Tithonia diversifolia as a green manure for soil fertility improvement in western Kenya: A review

Tithonia diversifolia, a shrub in the family Asteraceae, is widely distribu ted along farm boundaries in the humid and subhumid tropics of Africa. Gree n biomass of tithonia has been recognized as an effective source of nutrien ts for lowland rice (Oryza sativa) in Asia and more recently for maize (Zea mays) and vegetables in eastern and southern Africa. This paper reviews th e potential of tithonia green biomass for soil fertility improvement based on recent research in western Kenya. Green leaf biomass of tithonia is high in nutrients, averaging about 3.5% N, 0.37% P and 4.1% K on a dry matter b asis. Boundary hedges of sole tithonia can produce about 1 kg biomass (tend er stems + leaves) m(-1) yr(-1) on a dry weight basis. Tithonia biomass dec omposes rapidly after application to soil, and incorporated biomass can be an effective source of N, P and K for crops. In some cases, maize yields we re even higher with incorporation of tithonia biomass than with commercial mineral fertilizer at equivalent rates of N, P and K. In addition to provid ing nutrients, tithonia incorporated at 5 t dry matter ha(-1) can reduce P sorption and increase soil microbial biomass. Because of high labor require ments for cutting and carrying the biomass to fields, the use of tithonia b iomass as a nutrient source is more profitable with high-value crops such a s vegetables than with relatively low-valued maize. The transfer of tithoni a biomass to fields constitutes the redistribution of nutrients within the landscape rather than a net input of nutrients. External inputs of nutrient s would eventually be required to sustain production of tithonia when bioma ss is continually cut and transferred to agricultural land.