Variable Levels of Biological Activity in Sanborn Field after Fifty Years of Treatment

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Sanborn Field, where the predominant soil treatment has been manure additions and some fertilizers in continuous cropping and different rotations, provides an opportunity to study the properties and the performances of the soil organic matter as an index of the biological activity in relation to soil management practices. After fifty years of treatment the organic matter fraction of the soil might well be expected to have some ear marks of stability in nature and behavior which could be recognized by analytical study. The hypothesis for undertaking such a study is the belief that organic matter returned to the soil is a greater source of nutrients than commonly considered. Organic matter is readily recognized as the source of energy for the majority of soil chemical changes. As the source .of -carbon dioxide, we grant it a prominent place, and have been led to believe that in setting free this acidic agent through microbiological struggles, it serves to break the bases, or cations, out of the crystal minerals to be absorbed or held less firmly by the clay complex, and thus changed from the unavailable to usable forms for the plants. Skepticism toward such a belief in the mineral breakdown as the main source of nutrients for plants should arise from the acquaintance with the high ash content of the so-called humus or organic fraction taken .from the soil. Also, the stability of the colloidal mineral, or clay fraction, under rather drastic chemical treatment in the laboratory, indicates its insignificant change during a single growing season. Vftien organic matter decay reduces soil acidity, isn't such possibly the result more from oxidized alkali or alkaline earth residues left as ash than from basic residues liberated by rock and mineral breakdown under carbonic acid attack? By determining the more exact nature of the carbonaceous and nitrogenous fraction of the soil, and then by learning its rate of breakdown,—particularly during the growing season,—it will be possible to learn whether or not this fraction of the soil is the main, or almost sole, contributor of nutrients to the plant during the growing season. Sanborn Field is of the Putnam silt loam type in the Putnam-Vigo-Clermont association, whose profile consists of a silty surface underlain by an impervious horizon of compact clay. Its climatic equilibrium locates its surface acre nitrogen content at approximately twenty-five hundred to twenty-six hundred pounds. Its exchange capacity amounts to 1S-15 M.E. per 100 grams soil, and its degree of base saturation is about fifty per cent. The cropping systems on Sanborn Field include continuous crops and rotations while the list of soil treatments contains manure, ammonium sulfgpte, sodium nitrate, superphosphate, complete fertilizers, and limestone. Some of these treatments have extended through only the last twenty-five years, while some have been regularly followed since the establishment of the field fifty years ago. Attempts have been made to learn something not only about the changes in the supply of the organic matter, but also about the nature of the organic matter, particularly the differences in its composition as shown (a) by the carbon and nitrogen contents, (b) by the differences in the extracted humus, (c_) by the degree of Iginification, and (d) by the'differences in rates of its nitrification. The studies on the nature of the organic-matter of this old field were undertaken to learn more about the biochemical activity within, a soil as related to its treatment in the hope that such knowledge might contribute to more wise soil management.