Manure is often an under-valued resource. When well-managed and properly applied, it reaps many benefits. The benefits can be seen in the photo to the right, but are not captured in much of the field because of errors in application.
- Manure as a nutrient source can be a substitute for purchased fertilizers
- Manure increases crop productivity and can be used in gardens and flowerbeds, not just agriculture.
- The organic matter in manure improves water infiltration into the soil and reduces runoff and erosion. For this reason, when manure replaces synthetic fertilizers at agronomic phosphorus rates, there is often reduced risk of phosphorus runoff.
- Some manure types have a liming effect and reduce soil acidity.
- The P-Index rating is often reduced.
Manure tends to reduce sediment and water loss from a field, but the concentration of phosphorus in the runoff is typically high when soil test P is high. Manure application may, therefore, result in either increased or decreased phosphorus runoff.
Phosphorus runoff loss may be reduced with manure application due to reduced erosion and runoff in cases of agronomically moderate application rates and moderate soil test P. The combination of land-applying higher solids-manures and good conservation best-management-practices can significantly reduce erosion and runoff and maintain phosphorus losses at an acceptable level. However, if soil test P is excessively high, the reductions in erosion and runoff due to manure application are often not sufficient to prevent unacceptable losses of P.
*Cattle manure as fertilizer: One ton contains approximately 12 pounds of nitrogen. About 25% of this is immediately available to the crop. So, if 35 tons of cattle manure were spread over an acre of land, the available rate would be approximately 105 pounds of N for immediate crop use. If that amount of nitrogen had to be purchased, it could cost more than $68 per acre at 2008 prices ($0.65/lb), and this does not account for the other nutrients found in manure or the amount of nitrogen available to the crop in years to come. Phosphorus is also found in cattle manure. If you had to apply phosphorus as well as nitrogen, it could cost an additional $127.40 per acre if applied at the rate equivalent to the available amount (2.6 lb/ton of manure) in the manure using projected 2009 prices ($1.40/lb). In addition to nitrogen and phosphorus, manure contains zinc and sulfur, also important nutrients for crops.
The Nebraska Manure Value Calculator is a spreadsheet tool that calculates the value of a manure source for a specific field.
Other Nutrients in Manure
Potassium (K) and other nutrients essential for plant growth are supplied in manure. Some nutrients and heavy metals may reach toxic levels with repeated high rates of application. Heavy metals could be of greater concern when manure from animals that have consumed feeds containing heavy metals is applied directly to grazing land. However, heavy metals normally are not in animal manures unless the animals have been fed supplemental feeds that include zinc, copper, or selenium. Feedlot manure often contains significant amounts of calcium carbonate and can have a liming effect on acidic soils.
Calcium carbonate (lime) is a common additive to livestock diets. Manure can contain between one and four percent calcium carbonate, depending on the diet formulation. One way to account for the liming effect of manure is to monitor the pH of the soil over time. The pH of manured soils should increase, or become more basic. Used in conjunction with a good liming plan, the amount of agricultural lime needed can be reduced on manured fields.
A more direct way to determine the amount of calcium carbonate in manure is to test it for effective calcium carbonate (ECC), which is a commercial fertilizer test. Request an ECC test which will report the amount of calcium carbonate equivalent in the manure.
Potassium or Potash?
Potassium content in soil, plants, and animal rations is expressed as potassium (K) content but potassium in fertilizers and manure intended for land application is expressed as potash, or more correctly, potassium oxide (K2O).
- To convert K to K2O concentration, multiply by 1.2.
- To convert K2O to K, multiply by 0.83.
Liming effect of manure: Assume a feedlot manure sample test returns two percent calcium carbonate. If the manure was applied at 25 tons/acre and the manure was 20 percent moisture, the equivalent lime application would be 800 pounds (20 tons of dry manure/acre). In this example, one ton of dry manure contained 40 pounds of available lime. Therefore, if the producer's soil test showed a 2,500- pound lime requirement, he would only need a 1,700-pound application after the manure application.