Compost

Compost composition depends heavily on its feedstock, such as plant residues, yard waste, or food scraps, and on the composting process and degree of maturity. Nutrient analysis is far lower and more variable than mineral fertilizers, and is typically stated as a range rather than a fixed grade.

Nutrient content

Low and variable; typically well under 2% each of N, P₂O₅, and K₂O

Organic matter content

High; the primary contribution of the material

Nutrient form

Bound in organic matter, released as decomposition continues

Variability

Depends strongly on feedstock, process, and maturity

Compost supplies nitrogen, phosphorus, and potassium at low concentrations, along with secondary nutrients and micronutrients, most of them held within organic matter and released gradually as soil organisms continue to decompose the material after application.

• Nitrogen, phosphorus, and potassium are supplied at low, variable concentrations
• Nutrients are released gradually through ongoing decomposition rather than immediately
• Trace elements are often present, depending on feedstock

Compost is applied to build soil organic matter, improve soil structure and water-holding capacity, and support soil biological activity, in addition to contributing some nutrients. It is widely used in vegetable, horticultural, and other cropping systems, including organic production.

• Builds soil organic matter over repeated applications
• Improves soil structure, aeration, and water-holding capacity
• Supports soil microbial and biological activity

Because nutrient content and release rate vary so widely between compost batches, general practice is to test compost nutrient content where possible and account for gradual nutrient release when planning an overall fertility program, rather than treating it as equivalent to a mineral fertilizer of known analysis.

Well-matured compost generally poses lower risk of rapid nutrient loss than soluble mineral fertilizers, since its nutrients are released gradually, but excessive or poorly timed application can still contribute to nutrient runoff or leaching, particularly of phosphorus and, once mineralised, nitrate.