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Abstract
Soil nutrient depletion is one of the most serious threats to sustainable crop production in African agroecosystems where policy and economic factors have favoured extensification rather than intensification of agriculture. Initially, we assessed the relationship of household land use and socio economic factors on the soil fertility of over 2000 fields on 236 smallholder farms in the eastern and western highlands of Kenya. In the western highlands, soil organic matter was higher in pastures than other land uses, while the opposite was the case in the eastern highlands. Number of cattle per farm was not related to soil fertility in either region.
In the highlands of western Kenya, maize productivity across a cultivation chronosequence, which ranged from recent conversion from primary forest to 100 years in continuous cropping, declined rapidly as cultivation age increased from 3 to 25 years, and then gradually declined to a yield of 1.6 Mg ha -1 season-1. Long rainy season grain yield in the oldest conversions was only 24% of the young conversion grain yield (6.4 Mg ha -1). Soil pools of mineral nitrogen (N), strongly bound phosphorus (P) and plant available P decreased by 82, 31 and 36% and P adsorption capacity increased by 51% after 100 years of continuous cultivation. There was a greater yield increment response to urea N and triple super phosphate when applied together (ranging from 1 to 3.8 Mg ha-1 season-1) than when either fertilizer was applied alone, with the greatest responses on the oldest conversions. Tithonia diversifolia leaves or cow manure were applied at an equivalent rate of 25 kg P ha-1. With manure, additional N was needed to sustain yield as time under cultivation increased, but not with T. diversifolia.
Application of N and P together resulted in higher benefit to cost ratios (BCRs) on all conversions, with the greatest BCRs on the oldest conversions, suggesting that soil nutrient replenishment strategies should include both nutrients and consider time under cultivation. Despite BCR's >1 and positive net benefits of N and P fertilizer application, efforts to overcome nutrient depletion in these systems may be hampered by the high cost of fertilizer relative to the average income of farmers in this region.
