Composted materials can either be spread on, mixed with the soil, or used as a mulch around growing crops. The fertilizer should be mixed with the soil to prevent damage to seeds or roots of the young plants. Many people ask if it is possible and practical to substitute organic materials animal manures, compost, and various plant wastes for commercial fertilizer. It may be possible in some cases. The answer in each case depends on the particular soil under consideration.
Different soils have different needs.
Only a few soils have adequate quantities of humus and available plant nutrients so that they can produce good plant growth over a long period without amendments. The right kinds and amounts of commercial fertilizers, properly used, will increase plant growth in most soils including growth of roots and stubble that contribute to the supply of soil organic matter. Air space, root room, and water available to a plant depend largely on the soil structure. This, in turn, is closely related to the organic matter in the soil and a suitable distribution of mineral particles of different sizes to keep the soil open and porous, permit excess water to drain away, and air to enter. The plant absorbs mineral elements and water from the soil and carbon dioxide gas from the air.