Soil Fertility and Fertilizers

The mantra of organic farmers and gardeners is “feed the soil”. This is because, plant stress, diseases, and pest tolerance are tied to organic management of the soil. The soil in good condition for plant growth is composed of about 43-47% mineral solids (sand, silt & clay), about 3-7% organic matter and porous space containing 25% air and 25% water.

Ideal soil has good tilth for root development. It is well drained, contains appropriate types and amounts of nutrients and is free of harmful chemicals. Granite dust, rock phosphate (P) and limestone (Ca, Mg) provide minerals in their basic form and may be added to the soil directly. Organic materials (humus) provide nutrients and contributes to soil tilth and water holding capacity. Animal or green manures, along with compost are the most common sources of the needed organic material, with nutrients provided by various cycling processes.

• Compost

Humus is very important in healthy soil composition. It helps to maintain a loose soil and it also improves the water holding capacity. You can contribute to soil humus content by simply adding compost to your soil.

Compost may be added before planting, incorporated during the growing season, or may be used as a mulch. Compost might be added to the soil before planting at the rate of 2-4 bushels per 100 sq. ft., incorporated during the growing season so long as care is provided not to injure roots, or it may be used as mulch by applying ½-1” inch around the base of plants

• Green Manures

Another great way to keep your soils healthy and fertile is by using green manures. These are crops that are grown to feed the soil. Planted spring through early fall, they directly provide soil nutrients. Legumes fix nitrogen, other crops store calcium, magnesium or sulfur in their tissues. These nutrients are released as the cover crop decompose.

• Animal Manures

Animal manure, preferably composted, provides for soil fertility. The nutrient value of the manure is based on the type of animal, diet, and whether bedding is used or not. Soil application, for the home gardener, is about 200 pounds or 3 bushels per 100 sq. ft. for the lowest fertility soils. Fresh manure, by organic standards, may not be applied within 90 days of harvest for about ground crops and 120 days for below ground crops.

University of Illinois Extension. (2017). “Master Gardener Manual”