Gypsum

Gypsum is a crystal containing calcium and sulphur, with a pH about 6.5. Gypsum is used to break down clay in the soil through a simple chemical reaction. The calcium within the gypsum particle attaches itself to two clay particles to form a “crumb”. These crumbs stay separated, helping to improve drainage and aeration. More over, the sulphur part of gypsum (sulphate) can attach itself to sodium; combined the sodium sulphate is easily leached by water. Gypsum is used when the soil pH does not need to be altered from the ideal food growing pH of around 6.4. If the soil is too acidic, use lime (for clay soils) or dolomite.

 

More detailed information below is from the website http://www.gypsum-aus.com.au/gypsum.htm:

Gypsum is an excellent source of both Calcium and Sulphur - essential for both yield and grain quality (especially useful in combination with high analysis fertiliser) - but its benefits go much further. It also helps improve soil structure and balance the soil by both adding Calcium and displacing harmful Sodium.

When Gypsum is applied, the Calcium seeks out clay particles - sticking to them in much the same way that dust will stick to an old vinyl record (remember - before CDs). The value of calcium is that it can stick to two clay particles at once, causing them to clump together - improving soil structure.

One of the enemies of soil structure is Sodium and in soils with lots of this element (termed 'Sodic'), there may be little room left for Calcium on the clay particles. But Gypsum has another weapon - Sulphur. Gypsum is made up Of Calcium Sulphate, so when Calcium moves in to exchange places with Sodium, the Sulphate captures this Sodium and carries it away in the soil water.

Another problem with soil structure in Australian Soils is that many are high in Magnesium. This is a useful and important element, but with lots of Magnesium and little Calcium, soils set hard when dry, limiting root access and inhibiting crop establishment. Again, Gypsum can correct this situation - helping to restore the proper Calcium:Magnesium ratio.

Conditions that indicate the use of Gypsum include:

  • Low calcium or sulphur on a soil test (especially if Sodium or Magnesium levels are also high)
  • High soil pH
  • Signs of poor moisture infiltration or waterlogging
  • Soils that are very soft and sloppy when wet or very hard when dry (or both) . Crusting of the soil surface after rain that can cause poor germination
  • Burning of shoot and root tips caused by calcium deficiency.