Compost varies according to content, comporting procedures, age, storage conditions and particle size. Compost made mostly from wood products will have a much lower nutrient and soluble salt concentration than one made from leaves, grass clippings, sewage sludge, manures or food processing waste. Compost from efficiently managed systems optimizing microbial activity will tend to have higher soluble salt levels than compost from piles that are neglected and/or undisturbed.
Regardless of which organic materials used in making compost it will have a pH between 6.2 and 7.2 when ready for use. The effectiveness of compost as a soil amendment and in formulating potting mixes is dependent on particle sizes and soluble salt concentrations. Particles should not be recognizable and less than 2.5 cm in diameter for optimum plant response. Soluble salt concentrations of the compost will determine rates of application and fertilizer regime.