Nutrient Loss Reduction Strategy Research Library

To examine agriculture throughout Illinois, we used data on Major Land Resource Areas (MLRAs) published in 2006 by the U.S. Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS). MLRAs are geographically associated land resource units based on climate, soils, and land use. There are 15 MLRAs in Illinois, but several have only a small area in the state. Therefore, we combined the state’s MLRAs into a total of nine as described in Table 3.5 and shown in Figure 3.17.  

Information on planted crop acres and yields were obtained from the USDA National Agricultural Statistics Service (NASS) survey data by county for 2008-2012 and then summed by MLRA (Table 3.6). More than 12 million acres of corn and about 9 million acres of soybeans were planted each year in Illinois. The third most planted crop was wheat, but there was less than 1 million acres planted annually. Using tile-drained acres per county from David et al. (2010), we estimated that 9.7 million acres in the state were tile drained, with most in MLRA groupings 2, 4, and 7. We assumed all drained acres were in corn and soybean production. Corn and soybean yields were greatest in northern and central Illinois and least in southern Illinois.

Fertilizer nitrogen usage per county averaged over the 1997-2006 time period was obtained from David et al. (2010). David et al. (2010) used annual state-level fertilizer sales, with county usage determined by using NASS Census of Agriculture county-level data on fertilizer, lime, and soil conditioner expenditures from 1997 and 2002 and by estimating other years by interpolation.

Because statewide nitrogen fertilizer sales were 4 percent greater in 2008-2012 compared to the David et al. (2010) average during 1997-2006, we increased our MLRA-specific estimates by this percentage. To estimate the amount of nitrogen fertilizer used per acre of corn, we assumed 100 lb/acre/yr was used on wheat and that the remainder was applied to corn. No reduction in non-farm nitrogen, which is a small percentage of nitrogen sales in Illinois, was assumed. Manure nitrogen data came from David et al. (2010) and were adjusted for pastured cattle since manure from this source is not applied to production crops. David et al. (2010) looked at overall nitrogen balances in a region, and the manure numbers included all cattle, both grain-fed and pastured. Cattle on feed data were obtained from NASS by county to adjust the manure values for pastured cattle. We assumed all remaining manure nitrogen was applied to corn and that all was plant-available. Depending on the manure type, only some of the nitrogen would likely be available in the first year of application. However, this assumption has only a small effect on estimated nitrogen rates, because manure amounts were low in most of Illinois. Row crops were defined here as the sum of corn, soybean, and wheat acres.

We then estimated nitrogen application rates for acres where corn and soybeans are rotated and acres with continuous corn by adjusting the total fertilizer nitrogen application in combination with the acres of corn and soybean during 2008-2012 (Table 3.7). We assumed that continuous corn is typically fertilized at a rate of 40 lb/acre/yr greater than a corn and soybean rotation (University of Illinois, 2012a). The recommended amount of nitrogen fertilizer for both was determined using the Maximum Return to Nitrogen (MRTN) calculator (Iowa State University, 2013) assuming a 10:1 ratio of corn price to nitrogen fertilizer price. For the central and northern Illinois MLRAs, the estimated fertilizer-plus-manure rate for corn/soybean or continuous corn was about the same or less than the MRTN, suggesting that Illinois farmers are on average applying the recommended nitrogen fertilizer rate for corn. For the southern Illinois MLRAs, the rate was well above the MRTN. However, there are few corn acres in two of the MLRAs where this was true (8 and 9), and the estimates are subject to large errors.

To obtain nitrate-nitrogen yields per row crop acre, we integrated non-point source nitrogen yields in each HUC 8 across each MLRA to determine the average load of nitrate-nitrogen. The overall nitrate-nitrogen loads were then divided by row crop acres to get the nitrate-nitrogen yield per row crop acre (Table 3.8). This analysis assumes no nitrate-nitrogen is lost from non-row crop agricultural lands (e.g., pasture). The resulting values ranged from 3.9 to 7.4 lb/acre/yr in southern Illinois and 19.6 to 31.3 lb/acre/yr in central and northern Illinois. This pattern can be explained by tile drainage in all MLRAs except MLRA 3, the northwestern corner of Illinois. This is a karst region with high livestock density, and the high nitrate-nitrogen yield may be explained by these factors. To develop the nitrate-nitrogen yield losses that will be used to determine the reduction in yields due to changes in management practices, we partitioned the nitrate-nitrogen yield per row crop acre into losses from tile-drained land and losses from non–tiled land in MLRAs 1, 2, 4, 6, and 7. These MLRAs have substantial tile-drained acres and larger nitrate-nitrogen yields as a result. Nitrate-nitrogen yields from tile-drained MLRAs ranged from 26 to 43 lb/acre/yr, whereas yields from non-tiled land ranged from 3.9 to 11.8 lb/acre/yr, with the exception of MLRA 3, the northwest corner of Illinois discussed earlier, which had a nitrate-nitrogen yield of 31 lb/acre/yr.

Data on fertilizer phosphorus usage per county came from Jacobson et al. (2011), which was a mass balance study that estimated phosphorus usage for every acre in a county using 1997-2006 crop year averages. We adjusted this rate by dividing the total phosphorus applied to a county by the sum of corn, soybean, wheat, and hay acres (Table 3.9). Data on manure phosphorus also came from Jacobson et al. (2011) and was adjusted in the same way as fertilizer phosphorus. In addition, data on cattle feed were obtained from NASS by county to adjust the manure values, allowing pastured cattle manure phosphorus to be subtracted from overall manure phosphorus. Fertilizer phosphorus application rates on cropland ranged from 11 to 14.9 lb/acre/yr, with little variation across the MLRAs, and manure phosphorus rates ranged from 1.2t 5.4 lb/acre/yr. The largest manure phosphorus rate was in MLRA 3 in northwestern Illinois, where there was a high density of livestock. Total phosphorus yields per row crop acre ranged from 0.68 to 2.82 lb / acre/yr, with greater losses in southern Illinois and the least in northeastern Illinois.

These data will be the basis for applying nutrient reduction practices by MLRA across Illinois.

 Table 3.5. MLRAs in Illinois combined into nine categories for this analysis. The bolded MLRA numbers will be used throughout our analysis.

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Table 3.6. Summary of agricultural management by MLRA in Illinois showing corn, soybean, and wheat acres; drained acres; and average corn and soybean yields, all averaged for 2008-2012.

Table 3.7. Estimated fertilizer and manure nitrogen application for corn/soybean rotations and continuous corn by MLRA in Illinois along with the recommended rates using the MTRN approach.

Table 3.8. Tile-drained cropland acres and nitrate-nitrogen yields per row crop acre along with yields divided into tile-drained and non-tile drained land by MLRA in Illinois.

Table 3.9. Phosphorus fertilizer and manure inputs, row crop acres, and total phosphorus yields per row crop acre by MLRA in Illinois.