Studies in soil fertility with special reference to organic manures. I. The field experiments.

Abstract

The results for five field experiments with organic manures on three related soil types of the grey and brown soils group are reported. Four experiments were with irrigated summer crops and the fifth was a spring crop with supplemental irrigation. Pea trash and rice hulls were the main organic manures used, but chaffed lucerne and rice straw were also used in one of the experiments. Seedling emergence was delayed and reduced by rice straw when only one month was allowed for decomposition, but these effects were absent following three months of decomposition. Rice straw and rice hulls both induced more or less transitory symptoms of nitrogen deficiency in the absence of added nitrogen. Except in one experiment with pea trash on a near-virgin soil, pea trash, chaffed lucerne, and rice hulls all gave highly significant yield increases. Rice straw also gave yield increases when three months were allowed for decomposition. From an examination of the patterns of response in these experiments, together with data for soil water, nitrate nitrogen, and available phosphorus, it is concluded that phosphorus nutrition may be a dominant factor in yield determination, especially with rice hulls, and that nitrogen nutrition exerts a strong modifying influence in some circumstances. Hypotheses concerning the mineralization and biological fixation of nitrogen and phosphorus are discussed, but seem inapplicable to the specific conditions of these experiments. The origin of the additional phosphorus made available in the soil by organic manures with low content of that element emerges as a significant problem calling for solution. Current hypotheses on this point are considered, but it is contended that far more information than is usually obtainable from field experiments would be required to decide which of a number of possible mechanisms plays a significant role in the release or maintenance of this phosphorus.