Effects of Acidulated Rock Phosphate on Growth and Yield Performance of Selected Leafy Vegetables in Kiambu County
Githua, Fidelis W.
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African leafy vegetables are a good source of vitamin A, vitamin C, and foliate among others. They are also a complementing source of other vitamins such as thiamine, niacin and riboflavin, plus some dietary minerals including calcium, iron, potassium, zinc, copper and manganese. Decline in crop yields is mainly caused by loss of soil fertility. Phosphorus (P) is one of the critical elements that limit vegetable production. The situation is aggravated in smallholder agriculture where use of mineral fertilizers is limited or even non-existent, as peasant farmers, due to their low purchasing capacities, cannot afford high costs of these fertilizers. Rock phosphate (PR) provides an alternative to the expensive soluble P. Unfortunately, use of Rock phosphate (PR) to alleviate P deficiency in the soils remains a great challenge due to their low solubility. The current study was therefore conducted to assess growth, yield and quality responses of kales, cowpeas and amaranth to partially solubilised rock phosphate; a cheaper phosphorous source. Laboratory experiments were conducted to investigate phosphate dissolution ability from rock phosphate (RP) in soil as a function of incubation period, through application of organic acids, phosphoric acid and elemental sulphur at different incubation periods (0, 30, 60, and 90 days). Two grams of elemental sulphur and some organic acid solutions containing 0.4 mmolL-1 from each of Oxalic acid and phosphoric acid. The field experiment included four treatments: Triple Superphosphate (TSP); Rock phosphate plus Sulphur; Sole Rock Phosphate; and Control under three vegetable crops: Amaranth; Cowpea and Kales. The experiment was set up in a split plot design, where the 3 vegetables constituted the main plots while the rock phosphate, partially acidulated rock phosphate (RP+sulphur), triple super phosphate and control were sub-plots and replicated three times. Data were collected on the growth parameters and fresh weights of edible yield per plant. The soil pH and phosphorus were analysed before and after growing seasons as well as the phosphorus contents in plant tissues. All the growth and biochemical data was subjected to analysis of variance (ANOVA) using SAS. Mean separation was done using LSD at 5% probability level. The fresh root weight, leaf area, root dry weight, root length, shoot dry weight and shoot length were significantly responsive to the different phosphorus sources where the TSP treatment was superior to the other treatments while the control had the lowest growth rate as measured in all of the parameters. In the laboratory experiment, the acidulating agents showed significantly different dissolution rates of P from rock phosphate with sulphur showing the highest at 30, 60 and 90 days after incubation (37.5, 1175.3 and 1822.9 ppm respectively). In the field, the TSP treatment elicited the highest fresh weight, number of leaves and leaf area followed by the RP+sulphur treatment; then the rock phosphate treatment. The RP+sulphur treatment led to the highest reduction of soil pH while the highest pH was observed under the sole rock phosphate treatment. Higher and significant responses of the growth and yield parameters of the three crops to the different forms of P applied compared to the control where TSP had the highest then the RP+ elemental sulphur. The soil pH increased significantly on the rock phosphate treatment under the amaranth and kale crops from 5.7 before planting to above 6.0 while on the cowpea, the treatment showed the least change with a slight drop of pH from 5.7 to around 5.6 with the other treatments dropping to much lower units. Application of acidulated rock phosphate is a viable option to the more expensive soluble phosphate fertilizers among smallholder farmers.