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.[116] 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]).
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.