Influence of planted fallows and manure application on soil quality and maize yields on a Colombian volcanic ash soil

Due to increasing land-use pressure, Colombian volcanic-ash soils have been subjected to short-duration natural fallows, leading to severe degradation. The influence of planted fallows, with and without chicken manure application, on soil nitrogen availability, phosphorus and organic matter fractions, and on grain yield of maize was investigated on two on-farm experimental sites (BM1 and BM2) with the same soil type and recent cropping history. The soil at the two sites contained 33% sand and 39% clay, respectively, with a bulk density of 0.8 Mg m(-3). The soils had pH (H2O) of 5.1, organic carbon content of 50 g kg(-1), and exchangeable Al of 1.1 cmol kg(-1). The effect of chicken manure varied in the two sites. In BM1, Tithonia divers folia (with chicken manure), had a positive impact compared with the other fallow treatments. In BM2, Indigofera zollingeriana (with chicken manure), on average, performed better. Simple correlations between nitrogen (N) and phosphorus (P) contents in LL and LM soil organic matter (SOM) fractions, total N, NO3--N, NH4+-N, soil mineral N, potential nitrogen mineralization, NaHCO3 extractable Pi, NaHCO3 extractable Po, NaOH extractable Pi, NaOH extractable Po, and maize grain yield, were not significant. Higher values of maize grain yield ranged between 1.39-2.07 Mg ha(-1) (-CM, +CM) in T. diversifolia treatment, while lower values ranged between 0.64-1.49 Mg ha(-1) in rotation treatment (maize and bean). Multiple regression analyses to evaluate the additive effect of several measured soil and SOM parameters in explaining the variance in maize grain yield were not significant. Our study indicates that both P and N were probably immobilized. Thus, there is a need for mineral P fertilizer to be used in conjunction with planted fallows so as to avail more P to the growing crops, especially in these severely P-deficient soils.