Crops obtain required S from diverse sources including soil, crop residues, manures, irrigation water, rainfall, atmosphere, and soil amendments and fertilizers (Tabatabai, 1984). In the past, the primary sources of S for crops were from atmospheric deposition, soil, and fertilizers. A reduction of atmospheric S deposition in conjunction with increased use of high-analysis fertilizer has caused S deficiencies to become more common in crops (Ceccotti et al., 1998; Scherer, 2001). Other than fertilizer sources, soil and water could act as primary S sources for crop production. It is unclear if either one can meet crop requirements for S.
At the plant level, S is indirectly involved in photosynthesis and chlorophyll production (Ahmad and Abdin, 2000), which may be perceived as an increase in greenness in a crop plant. Similarly, improvement in vegetable crop yield and growth occurs when S was included in the fertilizer regime. In potato (Solanum tuberosum), Pavlista (2005) found that an application of 28 kg·ha−1 S improved tuber quality and yield compared with no added S. Rhoads and Olson (2000) found highest yields for cabbage (Brassica oleracea var. capitata) were consistently produced with 22 kg·ha−1 S. Similarly, Susila (2001) found that tomato and cabbage both responded to the addition of S to the fertilizer regime with rates ranging from 0 to 102 kg·ha−1 S. This response occurred regardless of S source and application method (preplant, drip injection, or a combination of both). Both Rhoads and Olson (2000) and Susila (2001) found S sources to be similar and yields were improved across all the studies with S applications within the recommended range for Florida [30 to 40 lb/acre S (Simonne and Hochmuth, 2009)]. None of these studies considered irrigation water SO4 concentrations as an S source for crop production.
Tomato production in Florida often occurs on deep Spodosols (fine sandy soils) with low organic matter (<2%), which are inherently low in both organic and inorganic S. Previous studies on tomato have found that some S sources such as ammonium sulfate [(NH4)2SO4], ammonium thiosulfate (NH4S2O3), ammonium nitrate [(NH4NO3) + (NH4)2SO4], and potassium sulfate (K2SO4) affected tomato yields similarly (Santos et al., 2007; Susila, 2001). Elemental S is a less expensive S fertilizer source when compared with other S sources that contain nitrogen (N). Elemental S can be an ideal S source for crop growth and yield because it is high in S analysis, insoluble in water, stable in damp and humid conditions, and has to be oxidized to SO4–S for plant uptake (Solberg et al., 2003). During the process of elemental S fertilizer oxidation by soil microbes, soil pH is lowered. The rate of elemental S oxidation by soil microbes is dependent upon several factors such as carbon substrate, N source, soil temperature, and moisture.
The concentration of S in the irrigation water used throughout North America varies widely. Little research is available about the relationship between S content of irrigation water and S requirement in a fertilization program (Olson and Rehm, 1986). Field sites without mineral-bound S sources may have insufficient S dissolved in soil water to move available S to plant roots (Eriksen et al., 1998). Studies on high SO4 irrigation water used water with 175, 646, 862, and 1743 mg·L−1 SO4 (Drost et al., 1997) and 15.0 and 38.8 mol·L−1 SO4 (Papadopoulos, 1986) have found mixed results in improving yield of broccoli (B. oleracea var. italica) and tomato. In Florida, S concentrations in drinking water wells can range from 0.07 to 460 mg·L−1 S (in the form of SO4) with higher concentrations coming from deeper wells (Sacks, 1996). Although these values cover drinking water, the concentration of S within irrigation water drawn from deep wells in Florida is unknown. It has been suggested that water pumped from the Florida aquifer and surface waters may contain appreciable amounts of S for crop production because of the S-like smell commonly associated with hydrogen sulfide volatilization. This often leads to the belief that S in the irrigation water is an S source for crop production (Hochmuth and Hanlon, 2000). Since S concentrations in irrigation water vary widely and current fertilizer practices use high-analysis fertilizer that often lacks S, the objectives of this study were to 1) investigate the effect of S fertilization rates on tomato growth and yield and 2) assess whether irrigation water could provide the necessary S for tomato production.
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