Suitability of organic amendments in production of Brassica oleracea in western Kenya

Challenges in management of soil fertility in small-holder farming systems in Kenya emanate from continuous nutrient mining with low replenishment. The situation is worse for vegetables production, since available fertilizer is mostly applied to major staple or cash crops. Organic amendments could alleviate the problem, but their appropriate use is hindered by varying soil conditions within a region. Farmers are mostly undecided on the use for which crop in their respective soil types. To determine the response of Brassica oleracea (kale), a common vegetable in Kenya, to organic amendments in different soils, an experiment was conducted in Kakamega County. The experiment was laid out in a randomized complete block design with the soil type (Nitisol, Alfisol and Ultisol) as the blocks, organic amendments (farmyard manure (FYM), biogas sludge and tithonia (Tithonia diversifolia) leaves and a control (no amendment) as treatments. All treatments were replicated four times. Kale biomass and nutrient uptake (nitrogen and phosphorus) were determined for three months. The results showed that the kale biomass was higher in FYM treatment than the control across all soil types (p≤0.05). In the Alfisol, all treatments were significantly higher than the control but no significant difference was observed among them. In the Nitisol, both FYM and biogas sludge treatments had significantly higher biomass. In the Ultisol, only FYM was significantly higher (p<0.001). All amendments showed significantly higher nitrogen and phosphorus uptake than the control in the Alfisol. In the Nitisol, biogas sludge and FYM showed significant phosphorous uptake. In the Ultisol, FYM showed significantly higher phosphorous uptake while nitrogen uptake in all treatments was significantly higher than the control. These results indicate that the suitability of organic amendments for kale production is dependent on the soil type. FYM can be recommended for all small scale kale growers as it showed best results.