Organic soil is free of man-made chemicals and consists of a large proportion (greater than 2 percent but no more than 10 percent) of organic material. Soil health depends on its organic composition. Unhealthy soil lacking essential nutrients will reduce a plant's growth, promote disease and may not grow vegetation at all.
Organic matter is required in soil to provide proper structure and for the release of nutrients. As organic matter in the soil decomposes, nutrients are released for the use of plants, according to the University of Wisconsin Extension. Nutrients from organic matter in the soil also feed microorganisms, which in turn break down more organic matter and provide organic matter and nutrients themselves when they die. Decomposing organic matter improves soil structure, increasing water retention and drainage.
The acidity, or pH level, of an organic soil determines its fertility, or ability to grow plant life. Soil pH affects a plant's ability to absorb nutrients. A pH that is too high or too low will inhibit nutrient absorption. According to Michelle Wander of the University of Illinois, most plants require a pH level of between 6.0 and 7.0. For highly acidic soils that have a pH level below 6.0, lime may be added to the soil to raise the pH. Soils that are too alkaline, or above the 7.0 mark, may be amended with sulfur to lower the pH.
Organic Soil Amendments
To keep a soil organic, organic soil amendments are required to improve the soil's structure and nutrient levels. Organic soil amendments include compost, manure, wood chips, grass clippings, straw, biosolids, sawdust and wood ash, according to Colorado State University. Organic amendments are made of either animal or plant waste. Soil amendments require careful selection for their rate of longevity in the soil, the texture it will add to the soil, soil salinity and nutrient content.
For an organic soil to stay healthy it needs a balance of the three main nutrients for plant growth: nitrogen (N), phosphorus (PO) and potassium (K). Store-bought organic fertilizers are available to provide easily measured amounts of nutrients to the soil. Animal meal, manure and other soil amendments may provide these nutrients in smaller amounts with more temporary results.
To prevent disease and manage pests without the use of chemicals, sustainable cultivation practices are required. Fallow periods, when no crops are grown, allow soil to recover after a harvest. Rotating crops, the practice of moving different crops throughout gardening spaces, removes the buildup of diseases specific to plants being grown and breaks up the habitat of pests in the area. Growing cover crops reduces topsoil erosion and adds organic material to the soil.