Multivalent cations (more than one positive charge) help hold soil particles together because they can have electrostatic (magnetic) attraction between two or more negative charge sites (soil clay and organic matter have a net negative charge). Soil structure is impacted by exchangeable cations (positively charged ions). So, if there is no need for fertilizer application of Ca or S, then gypsum application is simply not needed for fertilization reasons. These results are typical of research conducted for many years in that there was no yield increase to applied S, gypsum, or Ca. Table 2 gives results for recent S trials on corn and soybean conducted in 20 at six sites across Iowa. Research conducted for more than 35 years in numerous field trials across Iowa has shown only isolated and very small corn or soybean yield response to S fertilization (two positive and one negative). For calcareous soils (containing free lime) the soil system is saturated with Ca, and Ca supply and soil pH is controlled by the free lime.įor S, it's basically the same. Table 1 lists typical exchangeable Ca levels of several Iowa soils, and they are very high. ![]() Acidity problems will occur before a deficiency of Ca, so liming effectively takes care of Ca also. Research has not shown deficiency of Ca and normally any potential problem with low Ca levels is taken care of with application of limestone (CaCO3). If these forms are deficient in soil, then crop productivity will benefit if gypsum is applied. Gypsum is a fertilizer product and supplies the crop-available form of calcium (Ca 2+) and sulfur (SO 42-). The Ca 2+ ions simply interact with exchange sites in soil and sulfate remains dissolved in soil water. Therefore, it does not act as a liming or acidifying material. It adds calcium ions (Ca 2+) and sulfate ions (SO 42-), but does not add or take away hydrogen ions (H+). ![]() Dissolving gypsum in water or soil results in the following reaction: CaSO 4♲H 2O = Ca 2+ + SO 42- + 2H 2O. Gypsum is the neutral salt of a strong acid and strong base and does not increase or decrease acidity. Gypsum is sparingly soluble (the reason wallboard gets soft but does not immediately dissolve when it gets wet, at least if only damp occasionally). Gypsum fertilizer usually has other impurities so grades are approximately 22 percent Ca and 17 percent S. Usually, gypsum has water associated in the molecular structure (CaSO 4♲H2O) and is approximately 23.3 percent Ca and 18.5 percent S (plaster of paris). Refined gypsum in the anhydrite form (no water) is 29.4 percent calcium (Ca) and 23.5 percent sulfur (S).
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