Effect of Tillage Methods, Farmyard Manure and Potassium Rates on Some Soil Chemical Properties and Nutrient Contents in Cassava in Kagera, Tanzania

A study was conducted in Bukoba, Missenyi and Biharamulo districts, Tanzania for the objective of determining the effect of tillage methods, farmyard manure (FYM) and potassium rates on soil pH and the concentrations of N, P and K in the soils and cassava leaves. The treatments were arranged in the Randomized Complete Block Design (RCBD) using the split-plot design with three replications. Tillage methods (flat tillage, open ridging and tied ridging), were the main plots, and the fertilizer rates [farmyard manure (FYM) alone at 4 MT ha-1 alone or FYM alone at 8 MT ha-1, nitrogen (40 kg N ha-1) + phosphorus (30 kg P ha-1) + potassium at 40, 80 or 120 kg K ha-1 and the combination of FYM alone at 4 MT ha-1 or FYM alone 8 MT ha-1 + potassium at 40, 80 or 120 kg K ha-1] and the control, were the sub-plots. The inherent and post-harvest composite soil samples for determining soil pH and the concentration of N, P and K in each site were collected, processed and analysed. Post-harvest soil samples and cassava leaf samples were collected from the control plots and plots that received the combination of FYM at 8 MT ha-1 and potassium at 40 or 120 kg K ha-1 during the second cropping season. The leaf samples were oven dried at 70oC, grounded to pass through 0.5 mm sieve and analyzed. The results indicate that there was no a significant (P = .05) difference in the soil pH and the concentrations of N, P and K in the soils and cassava leaves among the tillage methods. There was a significant (P < .001) difference in the soil pH and the concentrations of N, P and K in the soils and cassava leaves between the control and the combined use of FYM and potassium rates. However, there was no a significant (P =.05) difference in the concentrations of N, P and K in the soils among the combined use of FYM and potassium rates, but there was a significant (P < .001) difference in the concentrations of N and K in cassava leaves among the combined use of FYM and potassium rates. The combined use of FYM at 8 MT ha-1 and potassium at 40 kg K ha-1, and the combined use of FYM at 8 MT ha-1 and potassium at 120 kg K ha-1 significantly (P < .01) increased the soil pH and the concentration of N and K in the soil. However, the combined use of farmyard manure at 8 MT ha-1 and potassium at 120 kg K ha-1 gave significantly (P < .001) higher concentrations of N and K in cassava leaves than the combined use of farmyard manure at 8 MT ha-1 and potassium at 40 kg K ha-1. Therefore, combined use of farmyard manure at 8 MT ha-1 and potassium at 120 kg K ha-1 is desirable for increasing the concentration of N and K in the soil and in cassava leaves. However, for the resource-poor farmers who cannot afford the high rate of K, the combined use of farmyard manure at 8 MT ha-1 and potassium at 40 kg K ha-1 could be used.