Because compost is primarily composed of humus, (decomposed organic matter), its structure is made up of many aggregates. Aggregates are materials made from two or more materials with significantly different physical or chemical properties. These materials remain separate and distinct on a macroscopic level within the finished structure.
On a microscopic level, compost has vast surfaces inside these particles where individual ions of minerals and fertilizers can cling. Compost is able to store nutrients that might otherwise wash through a sandy soil or be locked up in the tight spaces of a clay soil. These ions are given up to the roots as plants require them. The ions clinging to the surfaces of compost particles tend to be those that give soil an optimum "neutral" pH range of 5.5 to 7.5.
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simple soil test