Abstract

This study examined yield trends and changes in soil chemical properties during a 26-year field experiment, involving a winter wheat–summer maize rotation on a loessial soil in China. Mean yields of wheat and maize increased significantly with increasing rates of manure (M) or inorganic nitrogen and phosphorus (NP) application. However, over time, the yield of both crops declined under the NP treatments, although the change was only significant for wheat. Positive yield responses for both crops were recorded under M+NP treatments. Soil organic matter and soil total N increased under all treatments. The Olsen P level dropped significantly in NP plots and increased markedly in manured plots. There was a reduction in soil exchangeable potassium under nearly all treatments, particularly on NP, where it dropped from an initial concentration of 245 mg kg^-1 to ca. 110 mg kg^-1. We conclude that annual inputs of NP fertilizers at rates of less than 240 kg ha^-1 N and 52 kg ha^-1 P were not sufficient to sustain crop production in the system investigated. The practice of applying organic manure as well as NP should be modified to facilitate continued crop production and to provide environmental protection.

Key words: Chemical fertilizer, organic manure, soil organic matter, Olsen P, soil exchangeable potassium.