H.P. “Sonny” Viator, Richard Johnson and Maurice Wolcott
LSU AgCenter research has demonstrated that conventional, whole-field soil-sampling schemes and field-averaged yields do not satisfactorily describe the variations of yield and soil attributes present in several sugarcane fields. Over a three-year period, the research showed sugarcane tonnage varied up to 450 percent and sugar yield varied as much as 550 percent in different parts of commercial Louisiana sugarcane fields. The variability was present in all years of the study and was observed for all crop ages from plantcane to the sixth-year stubble crop.
GPS-guided grid-sampling techniques documented significant variability in soil acidity in several fields. With this technique, fields are sampled in a systematic, regular pattern and correlated with GPS readings so spatial differences in soil properties can be readily identified. In one field, for example, soil pH values ranged from 4.9 to 6.5 and the corresponding lime rates required to adjust the pH to 6.5 varied from zero to 2 tons per acre. Such documentation of within-field variability allows for the application of inputs only where necessary by using variable rate (VR) technology.
The initial research in sugarcane centered on variable-rate applications of lime and nitrogen. Beginning in 2001, soil electrical conductivity measurements (EC) were used to identify management zones for fertilizer application in several sugarcane fields. LSU AgCenter research has shown that electrical conductivity correlates with differences in soil texture, soil water content, salinity and other factors that directly influence plant growth. The AgCenter’s sugarcane nitrogen fertilizer recommendations are based on differences in soil texture, with lower nitrogen rates recommended for the sandier areas (low EC) and higher rates for the areas of greater clay content (high EC). Nitrogen fertilizer was therefore applied based on these recommendations. Figure 1 shows the spatial agreement between soil EC and textural differences associated with three soil classifications.
The objective of achieving approximately equal yields across all application zones was not consistently realized using five fertilizer application rate studies or “prescriptions.” Yields on the clayey areas of the fields were affected in most years by uneven soil moisture, a condition that is typical for clayey soil. Also, the variety used in all but one of the fields, LCP 85-384, yields poorly under stress. The failure to adjust for other yield-influencing variables, such as pH, may also have accounted for yield variations.
The 2006 variable rate nitrogen fertilizer prescription was applied to the variety HoCP 96-540 after the field was limed according to soil test results. After adjusting for differences in soil pH, a relatively smooth yield map was achieved for this VR prescription, with average zonal yields ranging from 37 to 39 tons of cane per acre. The results gathered from these experiments suggest prescriptions must consider the variability in all yield-influencing soil properties.
In 2005, a test was initiated to determine if soil electrical conductivity could be used to develop variable-rate lime application maps. The test also was designed to investigate alternate methods to estimate lime requirements for sugarcane grown on Louisiana soils. Before treatment, two soil EC measurements were taken – at zero-to-1-foot deep and at zero-to-three-feet deep. The soil samples also were taken based on a grid to compare soil EC levels with soil properties. Soil pH was measured and lime requirements were estimated based on three different testing methods. The lime was applied using both conventional, uniform application and variable-rate application, and both were compared to a no-lime control.
Considerable variability in soil acidity was noted in the grid soil samples, with soil pH varying from 4.1 to 8.1. The corresponding lime recommendations also varied, with the calculated lime rates ranging from zero to 3.5, zero to 3.3 and zero to 1.5 tons per acre for the different testing methods. Soil electrical conductivity measurements were correlated with soil pH levels from grid soil samples, with pH increasing with soil EC levels (Figure 2). Soil lime requirement estimates also demonstrated an inverse correlation with soil EC levels. The best correlations for both soil pH and lime requirement estimates appeared to be with the deep (zero-to-3-foot) soil EC estimates. Finally, the soil EC estimates also appeared to mirror changes in soil type in the test field. Further research is needed to calibrate these measurements on different soil types, but preliminary results are encouraging.
Sugarcane yield results from this study showed a significant advantage in the theoretically recoverable sugar (TRS) levels with several of the alternative lime requirement procedures. These results are promising because if similar yields can be obtained with the VR system while actually applying fewer inputs, then Louisiana sugarcane producers can show an overall increase in profitability.
These data suggest that sufficient variability exists in soil properties and in subsequent sugar yields to justify a precision management approach. In this approach, zones containing similar soil properties would be identified in each field using soil EC or other similar techniques. These areas could then be targeted for site-specific nutrient application using variable-rate application equipment. This practice would decrease the cost of soil sampling, while increasing the application accuracy of agricultural chemicals, ensuring sustainability and minimizing adverse environmental effects.
H.P. “Sonny” Viator, Professor and Coordinator, Iberia Research Station, Jeanerette, La.; Richard Johnson, U.S. Department of Agriculture-Agricultural Research Service, Sugarcane Research Unit, Houma, La.; and Maurice Wolcott, Research Associate, Department of Plant Pathology & Crop Physiology, LSU AgCenter, Baton Rouge, La.(This article was published in the spring 2008 issue of Louisiana Agriculture.)