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Extension > Agriculture > Dairy Extension > Manure > Grid soil sampling for small and medium livestock farms: Case studies in Minnesota

Grid soil sampling for small and medium livestock farms: Case studies in Minnesota

Les Everett, University of Minnesota Water Resources Center; Duane Starkey and Jose Hernandez, University of Minnesota Extension

Published in Dairy Star October 27, 2012

The economic and environmental benefits of grid soil sampling and precision nutrient application are not limited to crop producers with access to variable rate manure and fertilizer technology. Grid soil sampling can be used to guide conventional manure application in a way that can optimize the economic value of manure as a fertilizer replacement, produce more uniform yields, and reduce phosphorus in runoff to surface water. Case studies, listed below, are actual livestock farms in southeast Minnesota that compare manure application strategies using grid soil test maps of phosphorus (P) and potassium (K).

Livestock farms often have fields where manure was not applied uniformly in the past, resulting in areas with very high soil test P and K levels and other areas with low values. This can be a problem both for crop yields in low testing areas and unnecessary application of nutrients in high testing areas. Grid soil sampling provides a map of soil test values that can be used to target manure application, increasing its economic value. Targeting manure application can gradually reduce soil test values in the excessively high areas, reducing P in runoff. It can also build soil test values in very low testing areas, especially if supplemented with variable rate fertilizer application, resulting in improved yields.

A set of case studies found at www.manure.umn.edu represents a variety of livestock and manure types. Crop nutrient needs for one or more fields, manure quantity, manure nutrient composition, and nutrient availability are shown for each farm. Crop nutrient needs, obtained from tables in University of Minnesota Extension bulletins, are based on crop rotation, soil test values, and expected yields. Manure nutrient content is determined in these studies by manure testing.

While these case studies focus on the economic and environmental benefits of grid soil sampling, other management factors are also discussed for reducing P loss to streams, rivers and lakes. Manure application method (surface vs. subsurface) and timing affect how much P is available at the soil surface for runoff to carry in solution or attached to eroded particles. On sloping soils, eroded sediment is often the largest source of P in runoff, so soil conservation practices are important in keeping P out of water bodies as well as conserving soil. For case studies where soil erosion was a likely risk, the Minnesota Phosphorus Index with its included RUSLE2 erosion calculator was used to estimate potential loss of sediment and P. It is available at www.mnpi.umn.edu.

The case studies are divided into two groups. The Primary case studies are easier to analyze and draw conclusions from, while the Supplemental cases are more complex with less clear answers. All of the cases are real situations that farmers and their advisors experience in the field.

The economic comparisons among manure application strategies are based on the fertilizer substitution value of manure nutrients. Comparisons are made using the spreadsheet "What's Manure Worth?" MANURWKST.XLS (1.05 MB XLS), available at z.umn.edu/manureworth. Manure value is expressed both per acre and per volume or weight of the manure as applied under each strategy. Since one objective is to maximize the total value of the available manure, the value per volume or weight is emphasized.

Take a look at these case studies and consider how grid soil sampling may benefit your pocketbook and your environment by making better use of manure on your farm.

Primary cases

Case name No. Total manure vol./weight Field size & slopes Crop rotation Manure application strategies
Dairy farm #1: liquid manure 1 750,000 gal 50 acres 4 years alfalfa
4 years corn silage
N-based whole-field
N-based zonal
P-based zonal
Hog farm #1: liquid manure 3 1,000,000 gal 276 acres
1-18% slopes
2 years corn
1 year soybeans
N-based whole-field
N-based zonal
P-based zonal
Purchased poultry manure 7 200 tons 50 acres
3 fields
2-17% slopes
continuous corn (2 fields)
corn followed by alfalfa (1 field)
N-based, immediate incorporation, corn;
N-based, delayed incorporation, corn;
P-based corn;
P-removal 1st year alfalfa
Dairy farm #3: solid manure 8 1,000 tons 224 acres
3-12% slopes
1 year corn silage
1 year corn grain
1 year soybeans
N-based whole field
N-based zonal
P-based zonal

Supplemental cases

Case name No. Total manure vol./weight Field size & slopes Crop rotation Manure application strategies
Dairy farm #2: liquid manure 2 1,750,000 gal 95 acres
1-12% slopes
4 years alfalfa
4 years corn silage
N-based whole field
N-based zonal
P-based zonal
Hog farm #2: liquid manure 4 1,000,000 gal 157 acres
1-6% slopes
High P
2 years corn
1 year soybeans
N-based whole field
N-based zonal
P-based zonal
Beef farm #1: solid manure 5 5,000 tons 189 acres
2-12% slopes
High P
2 years corn
1 year soybeans
N-based whole field
N-based zonal
P-removal whole field
P-removal zonal
Beef farm #2: solid manure 6 1,200 tons 86 acres
3 fields
2-17% slopes
1 year corn
1 year soybeans
N-based whole field
N-based zonal
P-based zonal
P-removal whole field
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