Illinois Fertilizer
Conference Proceedings
January 27-29, 2003
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S.A. Ebelhar, E.C. Varsa, G.K. Robertson, T.D. Wyciskalla, C.D.
Hart, and J. Hamson1
One of the biggest problems with utilizing soil tests for fertilizer recommendations under a variable-rate application system is the uncertainty that the crops growing in that particular spot where the test is taken yields the same as the crops in other locations in the field. If a field averages 150 bu/acre corn, maintenance fertilizer 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 correctly identify 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 highyielding areas. This may prove to be economically as well as environmentally friendly.
The objectives of this study were to:
1) compare variably applied fertilizer P and K to uniform P and K applications, both agronomically and economically, in a corn and soybean rotation, and
2) assess whole-field fertilizer P and K applications (a) conventionally, based
on soil tests and average field yields under a buildup/maintenance program,
and (b) 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, and so there was no attempt to variably apply P at this location. At both locations, soybeans were grown in 2001 and corn in 2002.
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 a variable fertilizer application based both on a soil test map for buildup and normalized yield map for maintenance. Three years of previous crop history were 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. For 2002, we assumed a yield level of 115 and 150 bu/acre of corn for Belle Rive and Irvington, respectively.
Treatment 3 consisted of a uniform application or P and/or K based on a field-average 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 performed with a combine equipped with a yield monitor and GPS. Yield maps were created using the yield monitor data.
Soil tests are being monitored yearly for changes in P and K levels, with samples
taken in early spring each year. However, only the first year's soil samples
were used for determining the fertilizer recommendations and for the economic
analysis.
Irvington site
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 or 2002. This is due to very high soil test K levels and our cutoff 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 total potash applied at the Irvington location (Table 2) in 2001 and 575+ lbs less in 2002. The economics of this cost savings will be determined and discussed at a later date. Overall, there were no differences in grain yields for 2002 (similar to 2001; please see last year's report) among the three fertilizer application treatments, even though there were some differences in yield maps (Figures 4, 5, and 6). Field average yields were 68 to 73 bu/acre across the three treatments, which is considerably lower than expected. Wet weather at planting (creating poor stands) and very dry weather during the summer severely reduced grain production at this location in 2002.
Belle Rive site
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 (Figures 9 and 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 in both years (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 was the case with the site at Irvington, there were no differences in grain yields at Belle Rive for 2002 among the three fertilizer application treatments, even though there were some differences in yield maps (Figures 11, 12, and 13). Field average yields were 82 to 88 bu/acre across the three treatments, again reduced by poor growing conditions in 2002.
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 of soybeans and a savings of 575 lbs for
corn. 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.
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, 2002
Figure 4. Uniform fertilizer application yield map, Irvington, 2002
Figure 5. VRT-1 fertilizer application yield map, Irvington, 2002
Figure 6. VRT-2 fertilizer application yield map, Irvington, 2002
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, 2002
Figure 11. Uniform fertilizer application yield map, Belle Rive, 2002
Figure 12. VRT-1 fertilizer application yield map, Belle Rive, 2002
Figure 13. VRT-2 fertilizer application yield map, Belle Rive, 2002
Table 1. List of fertilizer P and K application ranges for Belle Rive and Irvington, 2001 - 2002
Table 2. Treatment effects on amount of triple super phosphate (TSP) and potash that were applied to each field and grain yields, 2001 - 2002 1 S.A. Ebelhar is an agronomist,
Department of Crop Sciences, University of Illinois; E.C. Varsa is a professor,
Department of Plant, Soil and General Ag., So. Illinois University; G.K. Robertson
is a farmer and private consultant; T.D. Wyciskalla is a researcher, Department
of Plant, Soil and Gen. Ag., So. Illinois University, C. D. Hart is a visiting
research specialist, Department of Crop Sciences, University of Illinois; and
J. Hamson is precision ag. coord., Agripride FS.
