Changes in Organic Carbon and Nitrogen in an Irrigated Soil during 28 Years of Differential Fertilization1

Abstract

A study was made of the changes in organic C and N and in the ratio of C:N in the soil of a long-term fertility trial with citrus at the University of California Citrus Experiment Station, Riverside, during 28 years of irrigation and differential fertilization, from 1927 to 1955. All changes in organic C and N and ratio of C:N in the soil were in the surface 0 to 6 inches. Check plots that received no fertilizers or organic materials contained about the same quantities of organic C and N in 1955 as in 1927. Treatment with urea and triple superphosphate did not increase the organic C and N in the soil. The (NH4)2SO4 treatment increased the organic C and N, probably as a result of extreme acidification of the soil. The Ca(NO3)2 treatment increased the organic C but not the organic N. Cover crops increased the organic C and N a small but statistically significant amount. Mineral N had no effect on the increases in C and N from the cover crops or from any of the added organic materials. There were accumulations of KCl-extractable and fixed NH4 in the soil of acidified plots but not in soil of pH near neutrality. The accumulation of organic C and N in the soil increased with increase in the rate of application of manure, the highest rate of application approximately tripling the organic C and N. The accumulations of C and N resulting from applications of alfalfa hay, bean straw, or cereal straw were not significantly higher statistically than those from the cover-crop treatment, and were much lower than those resulting from the application of equivalent weights of organic materials as manure.