Mineral Additions of Basic Elements
Composts are often deficient in Calcium (Ca) so add a small amount of lime (50-150g per cubic metre) and about five times as much (300-500g) agricultural gypsum (Calcium Sulphate). This helps to redress this shortage but also provides other physical and chemical benefits that improve the final product. Gypsum aids aggregation and prevents compost becoming slimy, displaces Sodium (Na) and converts Ammonia (NH4) into Ammonium Sulphate. Agricultural gypsum often contains significant quantities of Magnesium Sulphate so provides a total of three essential plant nutrients (Ca, Mg, S)
Compost, Calcium and Soil pH
It has been suggested that compost can have the neutralising effect equivalent to 10% of a Lime addition (WRAP 2003). Nothing could be further from the truth. Compost pH can vary from acidic to alkaline and many commercially made compost typically have a pH above 7.8. This high pH is due to the presence of basic cations [more?] one of which will be calcium (a 20 ton application of compost would typically supply less than 50kg of calcium) but the dominant cations, particularly when the pH exceeds 7.8, are likely to be Potassium, Sodium or Ammonia. At high concentrations these cations can be detrimental. They can, however, be easily displaced by adding Calcium as Lime or Gypsum. Consequently the use of compost with a high pH is more likely to increase Lime requirements than replace them. Using gypsum, either in conjunction with compost, or in the composting process can negate this problem.
Wide variations exist in the literature on the effectiveness of composts to sustain the phosphorus nutrition of crops. Phosphorus contribution is complicated as compost contributes directly to P availability and additionally by reducing soil P sorption and organic complexations that limit P solubility (Hue 1990). In crop trials compost applied at 10 ton per hectare increased P uptake equivalent to an application of 26kg P (as super-phosphate) ha-1 (Manna, Ghosh et al. 2001)
Free Cultural Works (CC-BY-NC-SA) Malcolm McEwen 2011