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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.

¹ Paper No. 930 University of California Citrus Experiment Station, Riverside, Calif.

² Assistant Chemist, Senior Laboratory Technician, and Assistant Chemist, respectively. The authors wish to acknowledge the assistance of members of the Department of Horticulture who directed the field trial from which soil samples were taken.

Received for publication May 31, 1956. Accepted for publication August 27, 1956.