AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335
Summer 1998, No. 5
Manure contains a number of essential plant nutrients. These include nitrogen, phosphorus and potassium, as well as a variety of secondary and micronutrients like calcium, magnesium, copper, manganese, iron and zinc. Unlike commercial fertilizers, which have a guaranteed composition, the nutrient content of manure is highly variable, depending on the type of livestock, feeding regime, method of collection, and storage.
Manure nitrogen is more complex than inorganic fertilizer nitrogen. Almost half the nitrogen in fresh manure is in ammonium form and is readily available to plants, if managed properly. The rest is in various organic compounds that are not plant available. Organic nitrogen must be converted to ammonium or nitrate nitrogen before it can be taken up by plants. The release of nitrogen from organic compounds is a microbial process that is regulated by soil temperature and moisture and continues for two to three years following initial application. Some 25 to 75 percent of the nitrogen in manure is available in the year of application, depending on the manure and how it is handled. About half of the remaining nitrogen is released the next year, and so on.
Nitrogen loss occurs as a result of conversion of the ammonium to ammonia gas, which escapes to the atmosphere. The most effective way to prevent nitrogen loss-whether in storage or in the field-is to reduce exposure to air. Immediate soil incorporation after spreading can save as much as 25 percent of the nitrogen. Similarly, injection of liquid manure into the soil also effectively minimizes ammonia losses. Compared to spring application and incorporation, fall and winter application can result in the loss of 50 percent more nitrogen through leaching, runoff, denitrification, and ammonia losses to the air.
Availability of phosphorus and potassium in manure, in the year of application, ranges from 50 to 100 percent. There are few environmental concerns associated with excess potassium, but excess phosphorus can increase the risk of polluting surface waters, if soil erosion and surface runoff are not controlled.
The value of manure nutrients should not be overlooked. Typical feedlot manure with 50 percent moisture contains about 21 pounds of nitrogen, 18 pounds of P2O5 and 26 pounds of K2O per ton. At prices of 20¢, 25¢ and 13¢ a pound respectively, for the nutrients, a ton of manure contains about $12 worth of plant nutrients. That's $120 per acre when applied at a normal application rate of 10 tons.
Nutrients are not the only value. Manure increases the soil's organic matter and physical structure, improving water holding capacity and infiltration, which reduces runoff. Manure can also reduce soil crusting problems and susceptibility to wind and water erosion.
Manure will not likely replace commercial fertilizer, but it can supplement and improve a fertility program. However, manure requires careful management. The problem is, it's difficult to time the release of manure nutrients with that of plant uptake. When using manure, avoid repeated annual applications to the same field and incorporate manure into the soil as soon as possible.
Manure is a good resource. Used wisely, it will be a great benefit. Used unwisely, it can pose great environmental risks.