There are ≈12,000 ha of calcareous soils dedicated to vegetable production in South Florida (U.S. Department of Agriculture, 2014). After fertilizer application, P can be fixed by free CaCO3 through surface adsorption and precipitation (von Wandruszka, 2006). As a consequence, supplemental P fertilizer is needed to alleviate the fixation effects (Mengel and Kirkby, 1987). However, oversupplied P can be released by runoff or leaching and contributes to eutrophication (von Wandruszka, 2006). Phosphorus leaching loss usually occurs in these vegetable production areas as a result of the coarse-textured soils, the porous limestone bedrock, and the shallow water table [0.15–1.8 m (U.S. Department of Agriculture, 1996)]. In the water bodies of the Everglades Protection Area in South Florida, the geometric mean of DRP concentration is required to be ≤0.015 mg·L‒1 (Florida Administrative Code, 2017). Thus, preplant soil testing, which is performed to evaluate soil-P availability, is essential for optimum crop production and protection of the Biscayne Aquifer in South Florida.
The most commonly used STP extractants for calcareous soils include Olsen, AB-DTPA, and Mehlich-3. The Olsen or sodium bicarbonate extraction method uses bicarbonate (HCO3‒) and hydroxide (OH‒) anions to extract labile P into solution (Olsen et al., 1954). AB-DTPA is a multielement extractant used in areas with calcareous soils (Soltanpour and Schwab, 1977). Similar to the Olsen method, the HCO3‒ in AB-DTPA serves to enhance phosphate solubility. Mehlich-3 was developed for extracting P and other macronutrients and micronutrients from a wide range of soils (Mehlich, 1984). The Mehlich-3 solution extracts P by dilute acid and fluoride (F‒), and acetic acid buffers the solution pH at 2.5 to prevent neutralization in calcareous soil (Wang et al., 2004). Rashid et al. (1999) observed significant correlations between P uptake by greenhouse-grown mung bean (Vigna radiata) and STP by Olsen, AB-DTPA, and Mehlich-3 in a calcareous soil. In our previous study, P uptake by snap bean (Phaseolus vulgaris) also correlated significantly with soil-P extracted by both AB-DTPA and Mehlich-3 (Zhu et al., 2016).
In addition to agronomic evaluation, STP was used to estimate P loss potential to water sources (McDowell et al., 2001; Sims et al., 2002). A split-line model was introduced to describe the relationship between STP and water- or calcium chloride-extractable P or DRP in leachate (McDowell et al., 2001). In the model, the intersection of the two lines is defined as a change point, which is a soil-P concentration above which soil P has a greater potential to be released to water. The rapid increase of P loss potential after the change point can be attributed to the saturation of soil-P fixing sites as STP concentration increases (McDowell and Sharpley, 2001). The values of change points depend on soil properties and management practices (Hesketh and Brookes, 2000; McDowell et al., 2001). An optimum STP concentration should be sufficient for crop production and should be less than the change point (Bai et al., 2013). For a more comprehensive risk assessment, a P index, which combines STP with site characteristics and P management practices, was developed to assess the potential of P losses from agricultural areas to water bodies (Lemunyon and Gilbert, 1993).
The STP interpretations using Mehlich-3 have been established for vegetables grown on acid–mineral soils in Florida (Freeman et al., 2014b). In calcareous soils, however, AB-DTPA was adopted and 10 mg·kg‒1 was proposed to be the critical level for vegetable production without calibrated interpretations (Li et al., 2000). Because no soil-test calibration has been performed in Floridian calcareous soils under field conditions, the STP interpretations and P recommendations are not available. Therefore, the objectives of this study were to compare three extractants—Mehlich-3, AB-DTPA, and Olsen—in 1) estimating P availability with different P application rates, 2) establishing STP critical level, and 3) calibrating the P requirement for tomato production on a calcareous soil.
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