Combined Effect of Organic and Inorganic Fertilizers on Soil Chemical and Biological Properties and Maize Yield in Rubona, Southern Rwanda

Effects of Calliandra calothyrsus Meissner, Tithonia diversifolia Hensley A.Gray and Tephrosia vogelii Hook.f green manure applied independently or combined with triple superphosphate (TSP) on soil chemical and biological properties that influence maize yield were evaluated on an Ultisol of Rubona, Rwanda. Treatments compared in randomised complete block design were the control, limestone at 2.5 t ha–1, TSP at 25 and 50 kg P ha–1, leaf of Calliandra, Tithonia and Tephrosia applied independently at 25 and 50 kg P ha–1 and each combined with TSP at equivalent rates of 25 and 50 kg P ha–1, respectively. Lime led to significant increases in soil pH followed by Tithonia combined with TSP at a rate of 50 kg P ha–1. All treatments significantly reduced exchangeable acidity and aluminium compared to the control. A combination of organic materials with TSP at a rate of 50 kg P ha–1 improved soil organic carbon, microbial biomass carbon (MBC) and phosphorus (MBP). Compared to TSP applied alone, only the combination of Tithonia with TSP increased labile inorganic P fractions by 1.6–52.2%. In the fourth season, application of Tithonia green manure combined with TSP at a rate of 50 kg P ha–1 resulted in higher maize yield (25% increase) than TSP and Tithonia (9% increase) applied alone. Plant quality residues, labile inorganic P fractions, soil organic carbon, MBC and MBP values were correlated with maize yield, confirming the crucial role of plant residues quality and the soil properties in improving maize yield.