Characterization of Organic Soil Nitrogen in a Hutton Series Soil from the Kwazulu‐Natal Midlands

Abstract

The labile soil organic nitrogen pool and the effect of old kraal manure on the pools of labile organic N in a Hutton series soil from Cedara in the Midlands of KwaZulu‐Natal in South Africa were determined. Four levels of kraal manure (0, 5, 10, and 15 t ha−1 oven dry basis) were applied in the field in a randomized complete block design with three replicates. Total soil N, microbial biomass N, and anaerobic mineralizable N were determined on soil samples taken a month after manure application. All three pools of soil organic N showed significant (p<0.01) increases due to manure addition. However, microbial biomass only showed significant changes if the difference in the manure rate was >10 t ha−1. Considering the work involved in measuring each pool and the variability among replicates it was apparent that anaerobic mineralizable N was the best pool for monitoring small changes in soil fertility. In addition, approximately 13% of the total soil organic N in the soil studied was potentially available to plants. Between 12% and 15% of the N added in manure was potentially available to plants, indicating that organic N in the well‐decomposed manure was more or less similar to that in the soil. With different manure treatments, there was also a significant correlation between microbial biomass N, and anaerobic mineralizable N although the former was more variable than the latter.