Browsing by Author "Wall, David Peter"

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Soil Tests for Winter Wheat Nitrogen Management in the Southeastern USA
(2009-02-06) Wall, David Peter; Ronnie Heiniger, Committee Member; Jeffery G. White, Committee Member; Carl Crozier, Committee Member; Randy Weisz, Committee Chair
ABSTRACT WALL, DAVID PETER. Soil Tests for Winter Wheat Nitrogen Management in the Southeastern USA. (Under the direction of Randy Weisz). Developing a system where soil tests could be used for winter wheat (Triticum aestivum L.) nitrogen (N) management and to determine N-fertilizer requirements in the southeastern USA would have many environmental and economic benefits. The objectives of this study were to: (i) evaluate the effects of sampling time, sampling depth, crop rotation, and N fertilizer application on the amino sugar-N test (ASNT); (ii) determine appropriate and reliable sampling depths for the ASNT and soil mineral N tests; (iii) explore the relationships between these soil N tests and winter wheat N uptake; (iv) calibrate these soil N tests to predict optimum spring N rates (OptN) for winter wheat in North Carolina. Ten study sites were established, to assess changes in ASNT values with sampling depth, crop rotation, and N fertilizer application. A repeated measures experimental design was implemented whereby soil samples were taken from each of the 10 study sites at specific times and depths during the course of a 19-month period. Amino sugar-N (ASN), NO3-N, NH4-N, and soil organic matter (SOM) levels were evaluated at each sampling. At all sites, soil ASN decreased with depth and showed significant variation over time. Soil ASN was influenced by crop rotation and tillage but was not significantly affected by fertilizer applications. When all sites were considered together, soil ASN was well correlated with SOM, however, they were not correlated across time within sites. This suggests that the ASNT is measuring a nitrogenous fraction of SOM that behaves somewhat independently over time. These results indicate that sampling to 30 cm at a specific sampling time will be necessary to capture temporal variability in ASN. Sixty-nine winter wheat N response trials were conducted for three growing seasons over a wide range of soils, drainage and previous crop management practices. A randomized complete block design with five replications and seven growth-stage 30 (GS30) N-fertilizer rates ranging from 0 to 168 kg ha-1 was used. Each February, soil samples were taken over three depths (0-10, 0-20, and 0-30cm) and analyzed for ASN, NO3-N, NH4-N and SOM levels. In the wheat N uptake study, the relationships between these soil N tests and N uptake from the unfertilized check plots (NUPZero) were evaluated. Considering all the experimental locations, the soil tests had poor relationships with NUPZero. When only the well-drained locations (n=58) were considered, the data aggregated into two distinct groups: 1) locations with low residual N, and 2) locations with high residual N and elevated levels of NH4-N and ASN. For locations with low residual N, the ASNT was able to predict NUPZero and grain yield without spring fertilizer applied (YLDZero) and was therefore a good predictor of soil N supply through mineralization. A combination of soil tests (NH4-N, ASNT and SOM) was required to predict NUPZero for all well-drained locations (R2 = 80). In the N fertilizer response study, the ASNT by itself was a poor predictor of OptN for all well-drained locations, however the ASNT was correlated with N uptake at the optimum spring N fertilizer rate (NUPOpt) and N uptake at GS-30 (NUPGS30) for locations with low residual N. The ASNT could not satisfactorily predict OptN for these low residual N locations, but could be used to predict the NUPOpt and wheat yield at the optimum GS-30 N fertilizer rate (YLDOpt) for well-drained fields. A combination of soil tests was required to predict OptN for all locations (i.e., NH4-N and SOM with resultant R2 = 0.59). Soil NO3 tests showed no benefit for winter wheat N management in North Carolina.