Effects of Swine Manure Applications on Soil Nutrient Levels and Phosphorus (AG-BU-08424) is a recently published bulletin that reports results of a study of liquid swine manure applications on 9 fields in southwest Minnesota. Fields have received manure every other year since 1998. Manure nutrient application rates and soil test levels for soluble salts, pH, phosphorus, potassium, sulfur, zinc, iron, manganese, copper, boron, calcium, and magnesium have been recorded each year. Beside tracking impacts on soil test levels, the study uses the Minnesota P Index to predict phosphorus losses from these fields to nearby water bodies.
The study found that the fields under study posed minimal risk for losses of phosphorus.
The bulletin provides detailed information on the annual manure application rates and resulting soil test levels. Listed below are some summary observations.
Monitoring sites observations:
- Soil test P did not increase linearly. In fields originally testing low for P, soil test P increased significantly after the first few manure applications and then increased only slowly from subsequent applications. In fields originally testing high for P, soil test P increased only slowly from manure applications.
- Per pig nutrient production estimated from manure test and gallons produced was similar to daily production estimates from the Midwest Plan Service.
- The proportion of ammonium to organic N was higher than typical values, making the total N slightly more available in the first year than commonly estimated.
- Manure application rates have trended downward over the observation period because of improvements in and adoption of manure application technology including the use of a tanker in place of a drag hose and improved controller equipment.
- With lower application rates (3,500 gallons/acre) and the use of dietary phytase, manure P application will not exceed two-year crop removal estimates for P.
- Trace mineral levels in the soil have not changed significantly over the observation period.
Phosphorus Index observations:
- Based on the MN P Index, the monitoring sites pose minimal risk of P delivery to the nearest water bodies.
- P loss risk from swine manure applications primarily depends on the rate and method of application. P loss risk can be minimized by controlling erosion, avoiding winter manure applications, limiting rates for any single manure application to the equivalent of about 200 lbs P2O5/ac, and limiting total applications over several years to amounts that prevent buildup of excessive soil P. The limit on single application rates will be lower if manure is not incorporated and in places with higher runoff, such as on soils with poor infiltration or in higher-rainfall regions of the state.
- Soil test P increases are moderate at N-based manure application rates.
- Soil test P levels depend on soil type. The elevated calcium carbonate levels in many soils of southwest Minnesota tend to buffer soil against soil test P increases. Although less available to plants, the P is still in the soil and can be carried via erosion to surface water.
- The MN P Index is useful for assessing the relative risk of P loss associated with animal agriculture in Minnesota. The Minnesota Phosphorus Index allows users to examine the effect of multiple P loss risk factors to estimate site-specific risk.
Authors of the bulletin are Bob Koehler, University of Minnesota Southwest Research & Outreach Center, and Ann Lewandowski and John Moncrief of the University of Minnesota Department of Soil, Water, and Climate.
The bulletin is available in several places, select what is most convenient for you:
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