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.
Scheme for Plot-tests of Fertilizers.
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.