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Manure Management Can Be Sticky – Part Two

Written by Meryl Rygg McKenna

Editor’s Note: This is part two of a two-part article. Look for the first installment in the April 23 Roundup.

In the right place and at the right time, manure can optimize crop yield and quality. Conversely, in the wrong place or used at the wrong time, manure’s nutrients can be considered pollution and have health and economic impacts to people, business sectors including agriculture and the environment in general.

Good management prevents phosphorus (P) runoff or nitrogen (N) leaching into ground and/or surface waters.

Concerns with manure

Land where manure has been repeatedly applied often has high-test levels of P, potassium (K) and micronutrients. While nitrogen is highly water soluble and can be washed off fields or leached through the soil by rainfall or irrigation, phosphorus and other nutrients attach to soil particles and tend to remain in the soil unless washed off fields by water erosion. Build-up of P may pose a threat to water quality and could have negative interactions with other nutrients, according to Tommy Bass, Extension associate specialist who advises on manure management.

Too much manure is often applied near feedlots, due to the cost of transporting it farther away from its source. This means fields near the source often already show high levels of P and K. Land that has not had much manure applied to it would benefit more overall, where manure N, P and K can be beneficial, providing a yield response from all nutrients.

Nitrogen leaching into groundwater is also a concern, especially in sandy or gravelly soils. Nitrate levels that can adversely affect humans or livestock have been detected in wells in Montana. According to Patrick Hensleigh, agronomist with the USDA Natural Resources Conservation Service (NRCS) Ecological Services in Bozeman, the NRCS uses a nitrogen risk assessment based on soils, precipitation or irrigation, management and other factors to assess the risk of nitrogen leaching from a field or farm.

Some growers believe that applying manure to alkali areas will improve the soil. However, Fehringer said repeated heavy applications of manure raise the soil test salinity level because manure contains salts. Adding it to areas that are already salty will make the situation worse. This happens more under pivot irrigation and on dryland fields, where leaching of salts is limited.

Soil compaction

Another concern Fehringer noted is that spreading manure can compact the soil, which is not good for plant growth. The type and speed of equipment used for spreading manure and the moisture content in the soil influence the extent to which the soil will be packed down. One option to minimize compaction is to load the manure into end-dump semi-trailers, dump it at the edge of a field, and load it from there onto a tractor-pulled manure spreader.

Whether by tractor or truck, spreading manure will likely leave tracks or ruts, which may require tillage to even out the soil. Spreading manure when the soil is dry will greatly lessen compaction.

Lastly, uniform application of manure is critical, just as a grower wants the fertilizer company to spread the whole field and not leave skips. Soil sampling after non-uniform application gives inaccurate test results of the situation in the field. Thus, crop yields will suffer. Sampling is best done shortly before application of manure.

Value of manure

Using a recent manure analysis of 15 pounds N, nine pounds P2O5, 18 pounds K and three pounds of sulfur (S) in a ton of a manure, and a recent fertilizer dealer cost of $0.40 per pound N, $0.50 per pound P2O5, $0.33 per pound K2O and $0.28 per pound S, Hensleigh estimated the manure’s value at $17 per ton. This estimate does not include the value of micronutrients or the increase of soil organic matter.

Fehringer noted, “When I have applied a value per unit of N, P and K contained in manure, they cost about 50 cents on the dollar versus buying them in commercial fertilizer. The major cost of manure is transporting and spreading it.”

The lower costs of N, P and K in manure hold true only if the soil needs those nutrients. When Olsen soil test P levels are over 60 parts per million (ppm), Fehringer quits having growers apply any commercial P. When K levels are over 400 ppm, he stops having growers apply K. So, if soil test P and K are above these levels and the grower applies manure, then the value of P and K from the manure is zero because those nutrients provide no additional benefit.

The nitrogen, organic matter and micronutrients are still beneficial, however. Since P tends to stay in the soils, it can build up to extremely high levels if not monitored. Fehringer said he has seen P test results as high as 435 ppm on a field that has received manure at high rates for over 30 years.

High soil P levels place an exclamation point on the need for a manure and nutrient management plan where nutrients can be allocated to fields where the needs are greatest and the grower can gain the greatest financial benefit, according to Karen Hoffman, NRCS state water quality engineer. Fields that have high P, K and micronutrient levels require only commercial nitrogen. With commercial fertilizer, producers only apply the nutrients that are needed. Bass added that applying manure at P rates and then rounding out N with a commercial product is also a sound strategy.

With so many economic, agronomic and management factors to consider, as well as the potentially daunting length and breadth of the permitting process, a conversation with a knowledgeable and trusted crop adviser may be the best place to begin.

For more information on certified crop advisers or to find one near you, go to certifiedcropadviser.org.