The decomposition of organic matter in soils at different initial pH

Four series of experiments were conducted on the decomposition of green crotalaria and green natal grass harvested at two different stages of growth in soils of varying degrees of acidity from pH 3.71 to pH 7.05. The results obtained may be summarized briefly as follows: 1. All plant materials decomposed more rapidly and more completely in soils at pH 5.94 to 7.05 than at pH 3.71 to 4.59. 2. The detrimental effect of acidity on decomposition was more pronounced in soils treated with natal grass than in soils treated with crotalaria. 3. The acidity of the soils treated with crotalaria was decreased markedly in the soils at low pH in the beginning of the experiment. This decrease in acidity of the soil was possibly caused by the liberation of ammonia or the dilution effect of carbonic acid or both. It can hardly be claimed that the full effect of the acidity of these soils on the decomposition of crotalaria was measured in these experiments. 4. The addition of nitrogen as potassium nitrate to the natal grass-treated soils in an amount sufficient to bring the total nitrogen content of these soils to that of the crotalaria-treated soils did not increase the decomposition of the natal grass. 5. The decreased nitrification in the soils at pH 3.71, 4.26, and 4.59 was undoubtedly brought about by the micro-organisms utilizing the ammonia in the decomposition of the carbonaceous materials and also partly by the detrimental effect of the increased acidity on the nitrifying bacteria. A deficiency of available calcium in the strongly acid soils could possibly have caused this decreased nitrification observed in the soils at low pH. 6. The plant materials containing the larger percentages of the hemicellulose, cellulose, and lignin fractions decomposed more slowly than those containing the smaller percentages of these constituents.