Effect of green manure application on cassava (Manihot esculenta Crantz) growth, yield quantity and quality in degraded Alfisols

The use of green manures as alternatives to minimize the usage of chemical fertilizers that are very expensive and also environmentally unfriendly is considered as a good agronomic practice. However, the effect of each green manure on soil properties and crop yield depends on its chemical composition. Hence field experiments were conducted in sandy soil during 2015/2016 and 2016/2017 to compare impacts of green manures (GM) and N15:P15:K15 fertilizer on soil properties, growth, root yield, mineral, starch and hydrocyanic acid (HCN) contents of cassava. The GM from leaves of: Neem (Azadirachta indica A. Juss.), Moringa (Moringa oleifera Lam.), Gliricidia (Gliricidia sepium (Jacq.) Kunth ex Walp.) and Leucaena (Leucaena leucocephala (Lam.) de Wit) were applied at 5 t ha-1, and the NPK fertilizer was applied at 400 kg ha-1. There was a no fertilizer (control). Application of GMs reduced soil bulk density and increased soil organic matter (OM), N, P, K, Ca, Mg, growth and fresh root yield of cassava compared with the control. The NPK fertilizer had no effect on soil bulk density and soil OM, but increased soil N, P, K, Ca, Mg, growth and fresh root yield of cassava compared with the control. Gliricidia increased growth and fresh root yield of cassava compared with other GMs and NPK fertilizer. In the first year (2015/2016) and compared with control; Gliricidia, Moringa, Leucaena, Neem and NPK fertilizer increased fresh root yield of cassava by 53, 33, 30, 29 and 28%, respectively. In the second year (2016/2017), these treatments increased fresh root yield of cassava by 85, 65, 61, 60 and 36%, respectively. The GMs and NPK fertilizer increased mineral and starch contents and reduced HCN content in the cassava tuber roots compared with the control. Use of GM has potential to improve soil properties, and growth, fresh root yield and nutritional contents of cassava than does NPK fertilizer. The Gliricidia treatment best improved soil properties and cassava productivity as indicated by the benefit:cost ratio. For those growing cassava for fresh root quantity Gliricidia is recommended as green manure. Moringa is recommended as green manure for those that desire fresh root quality.