Effectiveness of Phosphate Rock Application Methods and Frequencies on Soil Available P and Uptake by the Leafy Vegetable Amaranth

Low solubility of phosphate rock (PR) limits its direct application to leafy vegetables. However the low solubility and slow P releasing characteristics contribute to higher residual effect than water soluble P fertilizers. A pot experiment was conducted over three crop cycles at the Agricultural Research Institute Uyole (ARI Uyole) in Tanzania to determine the most appropriate PR application method and frequency for amaranth, a nutritious leafy vegetable. The experiment was laid out in a split plot design with three replications. Frequency of Minjingu phosphate rock (MPR) application constituted the main plots while the application methods were the sub plots. The methods were (i) control (no fertilizer material added), (ii) direct application of MPR, (iii) compost + MPR, (iv) crotalaria green manure + MPR, and (v) NPK (Nitrogen –Phosphorus-Potassium fertilizer) as standard. Frequencies were (i) once in the first crop cycle, (ii) during the first and third crop cycle, and (iii) during each crop cycle. Compost + MPR gave significantly higher fresh weight and dry matter amaranth yield than did other treatments. During the third crop cycle there was significant (P ≤ 0.01) interaction between frequency and method of MPR application on P uptake, plant P content, fresh weight and dry matter yield. There was positive and highly significant (P ≤ 0.01) correlation between soil available P, P uptake, concentration and amaranth yield. The results indicate that application of MPR once during three crop cycles especially when combined with manure could supply adequate levels of phosphorus thus replace the use of soluble P fertilizers in production of short season leafy vegetables such as amaranth.