Effects of mineral and organic fertilizers on crop productivity and nutrient use efficiency in smallholder farms of Southern Rwanda

Abstract

Smallholder farms in southern Rwanda are characterized by soil fertility depletion due to continuous cultivation without nutrient replacement, leading to poor crop productivity and threatening food security. Fertilizer application is therefore a pre-requisite to improve productivity in these soils. A study was conducted to determine the effects of combined mineral and organic fertilizers on maize (Zea mays L.) and beans (Phaseolus vulgaris L.) and nutrient use efficiency in farms selected from southern part of Rwanda. Simbi and Maraba were selected as representative sectors, and Umurera and Gasharu as representative villages in the two sectors, respectively. Farms were selected based on key socio-economic status (land size, livestock ownership, type of house, and labour availability). Two fields under either maize or bean crop were demarcated on each farm and one of them received fertilizer application comprising manure (1600 kg/ha) and mineral fertilizer (NPK 17-17-17) at a rate of 200 kg/ha. Maize plots were top-dressed with additional urea fertilizer at a rate of 50 kg/ha at flowering. Other two fields grown with the same crops without fertilizer application served as control treatment. In addition, a greenhouse experiment was run to assess crop response in semi-controlled environment. Soils were the most fertile in Simbi and in wealthier farms. Maize plants were significantly (P = 0.03) shorter in Simbi (1.7 m) than in Maraba (1.8 m). Grain yield was higher in Simbi (3.2 t ha^-1) than in Maraba (1.9 t ha^-1) and increased up to 3.4 t ha^-1 in fields with fertilizer application. Similar trend was observed in the greenhouse. In beans, fertilizer application significantly (P<0.001) increased the number of pods per plant, the number of grains per pod and bean yield of 1.4 t ha^-1. Agronomic efficiency (AE) was higher in Simbi than in Maraba and was the highest in wealthier farms. It was concluded that biophysical factors (field location and initial soil fertility status) greatly influenced crop yield and fertilizer.

Keywords: Bean, maize, fertilizer response, farm type, Rwanda