Low soil nitrogen and phosphorous levels are among several constraints limiting crop productivity in southwestern Ethiopia. The dominant soil types in the region are acidic nitosols that are often low in plant available phosphorus. Most farmers cultivate maize with minimal external inputs, hence resulting in sub-optimal yield levels. The effect of applying tithonia biomass and phosphorus fertilizer on agronomic efficiency of phosphorus, growth characteristics and yield of maize was investigated in a randomized complete block design with three replications. Tithonia (Tithonia diversifolia) biomass and Triple Super Phosphate (TSP) were used as organic and inorganic sources of phosphorus respectively. Significant treatment differences (P < 0.01) were observed for most of the parameters studied including agronomic phosphorus use efficiency and grain yield. Agronomic phosphorus use efficiency increased from 45 at the sole TSP to more than 125 when combined with tithonia biomass. The sole application of tithonia biomass resulted in 127% yield increment over the control, and combined application of 50% of the maximum rate of tithonia and 50% of the recommended rate of fertilizer resulted in grain yield increment of 88% over the control. Given the very high amount of tithonia biomass required to supply the recommended rate of phosphorus, the combined rate of tithonia biomass and inorganic phosphorus fertilizer (at 50% of the maximum rates), appears to be cheaper and appealing to smallholder maize producers in the region. The result therefore underscored the need to allot more resources and attention in exploring locally available and cheap sources of plant nutrients which could augment crop productivity in face of the mounting input prices which are prohibitively high for smallholder maize producers of the region.

Keywords: Agronomic efficiency; Biomass; Maize; tithonia diversifolia; TSP