Planting Pits’ Effects on Soil Nutrients in a Sorghum and Pigeon Pea Rotation in Semi-arid Areas of Eastern Kenya

Planting pits are rain water harvesting structures that trap water and nutrients in surface runoff and rain water falling directly into the pits. Planting pits have been promoted for improving crop yields without considering the nutrient dynamics. To contribute to this knowledge, a study was conducted to determine the soil nutrient content after four seasons of growing sorghum and pigeon pea in rotation in “Chololo” and “Five by Nine” pits. Two planting pits; “Five by Nine” and “Chololo” with a control without water harvesting replicated three times were arranged in a randomised complete block design. The study was done for four seasons in Embu and Tharaka-Nithi counties in semiarid Eastern Kenya. Soil pH, total organic carbon, total nitrogen, available phosphorus, exchangeable potassium, calcium, magnesium and sodium were determined. “Chololo” pits significantly increased total organic carbon by 0.06 mg kg-1 and total nitrogen by 0.4 mg kg-1 relative to without water harvesting in Machang’a. The potassium content significantly increased by 0.4 cmolc kg-1 and 0.54 cmolc kg-1 in “Five by Nine” and “Chololo” pits in Machang’a relative to without water harvesting. There was an insignificant effect on soil phosphorus, calcium, magnesium and sodium. After four seasons of planting pits, total nitrogen, potassium and calcium increased in both soils and phosphorus in Machang’a relative to the beginning of the study. Total organic carbon significantly decreased in “Chololo” pits and without water harvesting in Machang’a. Phosphorus significantly decreased in Nkarini whereas magnesium and pH decreased in both soils. Nutrients in “Five by Nine” and “Chololo” pits depended on the soils and crops grown and should thus be promoted together with periodic soil testing.