen from the soil solution, mainly in the form of nitrates. The problems presented by these nitrogen carriers are mainly bacterial and physiological, but certain features are of direct importance to the soil chemist and to a study of the soil solution. It is now known generally that there are many kinds of nitrifying and denitrifying bacteria in soils, and that probably every arable soil contains several species, or varieties at least of both kinds. With good tilth and consequent aerobic conditions, nitrifying processes prevail, and with poor tilth or in subsoils, anaerobic conditions and denitrifying processes prevail. Warmth, moisture, the reaction of the soil, and perhaps other factors markedly affect the activity of the organisms of the soil solution. Another important factor is that the absorptive powers of the higher plants are markedly affected by sunlight, so that, especially on bright and clear days, there is generally a higher concentration of nitrates in the soil solution in the morning than in the evening. This fact would seem to affect seriously the value of some recent and extensive investigations where it has been sought to classify soils by their content of water-dissolved nitrates. Nitric acid is more readily leached from soils than are most other acid radicals. Consequently nitrates, like other organic components of the soil solution, and unlike inorganic components, tend to vary greatly in concentration.

Oxidation in soils, and its connection with fertility, by Edward J. Russell: Jour. Agric. Sci., I, 261-279, (1905); Pt. II. The influence of partial sterilization, by Francis V. Darbishire and Edward J. Russell, =2=, 305-326, (1907).

The fixation of atmospheric nitrogen by bacteria, by J. G. Lipman, Bull. =81=, Bureau of Chemistry, U. S. Dept. of Agriculture, 1904, p. 146-160; A review of investigations in soil bacteriology, by Edward B. Voorhees and Jacob G. Lipman, Bull, =194=, Office of Experiment Stations, U. S. Dept. of Agriculture, 1907.