Simulation of Soil Moisture, Sorghum (Sorghum bicolor L.) and Sweet potato (Ipomea batatas L.lam) Yields using CropSyst Model in Matuu Sub-County, Kenya

Abstract

There has been declining crop yields in the ASALs of Kenya due to low soil fertility caused by continuous cropping without addition of external inputs and low soil water availability due to low and unreliable rainfall and poor water harvesting techniques. To increase crop yields, research on better use of available rainfall and interaction between climate, soil and management on crop production is required. CropSyst model was used to study the effect of soils, and management on cropping systems productivity. The aim of the study was to simulate soil moisture, sorghum and sweet potato yields under different tillage practices, cropping systems and organic inputs. The study was conducted in Matuu Sub-county in Kenya. The experiment was laid out in a Randomized Complete Block design with split-split plot arrangement and replicated three times. The main plots were tillage practices; furrows and ridges, oxen plough and tied ridges. Split plots were cropping systems; monocropping, intercropping and crop rotation. Split- split plots were organic inputs; farm yard manure (FYM), rock phosphate (RP) and FYM + RP. A control denoted that no organic input was used. In each plot, soil sampling was done to determine physical and chemical soil characteristics. The test crops were sorghum (Sorghum bicolor L.) and sweet potato (Ipomea batatas L.lam) rotated and/or intercropped with dolichos (Lablab purpureus) and chickpea (Cicer arietinum). The CropSyst model was calibrated using the measured soil texture, permanent wilting point, field capacity, bulk density and initial soil moisture at the experimental site. Sorghum and sweet potato growing degree days were used to calibrate the crop phenology in the crop file. Validation of the model was done using Root Mean Square Error (RMSE), percentage differences (PD) and wilmott index (WI) of agreement. CropSyst model was validated for soil moisture due to the low RMSE (0.5 to 1.3) and PD (less than ±15) values that were obtained and the WI index which was close to 1. CropSyst model Validation for the sorghum and sweet potato yield was achieved due to the low RMSE (0.629) and PD (less than ±3) values that were obtained and the WI index which was close to 1. In the sorghum based cropping systems, simulated soil moisture (101.91 mm) was significantly (P < 0.05) high in the interactions between tied ridges with sorghum/dolichos intercrop when Rock Phosphate(RP) + Farm Yard Manure (FYM) was applied and least (13.52 mm) in the interactions between oxen plough with sorghum mono cropping when no organic input was applied in the first season. In the second season, simulated soil moisture was significantly high (108.3 mm) in the tied ridges with sorghum/dolichos intercropping when FYM + RP was applied and least (15.4 mm) in the interactions between the oxen plough with sorghum monocropping when no organic input was applied. In the sweet potato based cropping system during the first season, soil moisture was significantly high in tied ridges (95 mm), sweet potato- dolichos rotation (75.32 mm), RP and FYM application (75.03 mm) and least in the oxen plough (32.49 mm), sweet potato monocropping (53.46) and control (52.52 mm). In the second season, soil moisture was significantly high in the tied ridges (100.24 mm) sweet potato-dolichos rotation (79.63 mm), RP + FYM (79.39 mm) and least in the oxen plough (34.36 mm), sweet potato monocropping (55.26 mm) and control (55.39 mm).