Effects of Phosphate Rock on Phosphorus Availability for Vegetable Crops in Cropping Sequence

This study was conducted to investigate the effects of cropping sequence and Minjingu Phosphate Rock (MPR) on soil phosphorus (P) availability and yield of selected vegetable crops. The study involved a researcher managed on farm field experiment conducted in Mbeya, Tanzania using a split plot design with three replications. Crop sequence (i) Cabbage - tomato - maize (ii) Maize - tomato -cabbage (iii) Cabbage - cabbage - maize (iv) Maize - tomato – maize, constituted the main plots. The P sources (i) control (no fertilizer), (ii) MPR, (iii) compost + MPR, (iv) crotalaria green manure + MPR (v) NPK (standard), were sub plots. The experiment was repeated three times on the same plots. In the fourth season, bean was planted in all plots without fertilizer to capture residual nutrients. Results indicated more percent increase in soil available P in plots where cabbage was included in the sequence (207.9%) as compared to when it was ommited (85.39%). Compost + MPR treatment significantly increased soil pH from 5.8 to 6.33 while in NPK treated plots it decreased to 5.49. After three seasons there was higher residual P in MPR treated plots (33.33 mg/kg) as compared with NPK (27.65mg/kg). Plots treated with NPK produced the highest maize yield while plots treated with MPR alone or combined with compost or crotalaria produced significantly (P ≤ .01) higher cabbage and tomato yield especially during the first and second seasons. Bean pod yield was higher in MPR treated plots. Therefore, the use of PR and including cabbage in crop rotations should be encouraged in organic vegetable farming systems to restore P on phosphorus deficient soils.