Illinois Fertilizer
Conference Proceedings
January 26-28, 2004
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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 of corn, then 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 (plus everything in between) areas 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 over-fertilize low-yielding areas and under-fertilize high yielding areas. Combining yield mapping and soil testing would reduce the amount of over-fertilizing of low yielding areas and under-fertilizing of high yielding 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, thus there was no attempt to variably apply P at this location. At both locations, soybeans were grown in 2001 and 2003, with corn grown 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 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 and 2003, 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. Soybeans (Asgrow 4702 variety) were planted on June 19 at Irvington and soybeans (FS 4426) was planted at Belle Rive on June 22. Planting was delayed by above average rainfall in May at both locations.
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.
The soil test K map and normalized yield map (Figure 1) were used to develop variable rate yield maps for VRT-1 and VRT-2 (Figure 2) for Irvington. Both treatments had a large acreage (15.4 acres out of 48) which received no fertilizer K in each of the three years. 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% 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 2003, 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 2003 (similar to 2001 and 2002, please see prior years’ reports) among the three fertilizer application treatments even though there were some differences in yield maps (Figure 3). Field average yields were 39–44 bu/acre across the three treatments, which is relatively better than expected considering the late planting. Because this study was laid out in replicated strips, further evaluations of the data can be made by analysis of variance procedures. Differences among treatments were less when compared by this method (Table 3) and there were no significant treatment effects. Over the three-year period, the VRT-1 treatment has had the lowest relative yields, but not significantly different than the other treatments.
The Belle Rive location had soil test P and K levels which indicated a need for additional fertilizer. The soil test P and K maps and normalized yield map (Figure 4) were used to develop the variable rate yield maps for VRT-1 and VRT-2 (Figures 5 and 6). Because there was not a large acreage at this site which needed no fertilizer P or K, the amount of fertilizer used was nearly equal among the fertilizer treatments 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.
Just like the site at Irvington, there were no differences in grain yields at Belle Rive for 2003 among the three fertilizer application treatments even though there were some differences in yield maps (not shown). The yield monitor at Belle Rive lost signal about half way through the harvest so only partial maps can be derived. We were able to save four of the six replicated strips, however. Field average yields were 24–27 bu/acre across the three treatments (Table 3), reduced somewhat by late planting and by poor growing conditions in 2003.
Soil test P and K levels have been monitored at each location with yearly samples taken in the spring prior to application of fertilizer treatments. Soil test levels from these samples have been summarized by breaking the data into categories of soil test levels ranging from very low to very high (Figure 7) as outlined in the Illinois Agronomy Handbook. At Irvington, soil test K levels with Uniform K application show an increase in frequency of samples >360 lb K/acre (very high). This indicates that a uniform application applies too much fertilizer to already high levels driving them higher. The VRT-2 application is doing what it is supposed to do, building up the frequency of samples in the 260–360 range. At Belle Rive, where P levels were relatively low, all of the treatments decreased the frequency of samples <20 lb P/acre, and both the Uniform and VRT-2 treatments are increasing the frequency in the 38-50 range, but none appear to be increasing the frequency in the 50–70 range. Soil test K levels are predominantly in the 100–200 lb K/acre range for all of the treatments but levels are increasing in the 200–260 range, most favorable in the VRT-1 treatments, but levels do not appear to be increasing as expected, an indication that our K rates are too low or that the handbook rate of 4 lb K20/lb soil test K to build-up K levels is not adequate.
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 and third years of our study, we found a fertilizer savings of about 900 lbs per year 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 show a favorable response to fertilizer additions, with the VRT-2 treatment appearing to be bringing the field up to uniform soil test levels, but slower than expected.
Table 1. List of fertilizer P and K application ranges for Belle Rive and Irvington, 2001–2003.
Table 3. Strip trial grain yields, 2001–2003.
Figure 1. Soil test K map and normalized yield map, Irvington.
Figure 2. Variable rate K application maps for Irvington, 2003.
Figure 3. Yield maps, Irvington, 2003.
Figure 4. Soil test P and K, and normalized yield maps, Belle Rive.
Figure 5. VRT-1 and VRT-2 P application maps, Belle Rive, 2003.
Figure 6. VRT-1 and VRT-2 K application maps, Belle Rive, 2003.
Figure 7. Changes in soil test levels over years for Irvington and Belle Rive.
1 S.A. Ebelhar is an agronomist, Dept. of Crop Sciences, Univ. of Illinois; E.C. Varsa is a professor, Dept. of Plant, Soil and General Ag., So. Illinois Univ.; G.K. Robertson is a graduate student; T.D. Wyciskalla is a researcher, Dept. of Plant, Soil and Gen. Ag., So. Illinois Univ., C.D. Hart is a research specialist, Dept. of Crop Sciences, Univ. of Illinois; and J. Hamson is a precision ag. coord., Agripride FS.
