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Fluid Journal : Fluid Journal 1996-1998
3 Fluid Journal Summer 1998 Figure 4. Loading of the different soil properties on the value of Principal Component used to predict corn yield. N along the Sterling landscape. Soil NO3-N ranged from 25 to 75 lbs/A of N (Figure 3). The 4-foot profile for NO3-N was spatially independent at the 22 sampling positions. This means that the soil NO3-N level at one location was not related to the soil NO3-N level 20 feet away at Sterling. These results are very disturbing because the whole theory of VRT is that soil parameters are spatially dependent -- i.e., the soil test level at point "A" is related to the soil test level of its close neighbors (point "B", "C," etc.). We are continuing to investigate this finding. In general, residual NO3-N was low over this landscape, which is expected in fields that have not been overfertilized during the previous season. At the Stratton I site. NO3-N at the 4-foot depth showed strong spatial dependence. A range of spatial dependence was 74. Residual NO3-N in the soil profile varied 1,200 percent from 19 to 222 lbs/A of N. On the summit and sideslopes, NO3-N content was low (approximately 35 lbs/A of N). However, NO3-N soil level was as high as 225 lbs/A of N on toeslopes. N mineralization. Kriged daily N mineralization rates varied from 0.23 to 0.96 lbs/A of N per day, depending on position along the Sterling landscape. At the Stratton 1 site, N mineralization rates varied from 0.19 to 1.6 lbs/A of N per day. Highest N mineralization rates occurred on the toeslopes, the most favorable environment in the landscape due to highest organic matter content and available soil water throughout the season. For both the Sterling and Stratton I landscapes, average daily N mineralization rates were 0.5 and 0.57 lbs/A of N, respectively. This means that these soils were producing (mineralizing) about 0.5-0.6 lbs/A of N that is available for plant use. N fertilizer response Sterling. N fertilizer applications increased corn grain yields at 53 percent of the positions along the landscape. No effect of applied N was observed at 33 percent of the positions, while the remaining 14 percent suffered reductions in yield (Figure 1). Yield response to N fertilizer was linear, meaning that under these conditions maximum yield was not achieved at the N rates used in this study, which was high (120 lbs/A of N). Stratton 1. Positive yield response to N fertilization occurred at 76 percent of the positions along the landscape. No response occurred at 1 8 percent of the positions, while only 6 percent showed reduced yields (Figure 2). Stratton 2. A quadratic yield response to N fertilization was observed on the landscape. N rate for maximum yield was 104 lbs/A of N. All sites. Considering the landscapes, a significant yield response to N fertilizer occurred at both the Sterling and Stratton sites. If it is assumed that N fertilizer-use efficiency is 50 percent, N requirement for dryland corn will vary between 1.12 and 1.6 lbs N/bu of total plant uptake or 0.9 and 1.23 lbs N/bu of grain uptake. These values agree with current N recommendation algorithms that are being used in this region. Soil productivity index When all soil parameters were evaluated, yields were higher where the profile NO3-N, surface organic matter, soil organic N, and mineralization rates were high, and lower in positions with high pH and lime content. The problem is that several soil properties that vary along the landscape contributed to variation in grain yield, either positively or negatively. One way to isolate the "cause and effect" relationship is by using a "soil index" that represents all soil properties of interest. This was calculated by principal component analysis, which identifies those factors contributing positively or negatively to corn yield. When considering the impact of soil
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