Fall 2000

Sampling and agitation of swine manure pits

by Jeffrey Lorimor, Department of Agricultural and Biosystems Engineering, and Kris Kohl, Northwest Iowa extension ag engineer

Manure experts recommend sampling your manure pit to get the best estimate of the actual nutrient content, rather than using book values. But producers know that the manure is not the same from the top to the bottom of pits, and they question how large the changes in concentration will be from the first load to last. Three recent studies done at Iowa State University examined sampling accuracy and how well agitation homogenizes manure.

The first study compared nutrient content in the top, middle, and bottom of manure pits from 278 samples taken across the state. Table 1 shows the results of the pit sampling. The last row of the table, labeled vertical profile, shows data from samples that were taken by inserting an open polyvinyl chloride tube to the bottom of the pit, to get an average sample throughout the depth of the pit.

Stratification of total Kjeldahl nitrogen (TKN) and phosphorus (P₂O₅) is evident in the bottom samples. Ammonia (NH₃-N) and potassium (K₂O) are uniform from top to bottom. As the last two lines of Table 1 show, a vertical profile sample provides an accurate estimate of the top-middle-bottom average.

In the second study, for 109 of the 278 samples, manure dipped off the surface of pits was compared with average vertical profile values from the same pits. A sample dipped from the top of a pit and tested for TKN averaged 95 percent of a vertical profile TKN from the same pit. On the average, a top-dipped sample was within 5 lb/1000 gal of the pit average. Analyzing the top-dipped sample for ammonia, and then adjusting it to provide a TKN estimate, did not work nearly as well. The variation was greater. The top-dipped sample tested for ammonia (and adjusted by a fudge factor) could only be expected to be within 9 lb of the actual profile TKN. Top sampling did not work for P₂O₅. An error of 11 lb/1000 gal could be expected.

In the third study, the first, middle, and last loads of six separate pits were sampled in northwestern Iowa as the pits were pumped after being vigorously agitated. Agitation varied from a minimum of one 85-horsepower pump running for 4 hours up to two 150-horsepower pumps running for 4 hours. Table 2 shows nutrient concentration results. The P₂O₅ showed some increased concentration from the middle load to the last load. Other nutrients, however, were uniform throughout the pumping process. These data show that with good agitation, uniform manure concentrations can be achieved.

Summary. This research indicates that producers can accurately estimate manure nitrogen and potassium levels and handle the manure so that the nutrient concentrations are uniform. For best results, collect a vertical profile sample. However, samples that are dipped off the top of swine pits and tested for TKN provide accurate estimates of average TKN values in the pit. Testing a top-dipped sample for ammonia, however, does not provide reliable information about TKN concentrations. With good agitation, the average TKN value can be expected on all loads from first to last.

P₂O₅ is the most difficult to sample. It concentrates in the bottom of pits and requires a vertical profile sample to get a good estimate. And it is still subject to higher concentrations late in the pumpout process, even with vigorous agitation.

Table 1. Manure nutrient samples at the top, middle, bottom, and vertical profile of swine pits.

T, top; M, middle; B, bottom.

Table 2. Average values from six pits before and during agitation and pumpout.

^aThe profile sample was taken before agitation.

Ames, Iowa 50011

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Iowa Manure Matters: Odor and Nutrient Management is published by Iowa State University Extension, with funding support from the USDA Natural Resource Conservation Service through Cooperative Agreement No. 74-6114-8-22. To subscribe or change the address of a current subscription, write to Angela Rieck-Hinz, 2104 Agronomy Hall, Iowa State University, Ames, Iowa, 50011-1010 or call 515-294-9590, fax 515-294-9985 or email: amrieck@iastate.edu. Please indicate you are inquiring about the Odor and Nutrient Management Newsletter. The newsletter's coordinators are Angela Rieck-Hinz, extension program specialist, Department of Agronomy, Wendy Powers, environmental extension specialist, Department of Animal Science, and Robert Burns, Department of Agricultural and Biosystems Engineering; the editor is Jean McGuire, the subscription manager is Rachel Klein, the production designer is Beth Kroeschell, and the web page designer is Liisa Jarvinen.