EFFECT OF ORGANIC AND INORGANIC FERTILIZERS ON THE PERFORMANCE OF WATERMELON (Citrulluslanatus Thunb.) IN COASTAL KENYA

Abstract

Watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai) is a high value vegetable crop that has gained importance in Kenya. Its production is constrained by low soil fertility, the use of blanket recommendations for inorganic fertilizers and pests and disease infestations leading to low yield and poor quality fruit. To address this problem, a study was conducted for two seasons at Kwale prison’s farm, Matuga Sub-county in Kwale County to determine the effects of organic (cattle manure) and inorganic fertilizers (Calcium ammonium nitrate and diamonium phosphate) on insect pest population, powdery mildew incidences, growth, yield and quality of watermelon. The specific objectives were: (1) to determine the effects of organic (cattle manure) and inorganic (Calcium ammonium nitrate and diamonium phosphate) on insect pest population and powdery mildew incidence in watermelon, (2) to determine the effects of organic (cattle manure) and inorganic fertilizers (Calcium ammonium nitrate and diamonium phosphate) on physiological characteristics of watermelon, (3) to determine the effects of cattle manure, nitrogen and phosphorous on growth, yield and quality of watermelon, and (4) to evaluate the profitability of using organic (cattle manure) Calcium ammonium nitrate and diamonium phosphate) in watermelon production. The experimental design was a 4 x 3 factorial arrangement of treatments embedded in randomized complete block design, with three replications. The two factors were: (1) Cattle manure (0, 2.5, 5.0 and 7.5 t ha-1), and (2) Inorganic fertilizers (three combinations of CAN and DAP) were 0 kg N ha-1 and 0 kg P2O5 ha-1 (N0P0), 50 kg N ha-1and 50 kg P2O5 ha-1 (N1P1), and 100 kg N ha-1 and 100 kg P2O5 ha-1 (N2P2). The parameters studied were days to flowering and maturity, vine length, fruit number, fruit weight, rind thickness, fruit firmness, chlorophyll content, stomatal conductance, powdery mildew incidence and insect pest population. Benefit cost ratio analysis was conducted to assess the profitability of using v vi both organic and inorganic fertilizers. Data was collected and subjected to Analysis of Variance (ANOVA) and means separated by Tukey’s honestly significant difference (HSD) at p ≤ 0.05. The results of this study showed that insect pest population and disease incidences increased with increasing fertilizer application. Chlorophyll concentration was also enhanced with increasing organic (cattle manure) and increasing level of inorganic (CAN and DAP) fertilizers dosage. Growth, yield and quality responses of watermelon to manure application, as well as the returns to investment in the watermelon enterprise, were dependent on the level of supplementation with inorganic fertilizers (CAN and DAP). Generally, the yield and quality of watermelon fruits improved with increasing level of supplementation of cattle manure with nitrogen and phosphorous. It can therefore be recommended that watermelon producers in coastal Kenya apply either use a high rate of manure (7.5 t ha-1), supplemented with the low rates of inorganic fertilizers (50 kg N ha-1 and 50 kg P2O5 ha-1) or the rate of manure (5 t ha-1), supplemented with the high rates of inorganic fertilizers (100 kg N ha-1 and100 kg P2O5 ha-