Effect of integrated cattle manure and phosphatic fertilizer application on orange fleshed sweet potato yield and quality

Abstract

Sweet potato (Ipomoea batatas L.) is a highly nutritious and a resilient crop with significant potential to enhance food security and reduce poverty in Kenya. However, its production has declined in recent years due to poor agronomic practices, particularly inadequate soil fertility management. Phosphorus deficiency remains a major constraint, as most farmers rarely apply phosphatic fertilizers, and research on optimal combinations of fertilizer and manure application to maximize yield and quality is limited. The objective of this study was to evaluate the effects of combined application of different levels of cattle manure and phosphorus (P) fertilizer on the yield and quality of orange-fleshed sweet potato, variety Vita. The study was conducted at two sites, Wambugu Agricultural Training Farm in Nyeri County and Kibirigwi Irrigation Scheme in Kirinyaga County. A 3 × 3 factorial experiment laid out in a Randomized Complete Block Design with three replications was used. The treatments were; cattle manure at 0 t/ha, 1.5 t/ha, and 2.5 t/ha, and phosphorus-based inorganic fertilizer (TSP) at 0 kg/ha, 50 kg/ha, and 75 kg/ha. Treatments were applied at planting. Data were collected on vine length, number of branches per plant, number of leaves, days to 70% maturity, tuber diameter, length, weight, and proximate analyses for quality determinants including dry matter content, vitamin A, and ash content. The results revealed significant differences (p < 0.05) in vine length, vine girth, number of branches, and leaf area index across treatments. Growth parameters improved progressively with increasing manure and TSP application, with the combination of 2.5 t/ha manure + 75 kg/ha TSP (T3S3) consistently producing the most vigorous plants. Vine girth under T3S3 reached 4.823 cm by 112 DAS, significantly higher than 2.507 cm under the control (T1S1). In terms of yield performance, root yields ranged from 1.07–1.09 t/ha in the control to 3.00–3.10 t/ha under T3S3, representing nearly a threefold increase. Intermediate treatments, such as 1.5 t/ha manure + 50 kg/ha TSP, also showed substantial yield improvement, underscoring the synergistic effect between organic and inorganic nutrient sources. Across sites, yields were consistently higher under integrated treatments, with greater responsiveness observed at Kibirigwi (nutrient-deficient soils) compared to Wambugu (moderately fertile soils). Nutritional analysis indicated that integrated treatments enhanced root quality, with significant increases in β-carotene, vitamin C, and crude protein compared to the control. The highest β-carotene content (10.03 mg/100 g) was recorded under T3S3, compared to 3.47 mg/100 g under the control. Similarly, T3S3 produced the highest dry matter and vitamin C concentrations, demonstrating that nutrient supplementation not only enhances yield but also improves the nutritional value of OFSP, thereby contributing to food and nutrition security. These findings confirm that integrated nutrient management enhances both agronomic and economic outcomes. Integrated application of cattle manure and TSP significantly improved sweet potato growth, yield and nutritional quality. The combination of 2.5 t/ha manure and 75 kg/ha TSP (T3S3) proved most effective across both sites, validating the superiority of integrated over sole nutrient applications. Farmers are encouraged to adopt integrated use of cattle manure and TSP for sustainable sweet potato production. Extension programs should promote awareness of the agronomic, nutritional, and economic benefits of integrated nutrient management. Future studies should assess long-term impacts on soil health, as well as adaptability of the approach across diverse agro-ecological zones to refine site-specific recommendations