Effect of organic and urea nitrogen fertilisers on soil properties and maize yield in highlands of Rwanda

Continuous cropping without nutrient restitution under smallholder farms in Rwanda has led to serious nutrient depletion and reduced maize (Zea mays) yields. This is further exacerbated by low soil organic matter and soil erosion. The smallholder farmers in Rwanda either do not obtain the necessary returns from fertiliser use to justify the costs or cannot afford to use inorganic fertilisers resulting in low inorganic fertiliser use. It is hypothesised that bio-slurry organic fertilisers are affordable, are of high quality and can be used to increase fertiliser use efficiency and crop yields for farming community in Rwanda. The objectives of this study were to determine the nutrient contents of bio-slurry as an organic fertiliser, to determine its effect on soil properties and to compare its effect and inorganic fertilisers on maize yield. A field study was carried out in Musanze district to test four levels of N fertilisers (0, 50, 100, 150 kg N ha-1) and three levels of bio-slurry (0, 10, 20 m3 ha-1) and their interactions. This constituted 12 treatment combinations laid out as a factorial experiment and arranged as a randomised complete block design with 3 replications using maize as the test crop. The experiment was established during the long rain season from September 2011 to March 2012. Slurry chemical analysis was done before planting to determine its nutrient content. Soil samples were taken before planting, at seedling, at tasseling and after harvesting to determine the following properties: organic carbon, total N, available P, soil pH, cation exchange capacity (CEC), soil texture and aggregate stability. The maize grain yield was determined at harvesting stage. Tukey’s honest significance test was used to separate the means. Analysis of variance of maize grain yield showed significant differences (P<0.05) due to different treatments. Soil total nitrogen in the soil at seedling and tasseling was almost at similar level in all treatments. The differences were observed after harvesting. The highest amount of soil total nitrogen of 0.34% was observed in the urea 100 kg ha-1 x bioslurry 20 m3 ha-1 treatment and the lowest of 0.27% was observed under the control. The highest grain yield of 8022 kg ha-1 was observed at 100 kg ha-1 x bioslurry 10 m3 ha-1 treatment and the least was 5002 kg ha-1 of the control and 4997 kg ha-1 of urea 100 kg ha-1 x bioslurry 20 m3 ha-1 treatment. Based on the results of this study, bioslurry is a good organic residue for soil nutrients management. The nutrient contribution of bioslurry was evidenced by increased maize yield and a slight change in soil physical and chemical properties. Resource poor farmers who cannot afford fertilizers may be encouraged to use bioslurry to improve the soil nutrient status. It is expected that the results obtained from this study should be used by extension staff for upscaling and farmers for increased production of maize to reduce food insecurity in Rwanda.