Influence of Tillage Methods, Farmyard Manure and Potassium Rates on Soil Moisture and Relationship with Cassava Root Yields in Kagera, Tanzania

Field experimental trial was established in Biharamulo, Bukoba and Missenyi districts, Tanzania in two consecutive (2018/19 and 2019/20) cropping seasons, aimed at determining the effects of tillage methods, farmyard manure (FYM) and potassium rates on soil moisture conservation and the relationship between soil moisture and cassava root yield using the split-plot design with three replications. Treatments: Tillage methods (flat tillage, open ridging and tie ridging) as the main plots and; the fertilizer rates [farmyard manure alone at 4 MT ha-1 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 at 4 or 8 MT ha-1 + potassium at 40, 80 or 120 kg K ha-1] and the control as the sub-plots were arranged in Randomized Complete Block Designed (RCBD). Farmyard manure was applied at planting and inorganic fertilizers were applied in two splits. Soil moisture was monitored starting from one week after the last rain event at an interval of two weeks up to the first rain event of the following season. Soil moisture samples (0 - 30 cm) were collected from the rows and ridges using a soil auger, placed in cores, weighed, oven-dried at 105 oC and reweighed after 24 hours to constant weight. The soil moisture content (%) in each soil sample was determined gravimetrically. Cassava root yields were recorded from each treatment during harvesting. The results indicated that the use of ridges conserved significantly (P < .001) more soil moisture (24 - 58%) than the use of flat tillage (7 - 27%) in both cropping seasons. The use of tie ridges conserved significantly (P ≤ .01) more soil moisture (11 - 58%) than the use of open ridges (10 - 57%) in the medium and high rainfall areas as opposed to the low rainfall area, which both conserved similar soil moisture. There was a significant (P < .001) difference in soil moisture conservation between the control and fertilizer types and rates. The use of FYM conserved significantly (P < .001) more soil moistures (16.54 - 63.48%) than the combined use of inorganic N, P and K fertilizer rates (12.27 - 53.60%). The use of FYM at 8 MT ha-1 conserved significantly (P < .001) more soil moisture (19.94 - 62.16%) than FYM at 4 MT ha-1 (16.80 - 58.33%). However, there was no significant (P = .33) different in soil moisture conservation between the combined use of inorganic N, P and K fertilizers and; the control. Results from regression analysis indicated significant (P < .001) association between soil moisture conservation and changes in cassava root yields. Thus, planting cassava on ridges, concurrently with the application of FYM at 8 MT ha-1 is recommended for adequate soil moisture conservation and increasing cassava root yield.