Between and Within-Farm Variability in Soil Fertility Management and Status in the Central Highlands of Kenya

The processes of nutrient depletion and soil degradation within smallholder farms of central Kenya are spatially heterogeneous, determined by both biophysical and socio-economic factors. A monitoring study involving nutrient stocks, flows and balances was conducted in central Kenya to explore between and within-farm variability in soil fertility management and identify spatial niches for targeting soil fertility management strategies. Focus group discussions were conducted and farms grouped into 3 farm types （rich, medium and poor）. Nine case-study farms - three from each of the farm types - were randomly selected from the 50 farms studied, for detailed resource flow mapping. The farms were visited to record movement of nutrients inputs using a monitoring protocol covering soil, crops, livestock, and socio-economic aspects of the farm. Soil in different plots were sampled at a depth of 0-20 cm and analyzed for texture, pH, C, N, available P, exchangeable K＋, Mg2＋ and Ca2＋. Results revealed that wealthy farmers added an average of 51.3 kg/ha N, 37 kg/ha P, and 244 kg/ha K, compared to 25.9 kg/ha N, 14.5 kg/ha P and 50.7 kg/ha K for the poor farmers. In all farm types, home fields received more nutrient inputs compared to the outfields. Consequently, maize grain yields, partial nutrient balances and soil nutrient stocks were significantly higher in wealthy farms and home fields compared to poor farms and outfields, respectively. These results imply that different soil management strategies are required to achieve similar yields on the different field and farm types and avert soil degradation.

Effects of Plant Age and Rock Phosphate on Quality and Nutrient Release of Legume Residue

An incubation experiment was carried out on plateau and slope fields to investigate the effect of plant age and rock phosphate （RP） on the organic resource （OR） quality and available N and P release of the legume residues, including standing biomass and surface litter. The legumes, Mueuna pruriens （L.） and Lablab purpureus （L.）, were treated with or without Togo rock phosphate （RP） and were sampled at 12, 18, 24 and 30 weeks after planting. Results showed that the application of RP significantly affected the P content of the legume residues on the plateau field for the first 18 weeks, but not the other OR quality parameters, nor their N mineralization, or P release parameters. Although application of RP led to higher P contents in both legumes on the plateau field, the P contents were still far below those observed on the slope field. For both species, the biomass age appeared to have a major impact on their N, P, and polyphenol contents, but not on the liguin content. At 24 weeks, both legume N and P contents dropped to about half their values at 12 weeks of age. Residue age also significantly affected N mineralization both with and without RP addition and the net Olsen-P with RP addition. The younger residues generally led to higher N mineralization and net Olsen-P content than the older residues. The best immediate responses to herbaceous legume addition were expected from younger materials, but often at the cost of the total biomass produced and the possibility to produce seeds. The production of seeds, however, could be potentially implemented on a small area of legumes, thus invariably allowing for maturity and seed production.