Soil Conditions in Relation to the Response from Green Manure Applications

HORACE J. HARPER IT IS generally assumed that cultivated soils in humid regions are commonly deficient in nitrogen and that the maintenance of crop production on these soils depends to a very great extent upon the use of cropping systems in which legumes are grown and/or liberal applications of various nitrogenous fertilizers are applied. In regions which are distinctly semi-arid in character and legume crops cannot be produced successfully, the problem of maintaining the nitrogen content of soil has not been solved; however, many investigators continue to hope that nonsymbiotic fixation of nitrogen will be adequate to meet the normal requirement of crops which can be grown under the climatic conditions which prevail. Between the semiarid region of the Great Plains where legume crops do not make a satisfactory growth under average conditions and the humid area in the west-central portion of the Mississippi Valley where a nitrogen deficiency in the soil frequently limits crop production, a zone of varying width occurs where the average climatic level for crop production is low with a marked seasonal variation in plant development. In this area the use of accepted soil-building methods which are commonly employed under a more favorable environment -for plant growth may require some modifications. " During the transitional period from virgin land to a point where all soils are deficient in nitrogen, the use of legume cropping systems for soil improvement cannot be recommended as a general practice on every farm, since economical increases in crop yield are not always secured following the growth of a legume crop when ̂ part or all of the crop residue is returned to the soil. Some important facts concerning organic matter and nitrogen changes in soil have been discovered by research in different areas, but their application is not always clear. Chemical analyses of cultivated land indicate that the organic matter and nitrogen content of a soil is decreased as a result of tillage and the production of soil-depleting crops. Soil biologists have studied the rate of nitrate accumulation in the field and in the laboratory, and when the nitrate is not removed by growing crops it will accumulate more rapidly in good soils than in poor soils. The soil chemist has also discovered that the carbon-nitrogen ratio affects the quantity of available nitrogen in a soil which is released for plant use as a result of biological activity.