SOURCE OF P, LIME APPLICATION SEQUENCE, AND RESIDUAL EFFECTS ON P AVAILABILITY TO WATERMELON

The value of watermelon [Citrullus lanatus (Thumb.) Matsum. & Nakai] p roduction in Florida was reported at more than $62 million in 1991-92. Watermelon has a relatively high phosphorus (P) requirement. Availabi lity of P from triple superphosphate (TSP) to some crops was reported to be greater than from diammonium phosphate (DAP). Also, lime and P a pplication sequence was reported to influence availability of DAP but residual effects have not been investigated. Our objectives were to de termine the influence of P source and lime and P application sequence on P availability to watermelon and determine the availability of resi dual P with respect to P source and lime treatment. Two crops of water melon were grown consecutively in the same soil in pots containing 2 k g of soil each. The first planting was made in the fall and the second in the spring. The Ap horizon of a Norfolk loamy sand (fine-loamy, si liceous, thermic, Typic Kandiudult) containing <5.0 mg kg-1 Mehlich-1 extractable P was used. Rates of P were 40 and 80 mg kg-1 from either TSP or DAP. Calcium oxide was added at a rate of 1 g kg-1 either 4 wk before or 4 wk after P application. The experiment consisted of two P sources, two P rates, three lime treatments and a control with no lime or P added. There were four replicates and the analysis of variance w as conducted on the 2x2x3 factorial treatment arrangement. Dry-matter yield and P content of watermelon tissue were determined. Availability of P from TSP was 23% greater than with DAP in the first corp (immedi ate effect) and 14% greater in the second crop (residual effect), aver aged over P rates and lime treatments, as shown by analysis of varianc e. Without lime, availability of P was about 23% greater with TSP than with DAP in both crops. Liming before P application increased P avail ability by 17% in the first crop and by 22% in the second crop over th e control (no lime). Liming after P application decreased P availabili ty by 12% in the first crop, but it increased P availability by 11% in the second crop.