Vegetable Garden: Soil Amendment: Precautions & Release Rates (Compost and Manure)

Manure, compost made from manure, and bio-solids may be high in salts that will interfere with crop growth. Do not add more than one inch per season without conducting a soil test to evaluate potential salt build-up.

Due to a health issue (E coli contamination), fresh manure additions should be made at least four months prior to the harvest of any edible crops. In other words, apply fresh manure only in the fall after crops are harvested.

Fresh manure or unfinished compost products may be high in ammonia. Avoid application of products with an ammonia smell; they could burn roots and leaves. Manure and compost may be source of weed seeds.

Gardeners need to understand that the nutrient release from compost and manure is slow, taking years. Adding compost or manure to improve soil tilth is not the same as fertilizing.

The typical nitrogen release rates from manure is only 30% to 50% the first year (fresh manure), 15% to 25% the second year, 7% to 12% the third year, 3% to 6% the fourth year, and so on. With compost and composted manure, the release rate is even slower, 5% to 25% the first year, 3% to 12% the second year and 1% to 6% the third year.

Since the nitrogen percentage of compost and manure products is typically only 2% to 4%, the amount of actual nitrogen release to support crop growth is very small.

For soil with 4% to 5% organic matter, the mineralization (release) of nitrogen from soil organic matter will likely be sufficient for crop growth.

For soils with 2% to 3% organic matter, the mineralization of nitrogen from soil organic matter will not likely be sufficient for heavy feeding vegetable crops. Supplement with 0.1 pound nitrogen fertilizer per 100 square feet.

For the typical garden soil with 1% organic matter or less, the mineralization of nitrogen for soil organic matter will be minimal. Add 0.2 pounds of nitrogen fertilizer per 100 square feet.