Soil nutrient status of smallholder farms in Kenya

Declining soil fertility remains a major cause of reduced crop production among smallholder farmers in sub-Saharan Africa. This is partly attributed to the blanket application of inappropriate fertilizers. Most fertilizer recommendations are based on soil analyses conducted in the 1990s, which have since become obsolete. In addition, the adoption of minimum tillage without considering the differential impacts of structural deformation has increased soil erosion, resulting in reduced nutrient levels as reflected by productivity indices (PI). To characterize the soil nutrient status of smallholder farms, six hundred soil samples were collected from thirteen counties in Kenya: Embu, Bungoma, Trans Nzoia, Kitui, Nandi, Machakos, Makueni, Tharaka Nithi, Nakuru, Kakamega, Kilifi, Kwale, and Taita Taveta. The samples were analyzed for soil pH, organic matter, and nutrient content. The results showed variation in the productivity index across the study regions. The Coastal region had a PI of 59.1%, while the Eastern and Western regions had PIs of 64.9% and 61.3%, respectively. The most limiting factors were soil pH, nitrogen, organic carbon, bulk density, and zinc. The mean nitrogen (N), phosphorus (P), and potassium (K) contents were 0.10%, 20.14 ppm, and 0.95%, respectively, in the Eastern region; 0.13%, 2.5 ppm, and 0.49%, respectively, in the Coastal region; and 0.20%, 11.20 ppm, and 0.61%, respectively, in the Western region. Nitrogen, phosphorus, and zinc were below the critical threshold levels in all three regions, whereas copper and potassium were within adequate ranges. Nitrogen showed a significant correlation (p < 0.001) with bulk density in the Eastern, Coastal, and Western regions, respectively, as follows: −0.1279x + 0.2753 (R² = 0.365), 0.1124x + 0.2462 (R² = 0.189), and −0.3292x + 0.5921 (R² = 0.462). These findings provide essential information for making informed and sustainable decisions regarding agricultural production in the study regions.