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Vegetable P fertilization recommendations in Florida are based on a soil test using the Mehlich-I (double-acid) extractant. For several Florida vegetables, including watermelon, there is a lack of crop correlation and extractant calibration data. Phosphorus fertilizer studies were conducted on sites with soils ranging in Mehlich-I P indices from 4 to 30 mg·kg-1. There was a quadratic yield response on soils testing 4 mg·kg-1 P with yield maximizing at about 70 kg·ha-1 fertilizer P. Watermelon did not respond to P additions on soils testing greater than 30 mg·kg-1 of Mehlich-I P.
Watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] was grown at two sites differing slightly in Mehlich-I (double-acid) -extractable P (6 and 10 mg·kg-l soil). Early and total yields responded positively to P rate; however, maximum yields were obtained with small amounts of P fertilizer. The linear-plateau critical P fertilizer rates were 26 and 27 kg·ha-1 at sites 1 and 2, respectively. These critical rates were lower than those currently used for recommending P fertilizer on soils that have very low P. Phosphorus concentrations of most-recently matured leaves at early fruit set were 2.5 and 2.8 g·kg-1 at sites 1 and 2, respectively, with 0 P, and 4.4 and 4.8 g·kg-1 with the 25-kg P/ha treatment.
The Mehlich-l (double-acid) soil test extractant is used widely on sandy soils in the southeastern United States. Research with eight major vegetables in Florida showed that the Mehlich-1 solution for phosphorus was correlated with crop yield and quality responses. Soils with a Mehlich-1 index for phosphorus greater than 30 mg·kg-1 required no phosphorus fertilization. Crop responses were not highly correlated with Mehlich-1 potassium index and it was difficult to predict crop response to potassium fertilization. The lack of predictive capability of the Mehlich-1 extractant for potassium raises questions over soil testing for potassium on sandy soils in Florida where potassium is mobile.
`Camelot' bell pepper was grown in a N fertigation study on sandy soil using polyethylene-mulched and fumigated beds. Portions of N (0%, 33%, 67%, 100% of total season N) were applied at bed formation. The remaining N was injected weekly into the drip irrigation system. Total N application treatments were 64, 128, 192, and 256 kg·ha–1. Early and total-season marketable fruit yields increased linearly with N rate. Preplant fertilizer proportion did not influence early yields, but late and total-season marketable fruit yields decreased linearly as preplant fertilizer proportion increased. Petiole sap NO3-N concentration increased with increasing N rates, but decreased linearly as preplant fertilizer proportion increased. Petiole sap NO3-N concentrations fell below critical levels for all N rates and preplant fertilizer proportions early in the season. Whole-leaf N concentrations were higher than critical values (>40 g·kg–1) throughout the season. Preplant fertilizer proportion had a significant linear effect on whole-leaf N concentrations for all sampling periods. Petiole sap was better correlated to yield data than whole-leaf N.
Tomato (Lycopersicon esculentum Mill.) was grown with drip irrigation on an Arredondo fine sand and on an Orangeburg fine sandy loam to evaluate the effect of N and K time of application on petiole sap, leaf-N and -K concentrations, fruit yield, and to determine N and K sufficiency ranges in leaf tissue. On the sandy soil, N—K at 196-112 kg·ha-1 were applied 0%, 40%, or 100% preplant with the remainder applied in 6 or 12 equal or in variable applications in 12 weeks. With the variable application rate, most nutrients were applied between weeks 5 and 10 after transplanting. On the sandy loam soil that tested high in K, only N (196 kg·ha-1) was applied as above. Petiole sap K concentration declined during the season, but was not greatly affected by treatment. Petiole NO3-N concentrations decreased during the season from 1100 to 200 mg·L-1, and the decrease was greater with preplant N treatments. On the sandy soil, marketable fruit yields were lowest with 100% preplant, intermediate with 100% drip applied (no preplant N), and highest with 40% preplant and 60% drip applied. With 100% drip applied, yields were higher with 12 even applications than with either six even weekly applications or with 12 variable N and K applications. With 40% preplant, timing of application had little effect on yield. On the sandy loam soil in 1993, yields were highest with 100% preplant, intermediate with 40% preplant and 60% drip applied, and lowest with all N drip applied. In 1994 when excessive rains occurred, yields were similar with all preplant and with split N applications. Petiole N concentration was correlated with tomato yield, especially at 10 weeks after transplanting. The best correlation between sap-N and total yields occurred between 4 and 6 weeks at Gainesville and between 4 and 10 weeks at Quincy.