For amending soil structure and reducing soil salinity, gypsum is best considered in a long term programme. Over a period of three years of annual or twice-yearly applications, soil structure can be improved significantly. Speed of action can be hastened by cultivating the gypsum in and by adding organic matter to the soil if that is practical. Application rates for improving soil structure can vary from 500 kg/ha to as 5 tonne/ha or greater. Smaller rates could be used in situations where a small improvement in soil is sufficient and/or sulphur requirement is being addressed.
Gypsum has calcium sulphate in the dihydrate form (two water molecules with each calcium sulphate) making it somewhat more soluble than the anhydrous form of calcium sulphate. A small amount of gypsum can dissolve at once in the soil solution (limit is 0.24 g per 100 mL of water at 20oC). Once the soil solution reaches the limit of calcium sulphate in solution, gypsum starts to precipitate again. Thus there is a fast provision of calcium and sulphate into soil solution for plant uptake or effect on the soil but the overall rate of release from gypsum is controlled by the rate with which the calcium and sulphate are taken or move from the soil solution.
As gypsum dissolves, one sulphate ion is released with each calcium ion. Since a sulphur atom is lighter than a calcium atom, the amount of sulphur released is around 75% of the amount of calcium by weight.
Calcium sulphate is much more soluble than calcium carbonate (lime) and over time will release calcium at a significantly faster rate than the weathering of lime. Time of release is variable depending on soil conditions including moisture level (dryer is slower), drainage (faster soil water velocity means more ability for the gypsum to dissolve as calcium moves downwards), soluble salt level (higher levels of some salts can make it slower), sodium level (higher sodium level makes it faster as the calcium goes on to the exchange sites allowing more gypsum to dissolve) and the general ability for the soil (and plants) to “absorb” calcium and sulphate from the soil solution – but for greater than 500 kg/ha application rates would generally be expected to be over a period of months and more like one year rather than a few months.
The sulphur fertiliser value is an added value on top of the calcium provision. The sulphate is bound with the calcium until it dissolves and so is released over the same time that the calcium is released though faster than the rate of release from elemental sulphur. This does indeed mean that release will not last as long in the soil (although the proportion that is incorporated organically by either plants or microbes can enter the organic cycle and thus have a prolonged effect). The somewhat faster speed of release than elemental sulphur is an advantage in some circumstances e.g. where a current crop is limited by low sulphur availability – and in particular soil conditions where release from elemental sulphur can be slowed even more e.g. dryer areas or thatched pasture). Gypsum offers this advantage of readily available sulphur while still being allowable organically. I see roles for both sulphur forms.
With a typical gypsum application for improving soil structure or reducing salinity, more sulphur is being added than is really needed by crop growth in that season. In many cases a fair proportion of the sulphur will be leached and this is actually one of the modes of action of gypsum. In high sodium soils, the action of gypsum is to leach out the sodium with the sulphate (two sodiums for each sulphate ion) – sodium is more easily leached than calcium and the calcium remains – this benefits soil structure through reduction in sodium levels and increase in calcium level (floculation effect). In heavy soils, the readily available sulphate helps bring the calcium further down the soil profile to benefit soil structure somewhat faster through the root profile and even into the subsoil – this will be one of the attributes studied in ongoing field trials.
The chemistry is interesting. As mentioned above, only a small amount of calcium sulphate can be in solution at once and any more just precipitates out as gypsum again. Thus the speed of release is affected and leaching is slowed (unless it is sodium sulphate being leached out from high sodium soils in which case it is quite desirable as soil conditions can improve with reduced sodium levels).