Chapter 3, Page 38).

The reserve food-store may then be placed under the following heads:

Hydrous or “zeolitic” silicates, from which dilute acids can take up the bases potash, soda, lime and magnesia. These silicates may be in either the gelatinous or powdery form; in the former case they may also occlude water-soluble substances.

Carbonates of lime and magnesia, which are readily dissolved by carbonated water as well as by the vegetable acids.

Phosphates of lime and magnesia, not very readily soluble in carbonated water, but more readily attacked by the acids of the soil and of plant roots; thus supplying phosphoric acid to plants. The more finely divided they are the more readily they are dissolved; some soils containing only crystalline needles of apatite (see chap. 5, p. 63) only are nevertheless poor in available phosphoric acid.

The natural phosphates of iron and alumina are practically insoluble in all solvents at the disposal of vegetation and though present in considerable amounts in some soils, (see chapter 19, page 355), may be considered as being permanently inert, and therefore not to be counted among the soil resources for plant nutrition. As yet no artificial process by which their phosphoric acid can be made available within the soil, has been discovered.

Water-soluble Ingredients.—As regards these it has already been explained that they are largely retained in the condition of purely physical adsorption, as in the case of charcoal or quartz sand, through which sea water filters and is thereby partially deprived of its salts. But these can be gradually withdrawn by washing with pure water alone, and still more easily when stronger solvents are used. Since the soil-water is always more or less charged with carbonic acid, and the roots themselves secrete carbonic as well as stronger acids in their absorption of mineral plant-food, there is no difficulty about explaining the manner in which such physically condensed ingredients are taken up.

Whitney (Bull. 22, U. S. Bureau of Soils) claims on the basis of a large number of (three-minute) extractions of soils made with distilled water, that these solutions are essentially of the same composition in all soils; that all soils contain enough plant-food to produce crops indefinitely; and that the differences in production are due wholly to differences in the moisture supply, which he claims is, aside from climate, the only governing factor in plant growth. The tables of analytical results given in Bull. 22 fail to sustain the first contention; the second is pointedly contradicted both by practical experience, and by thousands of cumulative culture experiments made by scientific observers; the third fails with the second, except of course in so far as an adequate supply of moisture is known to be an absolute condition both of plant growth, and the utilization of plant-food. It is moreover well known that it is not water alone, but water impregnated more or less with humic and carbonic acids, that is the active solvent surrounding the plant root.

Recognition of the Prominent Chemical Character of Soils. In a former chapter the soils formed from the several minerals and rocks have been discussed in a general manner. We can as a rule obtain some insight into the nature of any soil which we can trace to its parent rock or rocks, if we are acquainted with the composition of the latter.

Similarly, but in a much more direct manner, we can obtain a strong presumption as to the nature of any soil by determining the undecomposed minerals present in it. In all ordinary cases the presumption must be that the decomposed portion of the soil has been derived from the minerals still found in it. Of course it may happen in the case of lands derived from widely distinct and distant regions that no such characteristic minerals can be found; this is very commonly true of the soils forming the deltas of large rivers, in which sometimes the only remaining recognizable mineral is quartz in its several forms, with occasional grains of such hardy minerals as tourmaline, garnet, etc. Apart from such cases, the hand lens or the microscope permits us to recognize in most soils the minerals that have mainly contributed to their formation, thus also gaining a clew to their prominent chemical nature.

Such recognition sometimes involves, of course, a somewhat intimate knowledge of mineralogy; yet a little practice will enable almost any one to identify the more important soil-forming minerals, under the lens or microscope, according to the degree of abrasion or decomposition they may have undergone. The details of such researches lie outside of the limits of this treatise, but some general directions on the subject are given farther on.

See Appendix B.

Acidity, Neutrality, Alkalinity.—A test never to be omitted is that of the reaction of the soil on litmus or other test paper, to ascertain its acid, neutral or alkaline reaction. Should the latter occur quickly (by the prompt blueing of red litmus paper), “black alkali” would be indicated; but a blueing after 20 to 30 minutes means merely that a sufficiency of lime carbonate is present. An acid reaction (the reddening of blue litmus paper) of course indicates a “sour” soil (see chap. 8, page 122).

Chemical Analysis of Soils.—When the observations mentioned above give no very decisive results or inferences as to the soil’s chemical character, the more elaborate processes of qualitative and quantitative chemical analysis may be called in. It would seem at first sight that these ought to yield very definite results to guide the cultivator; yet such is by no means always the case. Both the previous history of the land, and the method of analysis, influence materially the practical utility of the results of chemical soil analysis.

The cause of this uncertainty becomes obvious when we consider the three groups of ingredients outlined above, viz., the insoluble or unavailable, wholly undecomposed rock minerals; the “reserve,” consisting of compounds not soluble in water but soluble in or decomposable by weak acids; and the water-soluble portion, either actually dissolved in the water held by the soil, or held by the soil itself in (physical) absorption. While the latter portion is directly and immediately available to plants, the amounts thus held are usually quite small, and (outside of alkali lands) would rarely suffice for the needs of a crop during a growing season. This demand must be materially supplemented by what can be made available from the soil minerals and the “reserve” by weathering, conjoined with the direct action of the acids secreted from the plant’s root-hairs upon the soil particles to which they are attached. It is obvious that the greater or less abundance of the plant-food in the soil-material upon which these processes may be brought to bear, must essentially influence the adequacy of the plant-food thus supplied. Moreover, the greater or less extent to which these sources may have been drawn upon previously in the course of cultivation, will similarly influence that adequacy, on account of the diminution of the readily available supply.

The investigations of King (On the Influence of Soil Management upon the Water Soluble Salts in Soils and the Yield of Crops, Madison, 1903) show that from some soils at least, a sufficiency of plant-food ingredients for a season’s crop may be dissolved by distilled water alone, if the soil be repeatedly leached and dried at 110°. Whether such a supply can be expected under field conditions, remains to be tested.

Water-soluble and Acid-soluble Portions most Important.—It thus seems that while the undecomposed rock minerals are indicative of the nature of the soil, but not directly concerned in plant nutrition, the most direct interest attaches to the water-soluble portion, and the acid-soluble reserve. Both of these can, of course, be withdrawn from the soil by treatment with acids of greater or less strength; and it would seem that if we knew just what is the kind and strength of the acid solvent employed by each plant, we could so imitate their action as to determine definitely whether or not the soil contains an adequate or deficient supply of actually available food for the coming crop.

We Cannot Imitate Plant-root Action.—In this, however, we encounter serious difficulties. The acids secreted by the plant roots are not the only solvents active in the dissolution of plant-food; as yet we know the nature of only a few; and even these, instead of acting for a long time (season) on a relatively small number of soil particles touched by the root-hairs, can in our laboratories only be allowed to act for a short time on the entire soil-mass. Clearly, the results thus obtained cannot be a direct measure of the amount of plant-food which a plant may take up in a given time; we can only gain comparative figures. These, however, can be utilized by comparison with actual cultural experience obtained in similar cases.

Cultural experience must, of course, be the final test in all these questions; and it is generally more fruitful to investigate the causes underlying such actual practical experience, than to attempt to supply, artificially, the supposed conditions of plant growth. The latter are so complex and so difficult of control, that the results obtained by synthetic, small-scale experiments are constantly liable to the suspicion that they are partly or wholly due to other causes than those purposely supplied by the experimenter.

Analysis of Cultivated Soils.—It is also clear that in view of the inevitable complexity of the conditions governing vegetable growth, we should whenever feasible proceed from the more simple to the more complex. The failure to conform to this rule in soil investigation has been the cause of an enormous waste of energy and work bestowed, at the very outset, upon the most complex problem of all, viz., the investigation of soils long cultivated and manured; lands which, having been subject perhaps for centuries to a great and wholly indefinite variety of crops and cultural practices, had thereby become so beset with artificial conditions that without a previous knowledge of what constitutes the normal regime in natural soils, the correlation of their chemical constitution, as ascertainable by our present methods, with their production under culture, became as complex a problem as that of motions of three mutually gravitating points in space. Neither can be solved by the ordinary processes of analysis, chemical or mathematical. Nevertheless, though it was at one time contended that the minute proportion of plant-food ingredients withdrawn from soils by cultivation could not be detected by quantitative analysis, numerous examples have shown that with our present more delicate methods this can in most cases be done, though not always after a single year’s crop.

Methods of Soil Analysis.—The more or less incisive solvent agents used in extracting a soil for analysis will of course produce results widely at variance with each other. When fusion with carbonate of soda, or treatment with fluohydric acid is resorted to, we obtain for each soil-ingredient the sum of all the amounts contained in each of the three categories—the unchanged minerals, the zeolitic “reserve,” and the water-soluble portion. It was early recognized that the results of such analyses bear no intelligible relation to the productive capacity of soils; for pulverized rocks of many kinds, or volcanic ashes freshly ejected and notoriously incapable of supporting plant growth, might be made to give exactly the same composition. The amounts of plant-food ingredients thus shown might be several hundreds or thousands of times greater than what one crop would take from the soil, and yet not an ear of grain could be produced on the material. The only case in which any useful information could be thus obtained would be that of the absence, or great scarcity, of one or more of the plant-food ingredients.

The next step was to use in soil analysis acids of such strength as to dissolve all the zeolitic (and water-soluble) portion, leaving the unweathered soil minerals behind; it being assumed that the prolonged action of the roots and soil-solvents would in the end act similarly to the acids employed, such as chlorhydric or nitric acids.

But here also the results of analysis very commonly failed to correspond to cultural experience in the case of cultivated soils; which frequently failed utterly to produce satisfactory crops even when the acid-analysis had shown an abundance of plant-food ingredients. Upon this evidence, this method of soil investigation was also condemned as being of little or no practical utility; and this has ever since been a widely prevalent view.

The preferable investigation of cultivated soils was due to the fact that they are practically the only ones available in the countries where the study of agricultural science was then being prosecuted; and the paucity of useful results there achieved discouraged the undertaking of similar researches where, as in the United States, the materials for the investigation of the simpler cases—those of unchanged, natural or virgin soils—were readily accessible. It was not apparent on the surface that the indefinitely varied conditions introduced by long culture would inevitably cause this lack of definite correlation between the immediate productive capacity of a soil and the composition of its acid-soluble portion, and that yet the same might not be true of natural, uncultivated soils, which have all been subjected, alike, only to the natural processes of weathering, and to the annual return of nearly the whole of the ingredients withdrawn by plant growth.

Following the failure of the treatment with strong acids to yield with cultivated soils results definitely correlated with cultural experience, numerous attempts were made to gain better indications by the employment of weaker acid solvents. The pure arbitrariness of such diluted solvents was equaled by the total indefiniteness and irrelevance of the results with different soils. Only two rational alternatives seem to remain, viz., either to push the extraction to the full extent beyond which action becomes so slow as to clearly exclude any farther effective action of plant acids; or else to use the latter themselves at such strengths as by actual experiment is found to exist in their root sap. The first alternative aims to ascertain the permanent productive values of soils; the latter to test their immediate productive capacity. Both alternatives are purely empirical, and derive their only claim to practical value from their accordance with practical experience (see chapter 19).

THE SOLVENT ACTION OF WATER UPON SOILS.

The almost universal solvent power of pure water has already been alluded to in chapter 2 (see p. 18), and illustrated by the analyses of drain and river waters. While these convey a general idea of the chief substances dissolved and carried off, the direct investigation of the solutions actually obtainable from the soil by longer treatment and with no more water than is compatible with the welfare of ordinary crops, necessarily gives somewhat different results. For when drains flow during or after heavy rains the water has not time to become saturated. The following data afford a clearer insight into the actual and possible solvent effects of water in the soil, and its possible adequacy to plant nutrition unaided by acid solvents.

Extraction of Soils with Pure Water.—Eichhorn and Wunder treated soils from Bonn, and from Chemnitz (Saxony) respectively for ten days and four weeks with about one-third of their weight of water; the solutions thus obtained contain in 1,000,000 parts:

==========================+=======+=========== | Bonn. | Chemnitz. --------------------------+-------+----------- Silica | 48.0 | 25.7 Potash (K₂O) | 115.4 | 7.5 Soda (Na₂O) | 11.0 | 30.4 Lime (CaO) | 128.0 | 83.6 Magnesia (MgO) | 38.4 | 37.4 Peroxid of Iron (Fe₂O₃) | Trace | 11.7 Alumina (Al₂O₃) | ? | ? Phosphoric acid (P₂O₅) | 31.0 | Trace Sulfuric acid (SO₃) | 100.2 | Chlorid of Sodium (NaCl) | 58.6 | 47.6 ---------------------------+-------+-----------

These figures differ widely in most respects from those given for drain and river waters. Potash especially is far more abundantly present in the Bonn-soil solution than in the drain water, and so is phosphoric acid; while lime is not widely different. Eichhorn therefore calculates that with a reasonably adequate supply of water, these ingredients would fully suffice for a full crop of wheat. The Chemnitz soil, on the other hand, does not yield enough plant-food for more than a very small crop upon the same assumptions.

Continuous Solubility of Soil-ingredients.—It seems to be impossible to exhaust a soil’s solubility by repeated or continuous leaching with water. This was demonstrated in 1863 and 1864 by Ulbricht and by Schultze; their general conclusions have quite lately been corroborated by King, as the result of extended and very careful investigations.

Schultze experimented on a rich soil from Mecklenburg, by continuous leaching with distilled water for six days, one liter passing every twenty-four hours, with the following results.

Vers. Stat. V. p. 207.

Ibid. VI. p. 411.

Proc. Ass’n Prom. Agr. Sci. 1904.

RICH SOIL FROM MECKLENBURG (Schultze.) 1,000,000 PARTS OF EXTRACTS CONTAINED: ==============+============+===========+==========+========== |Total matter|Organic and|Inorganic.|Phosphoric | dissolved. | volatile. | | acid. --------------+------------+-----------+----------+---------- First extract | 535.0 | 340.0 | 195 | 5.6 Second “ | 120.0 | 57.0 | 63 | 8.2 Third “ | 261.0 | 101.0 | 160 | 8.8 Fourth “ | 203.0 | 83.0 | 120 | 7.5 Fifth “ | 260.0 | 82.0 | 178 | 6.9 Sixth “ | 200.0 | 77.0 | 123 | 4.4 | ------- | ----- | --- | ---- Total | 1,579.0 | 740.0 | 839 | 41.4 --------------+------------+-----------+----------+----------

It thus appears that while the first extraction removed the main portion of the organic matter, the inorganic matters dissolved were not greatly diminished in subsequent leachings; and that phosphoric acid continued to come off to the last. The rich soil used in this case gave results corresponding in general to these from the Bonn soil, in the previous table. From a poorer soil similarly treated by Ulbricht, described by him as a ferruginous sand from Dahme, the leaching of which was continued for thirty days in periods of three days each, with a total of forty times its weight of water, the results were as follows:

SOIL OF LOW PRODUCTION FROM DAHME (Ulbricht). THE SEVERAL EXTRACTS CONTAINED IN 1,000,000 PARTS: ============+========+========+========+========+========+======== | First | Second | Third | Fourth | Fifth | Sixth |Extract.|Extract.|Extract.|Extract.|Extract.|Extract. ------------+--------+--------+--------+--------+--------+-------- Potash | 7 | 6 | 7 | 7 | | 3 Soda | 41 | 11 | 26 | 17 | | 8 Lime | 96 | 70 | 55 | 48 | 62 | Magnesia | 14 | 10 | 9 | 7 | 8 | Phosphoric | | | | | | acid | trace | 2 | trace | 1 | | + ----- + -- + ----- + -- | -- + -- Totals | 158 | 99 | 97 | 80 | 70 | 11 ------------+--------+--------+--------+--------+--------+--------

It will be seen that there is a considerable difference both in the total amounts of matters dissolved and in the phosphoric acid taken out by the water, as compared with the rich soil treated by Schultze. The uniformity of the amounts of potash removed at the successive leachings is remarkable.

King’s Results.—The same general features are again strikingly illustrated by King’s results, as given in the following table. King’s first leachings were always made by shaking up the soil with ten times its dry weight of water for three minutes, then after subsidence filtering the solutions through a Chamberland (porcelain biscuit) filter, and then (without evaporation) determining the ingredients dissolved, by very delicate, mostly colorimetric methods. Subsequent leachings were made by packing the soil around the filters and washing with five times the weight of water, taking about fifteen minutes each time; but drying the soil at 120 degrees C. between successive leachings.

WATER EXTRACTION OF SOILS OF LOW AND HIGH PRODUCTION, BY F. H. KING. PARTS PER MILLION. =======================+=======+=======+=========+=========== |Potash,| Lime, |Magnesia,| Nitric Extractions | K₂O. | CaO. | MgO. | Acid, | | | | N₂O₅. -----------------------+-------+-------+---------+----------- SOILS OF LOW PRODUCTION. Sassafras sandy 1 | 12.62| 74.39| 17.82 | 18.03 soil. 11 | 218.25| 135.35| 147.45 | 21.76 -----------------------+-------+-------+---------+----------- Norfolk, North 1 | 21.17| 58.30| 22.91 | 30.64 Carolina sandy soil 11 | 166.08| 162.98| 125.00 | 27.11 Average. | 192.60| 149.20| 136.23 | 24.44

SOILS OF HIGH PRODUCTION. Janesville, Wis. 1 | 25.35| 135.30| 51.72 | 55.10 Loam. 11 | 313.70|1120.30| 500.60 | 51.42 -----------------------+-------+-------+---------+----------- Hagerst’wn, Pa. 1 | 21.73| 165.25| 76.88 | 25.72 Clay loam. 11 | 301.55| 967.80| 463.15 | 96.04 Average. | 307.60|1044.05| 487.88 | 73.73 -----------------------+-------+-------+---------+-----------

=======================+==========+========+========+========+====== |Phosphoric|Sulfuric|Carbonic|Chlorin,|Silica Extractions | Acid, | Acid, | Acid, | Cl₂. | SiO₂. | P₂O₅. | SO₃. | CO₂. | | -----------------------+----------+--------+--------+--------+------ SOILS OF LOW PRODUCTION. Sassafras sandy 1 | 7.41 | 53.84 | 13.94 | 1 | 5.60 soil. 11 | 64.16 | 203.96 | 221.33 | 2 | 70.20 -----------------------+----------+--------+--------+--------+------ Norfolk, North 1 | 10.15 | 42.82 | 20.42 | 1 | 8.24 Carolina sandy soil 11 | 80.34 | 172.42 | 148.52 | 2 |122.20 Average. | 72.25 | 126.13 | 184.93 | 2.- |146.20

SOILS OF HIGH PRODUCTION. Janesville, Wis. 1 | 16.96 | 125.43 | 29.31 | 2.67 | 40.28 Loam. 11 | 418.85 | 592.75 | 472.95 | 0.00 |414.50 -----------------------+----------+--------+--------+--------+------ Hagerst’wn, Pa. 1 | 11.51 | 187.59 | 97.09 | 1.67 | 21.17 Clay loam. 11 | 136.21 | 502.82 | 620.00 | 0.00 |283.80 Average. | 277.03 | 547.79 | 546.48 | 0.00 |349.15 -----------------------+----------+--------+--------+--------+------

King’s observations show strikingly both the continuous solubility of the soil, and the differences between the solutions derived from soils of low and high productiveness; wholly negativing the contention of Whitney that the solutions from different soils are of practically the same composition. King also calls attention to the fact, shown in other experiments made in the extraction of soils without intermediate dryings, that the amounts extracted were very much less in subsequent than in the first extraction; doubtless because the evaporation from the soil particles had carried a large proportion of soluble matters to the surface, whence it was readily abstracted by the first touch of the solvent water. At each drying not only are the soluble matters again drawn to the surface, but heating a soil even to 100° renders additional amounts of soil ingredients soluble both in water and in acids. It can scarcely be doubted that the intense heating which desert soils undergo during the warm season is similarly effective; and thus the great productiveness of these soils under irrigation, and the marvelously rapid development of the native vegetation when rains moisten the parched soil, is in part at least accounted for by this immediate availability of a large supply of plant-food.

Bulletin No. 22, Bureau of Soils, U. S. D. A.

Composition of Janesville loam.—In connection with the above data given by King, it is interesting to note the composition of the soil in the above table yielding the highest proportions of soluble matter, when analyzed according to the method practiced by the writer (see chap. 19, p. 343). This analysis was made under the supervision of Professor Jaffa in the laboratory of the California Experiment Station by Assistant Charles A. Triebel.

Loam Soil from Janesville, Wisconsin; sample sent by Prof. F. H. King, Madison, Wis.

This soil is a light friable loam, resembling the northern Loess in color and texture; it is highly productive. It is underlaid at 5 feet by the drift gravel of that region, enclosing much calcareous material, which evidently has had a large share in the formation of this soil, just as is the case in southern Michigan.

The soil, when dried at 110° C, consisted of

CHEMICAL ANALYSIS OF FINE EARTH.

Insoluble matter 69.35 Soluble silica 10.89 Potash (KO₂) .59 Soda (Na₂O) .04 Lime (CaO) .83 Magnesia (MgO) .51 Br. ox. of Manganese (Mn₃O₄) .08 Peroxid of Iron (Fe₂O₃) 3.60 Alumina (Al₂O₃) 5.26 Phosphoric acid (P₂O₃) .06 Sulfuric acid (SO₃) .10 Water and organic matter 8.72 ------ Total 100.03

It will be noted that in accordance with the interpretation of analyses of soils as given in the next chapter, this is a high-class soil in every respect, except that its content of phosphoric acid is only just above the lower limit of sufficiency. But as is also shown below, in presence of a large supply of lime even lower percentages of phosphoric acid are adequate for long-continued production (see chap. 19, pp. 354, 365) by rendering the substance more freely available; and that this is true in this case is shown by the result of King’s leachings, in which this soil yields a maximum of 419 parts per million as against 80 and 64 parts in the poor soils, which at the same time yield only one fourth as much of lime. Unfortunately we have no full analyses of these other soils for comparison; although they have served as a basis of comparison for years in the Washington Bureau of Soils.

Solubility of Soil Phosphates in Water.—The solubility of the phosphate contents of soils has been elaborately investigated by Th. Schloesing fils. He found in the case of a number of soils investigated by him that the amount of phosphoric acid P₂O₅ in the soil-solution ranged from less than one millionth (or one milligram per liter of water) in a poor soil, to over three milligrams in a rich one. He also found that for one and the same soil the amount so found was constant, if about a week’s time were allowed for saturation. He calculates that while in general the amount of phosphoric acid capable of being supplied to the crop during a growing season of twenty-eight to thirty weeks would suffice for but few crops, the supply so afforded is in no case a negligible quantity, frequently amounting to more than half of the crop-requirements. Experiments with various crops prove that these dilute solutions are utilized by all of them, sometimes to the extent of completely consuming the content of the solution. The much smaller content of phosphoric acid in drain waters is accounted for by the lack of time for full saturation during the time that the flow lasts. Whitney, (Bureau of Soils, Bulletin 22) has extracted the soil-solution by means of the centrifuge from several soils; the contents of phosphoric acid thus found are in general of the same order as those shown in the preceding table by King, but much in excess of Schloesing’s figures; notwithstanding the fact that Whitney’s soils had been in contact with water for only twenty-four hours. The cause of this wide discrepancy is not clear.

Ann. de la Sci. Agron., 2de série tome I, pp. 416-349; 1899.

Practical Conclusions from Water Extraction.—As regards the practically useful conclusions to be drawn from the extraction of soils with pure water, the data given above, and especially the results obtained by King, seem to prove that there is a more or less definite correlation between the immediate productiveness of soils and the amount and kinds of ingredients dissolved; especially in the case of phosphoric acid, the adequacy of the supply of which for immediate production is assumed to be thus demonstrable by many French chemists. Moreover, a number of King’s results, tabulated in curves, exhibit a remarkable general parallelism of the curves showing totals of plant-food extracted by water, and actual crop production. This is the more remarkable since it is known to be, not pure water, but such as is more or less impregnated with carbonic acid at least, that is actually active in soil-solution and plant-nutrition. The farther development of this method may, it would seem, lead to definite conclusions at least in respect to the immediate productive capacity of cultivated, and perhaps also of virgin soils. But it is not likely to give any definite clew as to the durability of such lands.

ASCERTAINMENT OF THE IMMEDIATE PLANT-FOOD REQUIREMENTS OF CULTIVATED SOILS BY PHYSIOLOGICAL TESTS. PHYSIOLOGICAL SOIL-ANALYSIS.

As has already been stated, the quantitative analysis of cultivated soils by means of strong acids affords a presumptive insight into their immediate productiveness, and the kind of fertilizer needed to improve it, only in case of the extreme deficiency of one or several of the chiefly important plant-foods. The limits of deficiency of these in virgin soils have been discussed above; but since in cultivated soils amounts of soluble plant-food so small as to be beyond the limits of ordinary analytical determinations, when distributed through an acre-foot of soil may, when rightly applied, nevertheless produce very decided effects, the indications thus obtainable are not absolute. Thus a dressing of 150 lbs. of Chile saltpeter, containing only about 24 lbs. of nitrogen, is capable of causing the production of a full crop of wheat where otherwise, even under favorable physical conditions, only a fraction of a crop would have been harvested; provided, that all the other requisite ingredients were present to a sufficient extent and in available form. Yet the amount of nitrogen thus added would amount, in one acre-foot of soil to only .0008%, say eight ten-thousandths of one percent; which, with the amounts of substance usually employed in soil analysis, would be an unweighable quantity, and might easily be overlooked.

Since the amounts of potash and phosphoric acid actually taken out of the soil by one crop are in general of the same order of magnitude as the above, what is taken out by one or two crops will usually fall within the limits of analytical errors, especially of those incurred in sampling the soil. Yet that the changes caused by a number of successive crops can be proved, even by the ordinary methods, has been abundantly verified. For it seems that the losses of soil ingredients in cultivated lands exceed considerably those calculated from the actual drain represented by the crops.

Plot Tests.—There is, however, an obvious and apparently simple method by which every farmer might make his own fertilizer tests, on a small and inexpensive scale, the results of which may afterwards be put in effect on his entire land. It is to apply in proper proportions on plots (of say from one twentieth to one fortieth of an acre), the several plant-food ingredients usually supplied in fertilizers, singly as well as conjointly with each other, leaving check unfertilized plots around as well as among them. By comparison with these, the cultural results should at once determine which of the fertilizers can most advantageously be applied to the land. Such tests when carried out with all the proper precautions are often very decisive and practically successful. But they so frequently suffer from seasonal influences (such as scanty or excessive rainfall, cold or heat, etc.), inequality of soil conditions, failure to apply the fertilizers at the right time, or in the right way, the depredations of insects and birds, and other causes, that it generally takes several seasons’ trial to obtain any definite results. On level lands of uniform nature and depth, they are most likely to be successful; while on undulating or hill lands it is not only very difficult to secure uniformity of soil and subsoil on areas of sufficient size, but also to prevent the washing of fertilized soil, or fertilizer in solution, from one plot to the other by the influence of heavy rains or irrigation; thus wholly vitiating the experiments. In very many cases, especially in the arid region, the results of such trials have been practically nil, for the reason that physical defects of the soil, and not lack of plant-food, were the cause of unsatisfactory production.

A full examination of physical conditions, as outlined in previous chapters, should in all cases precede the application of fertilizers; such examination will at the same time serve to determine the greater or less uniformity of soil-conditions, which is of first importance to the cogency of fertilizer tests. As a matter of fact, few farmers possess the necessary qualifications to carry out such tests successfully, since their execution requires a certain familiarity not only with the principles and methods of experimentation, but also the faculty and practice of close and reasoning observation; which, unfortunately, is not as yet a part of instruction in our schools. The experience so often had in co-operative work between experiment stations and farmers is cogent on this point.

Those desiring to do such work, however, can make use of something like the plan given above; it being understood that in the case of clay soils, the unplanted paths left between the plots should be at least two feet in width; in the case of sandy soils the distance should be not less than three feet, and more if the plots are located on a slope. The crop from each plot should if possible be weighed as a whole; but if the plot be large and the crop measurably uniform, an aliquot part, such as one fourth, may be weighed instead. In regular experimentation the crops are weighed both in the green (freshly cut) condition, and after drying. Since the dry matter is the real basis of value in the case of most field crops, its weight is the most important; as the water-content of green crops may vary considerably. But in the case of vegetables as well as fruit crops, not only must the weight of the fresh crop be determined, but it should be sorted into the “marketable” and “unmarketable” sizes and qualities. Failure to do this may vitiate the entire experiment for practical purposes.

Pot Culture Tests.—The uncertainty attending plot culture tests on account of the difficulty of controlling seasonal and other external conditions, has resulted in the extended adoption of indoor culture tests, usually conducted in zinc or “galvanized” cylinders of a size sufficient to contain from twelve to twenty or more pounds of soil. These are kept in a green-house whose temperature and moisture-condition can be regulated at will, and where the soil-moisture is wholly under control. For investigations of the effects of various kinds of plant-food upon vegetable development, this method has served most satisfactorily and effectually, and striking photographs of results thus obtained are seen on all hands: for which reason, to save space, they have not been introduced into this volume. It seems at first sight that the same method should serve admirably to determine the manure-requirements of soils under controlled conditions.

It must, however, be remembered that the field conditions as regards subsoil, evaporation, ascent of moisture from below, penetration and spread of roots, etc., in other words, all the physical conditions so vitally concerned in crop production, except the temperature and moisture-condition of the soil, are wholly left out of consideration in this method. Hence the application of the results so obtained to actual field conditions can only be made with great caution, and are often widely discrepant with actual experience.

The method has of late been carried to an extreme by the U. S. Bureau of Soils in the proposition to supplant the large soil-pots heretofore used by small paraffined wire-cloth baskets, 3 × 3 inches in size, in which the soil to be tested is sown with seeds which are allowed to develop only for three to five weeks; it being claimed that the development occurring during that time is quite sufficient to indicate what will be the ultimate outcome in crop production. But practical experience has long ago demonstrated that these early stages of growth cannot be relied upon to show the crop results to be expected. Yet if this minute scale of pot-culture should, on further test, prove to give truthful forecasts even in a mere majority of cases, the facility with which it may be carried out will entitle it to favorable consideration. A great deal more proof is needed on this point than the confident claims of the Bureau indicate.

CHEMICAL TESTS OF IMMEDIATE PRODUCTIVENESS.

Testing chemical soil-character by crop analysis.—Another method for the determination of immediate soil requirements has been elaborated by E. Godlewski. The principle upon which this method rests is that plants growing in a soil deficient in available plant-food of any one kind will in their ash show a corresponding deficiency, or at least a minimum proportion of the same; and that in many cases, the nature of the deficiency manifests itself in the form or development of the plant, so clearly as to render chemical analysis unnecessary (see below, chapter 22).

Zeitschr. Landw. Vers. Oesterr., 1901.

To a certain extent the latter idea has been and is constantly being utilized in practice. It is essentially involved in the habit of judging of land by its natural vegetation; and by agricultural chemists and intelligent farmers, when they check excessive growth of stems and leaf (indicating excess of nitrogen) by the use of lime or phosphates; or prescribe the use of nitrogenous manures when a superabundance of small, unmarketable fruit is produced. From the coincidence of such indications with the results of the analyses of soils and ashes, very definite and permanently valuable indications as to the proper fertilization and other treatment of the land may be deduced.

Godlewski insists strongly, and with a good deal of plausibility, upon the importance of making such trials in the open field and not merely in pots. While this is true, it is also true that such field experiments suffer from the same liability to imperfection as the “plot fertilizer-test” plan just described; viz., that the season may exert a much more powerful influence than the fertilization, and the tests may lead to wholly erroneous conclusions unless the experiments are continued for a number of years, and under skilled supervision. But when once the normal ratio between the ash ingredients for a particular soil and climatic region have been ascertained, the data will be of lasting benefit to agriculture there, and perhaps, other things being equal, to the world at large.

H. Vanderyst has discussed the entire subject of physiological soil analysis elaborately in the Revue Génerale Agronomique of Louvain, 1902-3 (Exp’t St. Record, April 1904, Vol. 8, page 757) and shows in detail the conditions under which it may be successful. Among these he reckons as full a knowledge of the chemical characteristics of a soil as can be obtained by chemical analysis.

Chemical Tests of Immediately Available Plant-food.—It is scarcely doubtful that plants differ considerably in the energy of their action upon the “reserve” soil ingredients; hence no one solvent used by the analyst could represent correctly the action of plant-roots in general upon the soil, even if we could give that action the same time (a growing season) and opportunity afforded them in nature by the root-surface. We are forced to proceed empirically; and among the numerous solvents suggested for the purpose of soil extraction, that of Dyer, already mentioned, viz., a one per cent solution of citric acid, making allowance for such neutralization as may occur in the soil, has seemed to the writer to give results most largely in agreement with cultural experience. Walter Maxwell has recommended aspartic acid in lieu of citric, as approaching nearer to practical results, at least with sugar-cane.

According to the investigations of Dyer, on Rothamstead soils of known productiveness or manurial condition, it appears that when the citric-acid extraction yields as much as .005% of potash and .010% of phosphoric acid, the supply is adequate for normal crop production, so that the use of the above substances as fertilizers would be, if not ineffective, at least not a profitable investment. These figures refer to the ordinary field crops of England and to soils originally fertile and well supplied with lime. It can readily be foreseen that under other climatic and soil conditions, different figures may have to be established. So far as the writer’s experience goes, however, the above figures are very nearly valid for the arid climates as well; only the figures obtained for arid soils are usually far in excess of the above minimum postulates. Figures for lime and nitrogen are given in chapters 8 and 19. But the results obtained with the highly ferruginous soils of Hawaii show that under such conditions, figures far exceeding the minimum ones established by Dyer nevertheless coexist with need of phosphate fertilization.