The presence of nutrients, minerals, organic matter, microorganisms, and the texture and structure of the soil affect soil fertility. Fertility refers to the ability of soil to provide plants with the nutrients they require to grow and fight disease.
Decomposition of plant and animal wastes releases nutrients and minerals into soil, and microorganisms aid the decomposition process. Texture and structure influence soil's ability to retain nutrients, minerals and other components essential for fertility. Sandy loam, a combination of sand, silt and clay, offers the best balance of drainage and nutrient retention for most gardens. In addition, soil structure determines drainage rates, aeration and the ease with which plant roots move through the soil. Gardeners prefer soil that readily crumbles. Removing the components that support fertility and failing to replace them, usually the effects of intense cultivation and poor soil management, lead to soil depletion and poor soil quality. In turn, depletion causes poor crop yields.
In garden soil, organic materials, such as compost, animal manure and green manure, boost soil health by producing nutrients for plants, feeding insects and microorganisms, and improving soil structure. Organic fertilizers that consist of plant, mineral or animal sources improve soil fertility without changing the composition of the soil or introducing pollutants.