Illinois
Fertilizer Conference Proceedings
January 24-26, 2000
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E.C. Varsa, S.A. Ebelhar, S.K. Chong, S.J. Indorante,
and T.D. Wyciskalla1
Variable Rate Technology (VRT) and associated application systems provide a means of assuring that fertilizer applications are made only in amounts and locations where they are needed (Wollenhaupt et al., 1993). The basic logic of VRT fertilizer application is to increase fertilizer inputs to areas of high productivity and decrease fertilizer inputs to areas of low productivity (Smith, 1998). Agronomically, variable-rate systems provide a means of targeted application of fertilizer based upon detailed soil tests and other related databases. Economically, variable-rate systems allow fertilizer dollars to be spent on areas within a field where they will provide a response and to be saved where a response is unlikely. Environmentally, variable-rate systems help to prevent over-application of fertilizer where it could result in environmental problems. Research into successfully implementing VRT is in its developmental stages (Francis et al., 1996; Schefcik and Pete, 1996), and some economic benefits have been reported.
Improving efficient crop production through the use of VRT has received widespread attention in the fertilizer industry and farmer/producer publications. Advances in computer software technology and improvements in fertilizer application equipment that reliably delivers what is desired when and where it is desired is now available. However, considerable uncertainty abounds as to just how to fully utilize VRT in the management of fertilizers across farm fields. The introduction of VRT and yield monitors allows producers the opportunity to evaluate the effects of varying input rates on grain yield and, ultimately, profits. In order to fully utilize these technologies, information gained from small plot research must be adapted to farm field scale situations (Harrington et al., 1997).
A goal of these studies is to help fertilizer dealers and farmers better understand the interrelationships between soil chemical properties (soil tests and crop responses) and soil physical properties. In southern Illinois, field yields of corn and soybeans are closely related to the available water-supplying power of the soil. In drought years, field yield patterns can be closely tied to the presence of naturally occurring claypan layers that restrict root development for nutrients and water. Kitchen et al. (1996) reported that the depth to a claypan, as assessed by electromagnetic conductivity (EM) readings, closely correlated with crop yield response, especially in dry years. Additionally, soil management practices may also cause variability to the extent that identical land units (soil types) may act quite differently from one another when subjected to different management practices (Bouma and Finke, 1993).
Two southern Illinois farm fields have been identified with distinct variations throughout in terms of morphological properties but with somewhat uniformly low levels of soil test potassium. Soil sites within each field (called grid cells) with a wide range in depth to the claypan will be used as sites for crop response studies to increasing rates of K application.
The objectives of this research were to:
Evaluate Variable Rate Technology (VRT) as a management tool for K fertilization of corn and soybean on soils with a range in depth to the claypan.
Use grid sampling to determine critical need areas of each field for K fertilization on soils with varying depths to the claypan.
Determine the effect of K rates within field cells (squares) on K uptake and grain yield of corn and soybeans.
Compare the yield responses to K fertilization across grid cells (squares) that differ in depth to claypan and other soil properties.
Make preliminary assessments as to the most likely soil-site conditions that would be the most crop-responsive to variably applied K fertilizers.
The same two commercial farm fields that were utilized in 1997 and 1998 (one in Jefferson County, IL and the other in Pope County, IL) were utilized for the 1999 growing season for this research. Corn was grown using conventional tillage practices at the Jefferson County site in 1999, following soybeans in 1998. The Pope County site was planted to no-till soybeans in 1999, with the previous crop being no-till corn.
The Jefferson County site was planted on May 26, 1999, and the Pope County site was planted on May 10, 1999. Both locations had excellent seedling emergence, even though the Pope County site was planted in somewhat wet and cool soil conditions, which delayed emergence to approximately nine days after planting.
The same grid cells, 10 at the Jefferson County site and nine at the Pope County site, were used for the intensive variable K rate evaluations in 1999. Potassium rates of 0, 40, 80, and 120 lb K2O per acre as suspension grade muriate of potash (0-0-62) were applied in a 4-by-4 Latin Square design (4 K rates x 4 replications). The individual plots, 15 feet wide by 30 feet long, were established in the same areas as the previous year. The K rate treatments were applied as a broadcast spray two days after planting at the Jefferson County site and eight days after planting at the Pope County site but before emergence at both locations.
Corn ear-leaf samples from the Jefferson County site and soybean trifoliate-leaf samples from the Pope County site were collected from each plot to assess K concentrations in the plant tissue as a function of site and K treatment effects. For harvest, each individual plot had the center two rows by 20 feet harvested for grain yield and moisture content. The Jefferson County plots were harvested on September 30, 1999, and the Pope County plots were harvested on October 27, 1999. After individual plots were harvested, the whole field was later harvested with a combine equipped with a yield monitor and GPS guidance system for whole-field yield variability assessment.
Following harvest at each location, surface soil samples (0-6 inch) were collected from each individual plot for final soil fertility assessment marking the end of this three-year study. All analyses are not completed, but preliminary data show soil test pH and P to be within sufficient ranges, whereas the soil test K values (which were "low" at the beginning of this study) to be "building up" on those plots that received higher rates of applied K2O. In the fall of 1999, georeferenced soil electrical conductivity measurements were collected using the Veris EC 3100 at both locations. Data are preliminary, but there is an apparent correlation between soil electrical conductivity, topography, soil moisture, and associated crop yields. Quantification of these relationships is continuing.
At the Jefferson County location, application of fertilizer K resulted in an overall increase in the corn ear-leaf K concentrations such that a positive linear response was observed (Table 1). Six of the 10 grid cells showed a positive linear response to applied K, while the remaining four cells were nonsignificant. Potassium composition in the tissue was highly variable between cells as well as within cells, ranging from 1.18 percent K for the control plot of cell 20 to 2.02 percent K for the 80 lb K2O per acre treatment in cell 29. The suggested critical value for corn ear-leaf K composition to achieve optimum growth is 1.70 percent. Ear-leaf K concentrations exceeded the critical level in five of the 10 cells for the 80 lb K2O per acre treatment and in six of 10 cells for the 120 lb K2O per acre treatment. The grid cells that had a significant linear response to K fertilizer showed a 22 percent increase in leaf K, with the highest K rate applied compared to the control.
Overall, the K rate effects on 1999 corn yields at the Jefferson County site were nonsignificant (Table 2). The long drought period during pollination and grain fill, and possibly residual fertilizer K from the previous year, were likely responsible for the small grain yield differences that resulted from applied K treatments. Four of the 10 grid cells expressed a general trend toward increased yield with applied K, while the remaining six cells showed a trend toward decreased yields.
The 1999 K rate effects on soybean trifoliolate-leaf K concentrations at the Pope County site showed linear significance across all grid cells (Table 3). A 26 percent increase in trifoliolate-leaf K concentration was observed for the 120 lb K2O per acre rate over the control. For the control plots, one of the nine grid cells showed insufficient K (less than the established 1.70 percent K critical level) in the trifoliolate-leaf tissue. As an average over all cells, trifoliolate-leaf K composition ranged from 1.97 percent for the control to 2.48 percent for the 120 lb K2O per acre rate.
Soybean yield responses to applied K did not follow the pattern observed with tissue K analyses. Seven of the nine grid cells were nonsignificant in their yield response to added K (Table 4). The remaining two cells expressed a positive linear response to applied K. Of particular note was the fact that the two responding cells to K were also the two cells that had the greatest rooting depth to claypan and fragipan layers. This additional rooting volume for nutrients and water may have contributed to the higher yields observed in these cells and to the applied K. The overall lack of a yield response in the remaining seven cells may be attributed to an extended drought period during flowering and pod fill, and possibly to residual soil K from the previous year's application of K. Yield averages for all nine cells were 22, 23, 24, and 23 bu/acre for the 0, 40, 80, and 120 K2O per acre K rates, respectively. The trend for reduced yield at the highest K rate may have been a result of a nutrient imbalance caused by the higher level of applied K.
Two field locations, the same as were utilized in 1997 and 1998, were utilized in 1999 for variable rate K studies on corn and soybean. One field was in Jefferson County, IL, and the other was in Pope County, IL. Both fields had soil types and topography typical for the soils of their respective areas. Soil test K levels were "low" at both sites, averaging 141 lb K per acre in Jefferson County and 147 lb K per acre in Pope County. Ten grid cells (squares) with differing K levels and depths to claypan were used as sites for K rate studies at Jefferson County, and nine squares were utilized at Pope County. Potassium rates evaluated were 0, 40, 80, and 120 lb K2O/acre.
Corn yields obtained in 1999 at Jefferson County were strongly affected by drought factors such that there were mostly nonsignificant correlations between yield and soil test K level and between yield and added fertilizer K. Corn ear-leaf K composition showed a significant positive linear response in most of the grid cells with added K. However, in those cells with significant leaf K increases, there were not corresponding yield increases. Data analysis is ongoing to determine what factors were most strongly correlated to yield at this drought-stressed site in 1999.
At Pope County, soybean trifoliolate-leaf K composition responded linearly to applied fertilizer K in all cells. These leaf K increases did not translate into increased soybean yield. Only two of the nine grid cells showed a positive correlation between applied K, trifoliolate-leaf K composition, and soybean yield. Those two responding cells also had the greatest depths to claypan and fragipan layers. Extreme drought conditions throughout the growing season, excessive crop stress, and residual K from the previous year's application of K all may have contributed to the nonresponsiveness of this site.
When looking across all years at the Jefferson County location, crop yield response to applied K did not follow the general trend expressed in the leaf K concentrations (Table 5). Leaf tissue K was linear, nonsignificant, and linear for 1997, 1998, and 1999, respectively, to applied K. The corresponding crop yields across all three years were nonsignificant. Crop yield response to applied K was somewhat better in 1997 at the Pope County location, with an overall linear response in both leaf tissue K concentrations and grain yields (Table 6). The 1998 crop year elicited a linear response for leaf tissue K, but the corresponding 1998 crop yield was nonsignificant.
Both locations had atypical weather patterns. The 1997 and 1998 seasons had excessively wet springs followed by a summer drought. However, 1999 had nearly ideal spring planting conditions, with adequate rainfall followed by a severe drought throughout most of the growing season. Since growing conditions resulted in crop stress and reduced K uptake, residual K may have contributed to crop yields being nonresponsive to applied K.
In the fall of 1999, georeferenced soil electrical conductivity measurements were collected using the Veris EC 3100 tool at both locations. Although data are preliminary, there is an apparent correlation between soil electrical conductivity, topography, soil moisture, and associated crop yields. Similar observations were also reported by Calvin and Kerkman (1997) and Sudduth et al. (1997) on comparable soils. Additional evaluations using the Veris 3100 are ongoing.
Table 2. Potassium Rate Effects on Corn Grain Yields at the Jefferson County, IL Site, 1999.
Table 4. Potassium Rate Effects on Soybean Grain Yields at the Pope County, II. Site, 1999.
Table 5. Leaf Tissue K and Crop Yield Responses at the Jefferson County, IL Site, 1997-1999.
Table 6. Leaf Tissue K and Crop Yield Responses at the Pope County, IL Site, 1997-1999.
The authors wish to express their deep appreciation to Matt McCauley of the Natural Resources Conservation Service for his assistance in collecting the electrical conductivity data using a Veris EC 3100 tool owned by the USDA-NRCS.
1 E.C. Varsa is Associate Professor, Plant, Soil and General Agriculture Dept., SIUC; S.A. Ebelhar is Agronomist, Dixon Springs Agricultural Center, Univ of Illinois, Simpson, IL; S.K. Chong is Professor, Plant, Soil and General Agriculture Dept., SIUC; S.J. Indorante is Soil Scientist, Natural Resources Conservation Service, Carbondale, IL; and T.D. Wyciskalla is Researcher, Plant, Soil and General Agriculture Dept., SIUC.
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