Both fresh and composted organic materials are useful for amending soils. Fresh organic material is rapidly decomposed by microorganisms in a compost pile or in the soil. The microorganisms use the organic material as a food source and release carbon dioxide to the atmosphere. As decomposition proceeds, the remaining organic compounds are more resistant to decomposition. (They have less "food" value to microorganisms.)
Decomposition is usually very rapid for the first 30 days after application of fresh leaves, fruits, or other vegetative material to soil. When decomposition of fresh organic materials takes place in soil, the sticky exudates (fluids) produced by soil organisms help glue soil particles together, improving soil structure. The volume of material will be reduced rapidly as decomposition takes place.
Soil microorganisms require nitrogen for their growth, so the process of degrading fresh organic matter in the soil sometimes causes a nitrogen deficiency for plants. If you use fresh plant material, allow it to decompose in the soil for several weeks before planting into it. Also keep in mind that very woody materials, such as sawdust or sawdust-bedded manures, may cause nitrogen deficiency in soils for a long time, even after composting.
When organic materials are composted before use, the rapid decomposition phase takes place in the compost pile instead of in the soil. Organic matter supplied by compost lasts longer in soil than fresh organic matter because much of the decomposition has already occurred. However, composted organic matter is a poorer food source for soil organisms compared to fresh organic matter, so less of the sticky exudates that build soil structure are produced in the soil after compost application.
On the other hand, composted materials have fewer weed seeds and are less likely to carry plant disease organisms. Composted manures are preferred over fresh manures when contamination of food crops with human pathogens such as E. coli is a concern.
In a garden situation, fresh or composted materials may be used. If you are establishing a raised-bed garden, compost is preferred because it will lose volume less rapidly and because it has less potential to compete with plants for nitrogen.
