Illinois Fertilizer
Conference Proceedings
January 25-27, 1999
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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 early stages (Francis, et al., 1996; Schefcik, et al., 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 interaction of soil chemical properties (soil tests and crop responses) with soil physical properties. In southern Illinois, field yields of corn and soybeans are 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 claypans that restrict root development for nutrients and water. Kitchen et al. (1996) reported that the depth to claypan, as assessed by electromagnetic conductivity (EM) readings, closely correlated with crop yield response, especially in dry years. Additionally, soil management practices may cause further variability to the extent that identical land units (soil types) may act quite differently from one another when subjected to different management practices (Bouma, et al., 1993).
Two southern Illinois farm fields have been identified that have distinct variations throughout in terms of morphological properties but have somewhat uniformly low levels of soil test, potassium. Soil sites within each field (called grid cells) with a wide range in depth to claypan will be used as sites for crop response studies to increasing rates of K application.
The objectives of this research were to:
1. Evaluate Variable Rate Technology as a management tool for K fertilization of corn and soybean on soils with a range in depth to claypan.
2. Determine through grid sampling critical need areas of each field for K fertilization on soils with varying depths to the claypan.
3. Determine the effect of K rates within field cells (squares) on K uptake and grain yield of corn and soybeans.
4. Compare the yield responses to K fertilization across grid cells (squares) that differ in depth to claypan and other soil properties.
5. 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 (one in Jefferson County, IL and the other in Pope County, IL) were utilized for the 1998 growing season for this research. Soybeans were grown using conventional tillage practices at the Jefferson County site in 1998, following corn in 1997. The Pope County site was planted to no-till corn in 1998, with the previous crop being no-till soybean.
Due to excessive amounts of springtime rainfall at the Jefferson County site, planting was delayed until June 13. Within 24 hours of planting, a severe storm hit the site, resulting in soil compaction and poor stands. The site was then replanted on June 28. The Pope County site was planted on May 10 and had excellent seed emergence.
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 1998. Potassium rates of 0, 40, 80, and 120 lb K20 per acre as 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 within five days after initial planting 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. 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.
Large, intact soil cores (1.75-inch diameter by 36 inches long) were collected from the center of each grid cell for detailed soil classification. Cores at the Jefferson County site were collected prior to tillage and soybean planting. The Pope County site cores were collected after corn harvest in 1998. Through intense classification, it was determined that the Jefferson County site was predominantly a Wynoose/Bluford silt loam soil, and the Pope County site was dominated by a Grantsburg silt loam soil. After classification, the cores from both sites were subdivided by soil horizon layers for chemical analysis. Preliminary soil test pH results indicate a sharp drop in the soil pH (from pH 6.8 to pH 4.4) from the plow layer to the strongly developed, underlying Ehorizon at the Jefferson County site. This drop in pH warrants a close evaluation for soluble or extractable aluminum. In addition, further soil testing on all grid cells at the Jefferson County site indicated that the east portion of the field, which was in corn prior to the 1997 corn crop, was deficient in magnesium (Mg). The west portion of the field did not show this deficiency in the soil test Mg values. Therefore, for the 1998 growing season, 50 In per acre of Mg as magnesium oxide was applied broadcast, prior to tillage operations.
Application of fertilizer K resulted in changes in soybean leaf K concentrations that were generally nonsignificant (Table 1). Three of the ten grid cells showed a positive linear response to applied K, while a fourth grid cell expressed a quadratic response. Sufficiency ranges for K in the leaf tissue (minimum: 1.70%), for all grid cells across all K rates were adequate (ranging from 1.69% to 2.32%). The grid cells that showed a significant response had approximately an I 1 % increase in the leaf K concentration for the 120 lb K20 per acre rate over the control.
Overall, the K rate effects on 1998 soybean yields at the Jefferson County site were nonsignificant (Table 2). The late planting date, drought during pod fill, and possibly residual soil K from the previous year (further soil analyses are being conducted) were likely responsible for the small grain yield differences that resulted from K treatments. Five of the ten grid cells expressed a general trend toward increased yield with applied K, while the remaining five cells showed a trend toward decreased yields.
The 1998 K rate effects on corn ear-leaf K concentrations at the Pope County site were significant across all grid cells (Table 3). A 32% increase in ear-leaf K concentration was observed in the 120 lb K20 per acre rate over the control. For the control plots, seven of the nine grid cells showed insufficient K (less than 1.70%) in the ear-leaf tissue. As an average over all cells, ear-leaf K composition ranged from 1.66% for the control to 2.20% for the 120 lb K20 per acre rate of K.
Corn yield responses to applied K did not follow the pattern observed with tissue K analyses. Eight of the nine grid cells were nonsignificant in their response to added K (Table 4). The ninth cell expressed a negative linear response to applied K. The overall lack of a yield response may be attributed to a near-ideal growing season, possible residual soil K from the previous year's application of K, and a large soil volume (in relation to restrictive layers) from which to draw needed moisture and nutrients. Yield averages were 172, 174, 175, and 169 bu/acre for the 0, 40, 80, and 120 K20 per acre K rates. The reduced yield at the highest K rate may have been a result of nutrient imbalance caused by the higher level of applied K.
The same two field locations used in 1997 were utilized in 1998 for variable rate K studies on corn and soybean, one in Jefferson County and the other 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/acre in Jefferson County and 147 lb K/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 K20/acre.
Soybean yields obtained in 1998 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. Soybean trifoliolate-leaf K composition increases were nonsignificant in most of the grid squares with added K, but in those cells with significant leaf K increases, there was not a corresponding yield increase. Data analysis is yet incomplete to determine what factors were most strongly correlated to yield at this drought-stressed site in 1998. At the Pope County location, corn ear-leaf K composition responded linearly to applied fertilizer K in all cells. These leaf K increases did not translate into increased corn yield. Nearideal growing conditions and little to no crop stress throughout the growing season was attributed to the nonresponsiveness of this site.
Table 2. Potassium rate effects on soybean grain yields at the Jefferson County site, 1998.
Table 3. Potassium rate effects on corn ear-leaf K concentrations at the Pope County site, 1998.
Table 4. Potassium rate effects on corn grain yields at the Pope County site, 1998.
The authors wish to express their deep appreciation to Mr. Matt McCauley of the Natural Resources Conservation Service for his assistance in collecting large, intact soil cores for detailed soil classification at each location
1 Associate Professor, Plant, Soil and General Agriculture Dept., SIUC; Agronomist, Dixon Springs Agricultural Center, Univ. of Illinois, Simpson, IL; Professor, Plant, Soil and General Agriculture Dept., SIUC; Soil Scientist, Nat. Resources Conservation Service, Carbondale, IL; Researcher, Plant, Soil and General Agriculture Dept., SIUC.
Bouma, J. and P.A. Finke. 1993. Origin and Nature of Soil Resource Variability. In Soil Specific Crop Management: a Workshop on Research and Development Issues. pp 3-14. American Society of Agronomy, Madison, Wisconsin.
Francis, D.D., T.M. Blackmer, and J.S. Schepers. 1996. Moving toward site-specific management. In Proceedings of the Great Plains Soil Fertility Conference. Vol. 6. pp 1-7. Potash and Phosphate Institute, Manhattan, Kansas.
Harrington, P.D., E.D. Nafziger, and R.G. Hoeft. 1997. On-Farm Evaluation of Variable-Rate Nitrogen Fertilizer Response Using Farmer-Owned Equipment. In 1997 Illinois Fertilizer Conference Proceedings. pp 57-64. University of Illinois, Urbana, Illinois.
Kitchen, N.R., K.A. Sudduth, S.T. Drummond, and S.J. Birrell. 1996. Spatial prediction of crop productivity using electromagnetic induction. In Missouri Soil Fertility and Fertilizer Research Update 1995, pp. 83-87. University of Missouri, Agronomy Miscellaneous Publication No. 96-13.
Schefcik, M.E. and J. Pete. 1996. Precision farming, an approach to problem-solving. In Proceedings of the Great Plains Soil Fertility Conference. Vol. 6. pp. 14-18. Potash and Phosphate Institute, Manhattan, Kansas.
Smith, J. 1998. Enhancing Environmental Stewardship Via Site-Specific Agriculture. Fluid Journal, Vol.6, no. 2, issue 21. pp 10-12.
Wollenhaupt, N.C., R.P. Wolkowski, and H.F. Reetz. 1993. Variable-rate fertilizer application: update and economics. In Proceedings 21st North Central Extension-Industry Soil Fertility Conference. Vol. 9. pp. 139-150. Potash and Phosphate Institute, Manhattan, Kansas.
