Crop Nutrient Assimilation Capacity

Crop nutrient assimilation capacity was based on estimated nutrient removal or fertilizer need for all row crop acres suitable for manure application based on current MDNR regulations. These restrictions made 67,900 acres (over 20%) of cropland acres unsuitable for manure application. Total assimilation of the county was estimated to be 33,170,000 lbs. N and 10,254,000 lbs. P₂O₅.

Cropland and crop type:

Define cropland capable of receiving manure as land designated cropland (row crops) by Landstat imagery. The Landstat image estimated 291,616 acres of row cropland in the county. Missouri Agricultural Statistics estimated row-crop production on an average of 265,590 acres from 1989-1996 in Saline County (Table 1).

Landstat imagery does not differentiate between different row crops. We assumed that all land in row crop was 40% corn, 47% soybean, and 13% wheat based on the 8-year means in Table 1. Sorghum accounted for less than 1% of row cropland in the county and so was not included.

Landstat imagery categorizes 125,850 acres as rural grassland. This imagery cannot differentiate between hay, pasture, and fallow land. Missouri Agricultural statistics only track hay harvest which averaged 24,900 acres in the county from 1989-1996 (Table 1). In this analysis we assumed manure only was applied to row cropland. This will underestimate land available for manure application in the county.

Table 1. Acres of major row crops and hay in Saline County from 1989 to 1996 based on Missouri Farm Facts published by Missouri Agricultural Statistics Service.

Cropland capacity to use nitrogen and phosphorus:

Nitrogen application rate for corn and wheat cropland were calculated based on University of Missouri soil test recommendations (Table 2). Soybean nitrogen application rate was based on its nitrogen removal capacity (Table 2). Legumes, such as soybean, do not require fertilizer nitrogen to optimize yield but will use applied nitrogen in lieu of nitrogen fixation if it is applied. Phosphorus application rate for all crops was based on the nutrient removal capacity of the crops (Table 2).

Cropland capacity to use nitrogen or phosphorus was calculated as a weighted average of the three crops based on % crop types outlined in the previous section. Table 3 reports nitrogen and phosphorus removal rate used for selected agricultural soils in Saline County.

Table 2. Equations used for nitrogen and phosphorus need/removal capacity of row cropland.

 Table 3. Calculated capacity of corn, soybean, and wheat to use nitrogen and phosphorus for selected agricultural soils in Saline County and the weighted average used in the analysis.

Yield goal is a critical component of all estimates of land capacity to use nutrients (Table 2). We determined yield goal corn, soybean, and wheat for each soil type in the county using the productivity index developed by NRCS increasing the yield on the benchmark soil. This used the index approach of Soil Survey to establish differences in yield potential among different soil types while updating the reported yields to levels closer to those anticipated by farmers in the county.

The increase is necessary because of weaknesses in the Soil Survey for providing current yield estimates. Evidence for the need to adjust Soil Survey yield is provided in University of Missouri corn and soybean crop variety trials in Saline County (Table 4). Mean yield for the represented soil types exceed the Soil survey tabular values by 44 bushels for corn and 16 bushels for soybean, increases of 33 and 39% respectively above the Soil Survey tabular values. Table 5 compares the Soil Survey yield and the yield used for some common agricultural soils in Saline County.

The Soil Survey estimates have value for determining the relative productivity of different soil types in the county but are likely to underestimate expected yields in the county. The values in the most current soil survey for Saline County (1993) were based on an index calibrated in 1989. Over the intervening 10 years yield capacity of row crops will increase through improvements in plant genetics and management. Yield increases for a 10-year period would be estimated to be 18 bu/acrefor corn, 4 bu/ac for soybean, and 6 bu/acre for wheat based on historic trends.

Fertilizer use in the county over the past five years has averaged less than the assimilation capacities we calculated (Table 6). This is expected for nitrogen because soybean has the capability of assimilating manure N but would not receive commercial fertilizer N. Corn and wheat represent 53% of the cropland in the county; 53% of calculated nitrogen assimilation capacity is 17,560,000 lbs. However our assimilation estimate does not account for the 20% of the cropland that cannot receive manure. Our nitrogen assimilation estimate may be on the high side. Reasons for a high nitrogen estimate may be our inability to account for legume nitrogen credits and manure nitrogen credits. Reasons for a higher calculated phosphorus assimilation than fertilizer use is soils with high soil test have the capability to remove phosphorus but may not receive purchased fertilizers.

Table 4. Acres of major row crops and hay in Saline County from 1989 to 1996 based on Missouri Farm Facts published by Missouri Agricultural Statistics Service.

 Table 5. Comparison of adjusted yield for corn, soybean, and wheat used for yield goal calculation with 1993 Soil Survey of Saline County yields.

 Table 6. Purchased fertilizer nutrients in Saline County based on Missouri Fertilizer Tonnage Report, University of Missouri and calculated nutrient assimilation capacity of the county.

 Regulatory restrictions
Slope: Eliminate all land with a slope greater than 10%.

Separation distances: Create buffers of this size from the outer edges of the following features (remove for land application).

- Losing streams, sinkholes, caves, wells, abandon wells, water supply structures, 300 feet.
- Permanent flowing streams, 150 feet.
- Intermittent flowing streams, 50 feet.
- Property lines (when available), 50 feet.
- Dwellings or public use areas (including roads), 100 feet.

Flooded ground: Remove land that has that has a flooding frequency of frequent in the soil and water features of the Soil Survey of Saline County.

Minimum field size:

The nature of landstat imagery is small areas are sometimes erroneously identified as being crop land. In addition the application of buffers, etc may create small parcels that may not be appropriate for manure application. Remove contiguous crop areas smaller than 5 acres.

References:

1990 to 1997 Missouri Farm Facts. Missouri Agricultural Statistic Service.

1993 to 1997 Missouri Corn Crop Performance. Agriculture Experiment Stn., Univeristy of Missouri-Columbia.

1993 to 1997 Missouri Soybean Crop Performance. Agriculture Experiment Stn., Univeristy of Missouri-Columbia.

Genetic contributions of yield to yield gains of five major crop plants. 1984. W.R. Fehr, ed. Crop Science Society of American, Madison, WI.

Guide to Animal Feeding Operations. 1997. Missouri Department of Natural Resources.

Missouri Soybean Field Guide, University of Missouri, Columbia, Missouri.

Productivity of Missouri Soils. Natural Resource Conservation Service.

Soil Survey of Saline County, Missouri. 1993. USDA Soil Conservation Service.

Soil Test Interpretations and Recommendations Handbook. 1992. University of Missouri, Columbia, Missouri.