Animal manure is a secret ingredient which many home gardeners overlook. Manure offers many benefits, the most notable being increased available soil phosphorus and enhanced water infiltration rates. Chemical reactions with soil-and-water calcium alter the phosphorus to forms that are not available or only slowly available to plants. When manure decays, organic phosphates are released and the total phosphorus level available to plants increases. Organic phosphates may persist for many years after manure applications have stopped. The organic phosphorus also moves much deeper into the soil than do inorganic forms of phosphorus fertilizers.
Manure application also increases water infiltration dramatically compared to non-manured plots. In fact, studies indicate that manured plots can be properly irrigated five times faster than non-manured plots. Manure also keeps soils from sealing tightly while crops are growing.
There are three problems which can cause manure to be more of a problem than an asset. A cheap source of manure may be hard to find with so many gardeners scouring the countryside.
Secondly, one must consider the source of the manure because undigested weed seeds cause problems in a garden. Thirdly, large amounts of salts in the manure can kill garden plants if large quantities are used. Salt content is a major problem in using sewage sludge (too many water softeners polluting a potentially excellent manure source). Saline plant toxicity can also be a problem when using feedlot manure if cattle are fed high salt rations.
How much manure should be added to a garden? Bag instructions recommend 40 pounds per 100 square feet of garden area. That is a good, safe recommendation; however, some very successful gardeners apply the manure at 80 pounds per 100 square feet (8 pounds per tomato plant located 3 feet apart). Actually, the type of manure determines the rate which can be safely used. Barnyard manure can be used in greater quantities because it does not contain as much ammonia. Vapors of ammonia in fresh manure, especially rabbit or poultry manure, can severely damage plants. Thoroughly incorporate all fresh manure into the soil several weeks before planting.
Certain tomato varieties, such as Merced and Surefire, and most peppers respond well to heavy manure applications. However, adding excessive amounts of manure on a yearly basis is not encouraged since nutrient release occurs over a period of time and nutrient toxicity could eventually occur.