Illinois Fertilizer
Conference Proceedings
January 21-23, 2002
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S.A. Ebelhar, E.C. Varsa, K. Robertson, T.D. Wyciskalla, C.D. Hart,
and J. Hamson1
One of the biggest problems with utilizing soil tests for fertilizer recommendations under avariable rate application system is the uncertainty over whether crops growing in the particular spot where the test is taken will yield the same as crops in other locations in the field. If a field averages 150 bu/acre corn, maintenancefertilizer recommendations call for replacement of the P and K removed by the crop across the whole field. In reality, fields have a wide range in yield (for example, from 90 to 210 bu/acre). Using yield maps to correctlyidentify where the high- and low-yielding areas (plus everything in between) are located would allow a more exact replacement of nutrients removed by a previous crop. Relying solely on soil tests for fertilizer recommendations tends to overfertilize low-yielding areas and underfertilize high-yielding areas. Combining yield mapping and soil testing would reduce the amount of overfertilizing of low-yielding areas and underfertilizing of high-yielding areas. This may prove to be economically as well as environmentally friendly.
The study had two objectives. The first was to compare variably applied fertilizer P and K to uniform P and K applications, both agronomically and economically, in a corn and soybean rotation. The second was to assess whole-field fertilizer P and K applications conventionally, based on soil tests and average field yields under a buildup/maintenance program, and also using VRT, based on soil tests and yield map history to build a recommendation map for P and K application.
Field studies were started in 2001 at two locations in southern Illinois. The first location, Irvington, was a 48-acre field with a variable soil test K history, and the second location, Belle Rive, was a 26-acre field with a variable soil test P and K history. The field at Irvington had such a high soil test P level that no P fertilizer was required; thus, there was no attempt to variably apply P at this location. At both locations, soybeans were grown in 2001.
There were three fertilizer application treatments. Treatment 1 (VRT-1) consisted of a variable fertilizer application based on a soil test map for buildup but whole-field average yield for maintenance. Treatment 2 (VRT-2) consisted of variable fertilizer application based both on a soil test map for buildup and normalized yield map for maintenance. Three years of previous crop history was normalized by expressing the yields as a percentage of the average yield for that crop that year. This normalized yield map was used as a productivity map to adjust yields higher or lower than the field average for the crop to be grown. For 2001, we assumed a yield level of 35 and 45 bu/acre of soybeans for Belle Rive and Irvington, respectively. Treatment 3 consisted of a uniform application of P and/or K based on a fieldaverage soil test and buildup/maintenance applied uniformly for the whole field. Table 1 shows the range of fertilizer P (as 0-46-0) and K (as 0-0-60) spread across the two locations and the field average soil test P and K levels.
A randomized complete block design was used with field-length strips of each of the three treatments serving as plots. Plot widths were determined by the width of the fertilizer application equipment (60 feet for our study). There were six replications at Belle Rive and eight replications at Irvington. Harvest was done with a combine equipped with a yield monitor and GPS. Yield maps were created using the yield monitor data.
Soil tests will be monitored yearly for changes in P and K levels, with samples being taken each year in early spring. However, only the first year's soil samples were used for determining the fertilizer recommendations and for the economic analysis.
The soil test K map (Figure 1) and normalized yield
map (Figure 2) were used to develop the variable rate
yield maps for VRT-1 and VRT-2 (Figure 3) for Irvington.
Both treatments had a large acreage (15.4 acres out of 48) that received no
fertilizer K in 2001. This is due to very high soil test K levels and our cut-off
of K application when soil test levels exceeded 360 lb/acre. There was not a
huge difference between the two VRT maps, an indication that yield variability
across the field was not great. This means that the yields across the field
were close to the field average yield (most were within 15 percent of mean).
Utilizing either the VRT-1 or VRT-2 method would have resulted in 900+ lbs less potash applied at the Irvington location (Table 2). The economics of this cost savings will be determined and discussed at a later date. Overall, there were no differences in grain yields for 2001 among the three fertilizer application treatments, even though there were some differences in yield maps (Figure 4, Figure 5, and Figure 6). Field average yields were 46-47 bu/acre across the three treatments.
The Belle Rive location had soil test P and K levels that indicated a need for additional fertilizer. The soil test P and K maps (Figure 7) and normalized yield map (Figure 8) were used to develop the variable rate yield maps for VRT-1 and VRT-2 (Figure 9 and Figure 10). Because there was not a large acreage at this site that needed no fertilizer P or K, the amount of fertilizer used was nearly equal among the fertilizer treatments (Table 2). There was not a huge difference between the two VRT maps either for P or K, an indication that yield variability across the field was not great. As with the site at Irvington, there were no differences in grain yields at Belle Rive for 2001 among the three fertilizer application treatments, even though there were some differences in yield maps (Figure 11, Figure 12, and Figure 13). Field average yields were 39 bu/acre across the three treatments.
When soil test levels are above the critical level where no fertilizer is needed for a large enough acreage, less fertilizer may be required using a VRT method of application. In the first year of our study, we found a fertilizer savings of about 900 lbs for a 48-acre field. Whether this is an economical cost savings has yet to be determined. At our other location, there was no difference in fertilizer application among the three treatments for either P or K. Yield differences were very small among the treatments. Soil test changes and impacts on grain yields will be determined throughout the next few years of this study.
Table 1. List of fertilizer P and K application ranges for Belle Rive and Irvington, 2001
Figure 1. Soil test K map, Irvington, 2001
Figure 2. Three-Year normalized yield map, Irvington, 2001
Figure 3. Variable rate K application maps for Irvington, 2001
Figure 4. Uniform fertilizer application yield map, Irvington, 2001
Figure 5. VRT-1 fertilizer application yield map, Irvington, 2001
Figure 6. VRT-2 fertilizer application yield map, Irvington, 2001
Figure 7. Soil test P and K maps, Belle Rive, 2001
Figure 8. Normalized yield map, Belle Rive, 2001
Figure 9. VRT-1 P and K application maps, Belle Rive, 2001
Figure 10. VRT-2 P and K application maps, Belle Rive, 2001
Figure 11. Uniform fertilizer application yield map, Belle Rive, 2001
Figure 12. VRT-1 fertilizer application yield map, Belle Rive, 2001
Figure 13. VRT-2 fertilizer application yield map, Belle Rive, 2001
1 S.A. Ebelhar is Agronomist, Dept. of Crop Sciences, Univ. of Illinois; E.C. Varsa is Professor, Dept. of Plant, Soil and General Ag., So. Illinois Univ.; K. Robertson is a farmer and private consultant; T.D. Wyciskalla is Researcher, Dept. of Plant, Soil and Gen. Ag., So. Illinois Univ., C.D. Hart is Visiting Research Specialist, Dept. of Crop Sciences, Univ. of Illinois; and J. Hamson is Precision Ag. Coord., Agripride FS.
