Variable‐rate fertilizer (VRF) application requires knowledge of the spatial variability of soil test P and K within a field. The objectives of the study were to: (i) evaluate cell (area) vs. point soil sampling, both on a grid basis, and (ii) compare methods for mapping location specific soil test data. Soils in two central Wisconsin fields were sampled with two methods; study site soils included two alfisols and one entisol. The grid‐cell method involved dividing fields into 318‐ft square cells and compositing soil cores to give one sample per cell. The grid‐point method involved soil sampling at grid intersection points spaced on a 106‐ft square grid. Soil test P and K maps were constructed with nine mapping methods, including: field average, field median, CELL A (five soil cores), CELL B (72 soil cores), Delaunay triangulation, polynomial trend surface, inverse distance squared gridding, point kriging, and block kriging. Mapping accuracy was determined with map overlay comparisons where the Delaunay triangulation contour maps (106‐ft data) served as the control maps. On average, the CELL A and CELL B methods improved mapping accuracy by 14 and 33 percentage points over the field average. Even with the CELL B method, on average, 38% of the two fields would receive an incorrect application of fertilizer, indicating that cell mapping methods are not acceptable for making VRF management maps. Grid‐point sampling improved soil test P mapping accuracy by 20 percentage points over the CELL A method, even with point sampling in a 318‐ft grid. Mathematical procedures used to create a 53‐ft grid of data, from field sample data, did not improve map accuracy. Soil samples should be collected on a triangular or unaligned systematic grid. Sample spacing will depend on field variability, but probably should not exceed 300 ft. Research Question Traditionally, farm fields are soil sampled to determine the field average soil test P and K. One rate of fertilizer is recommended and applied, a rate that generally results in over‐ or under‐fertilization of portions of the field. Computer‐controlled fertilizer application equipment allows fertilizer rates or blends to be changed over short distances. Variable‐rate fertilizer (VRF) application requires accurate maps of the spatial variability of soil fertility levels. The objectives of this study were to: (i) evaluate cell (area) vs. point soil sampling, both on a grid basis, and (ii) compare methods for mapping location specific soil test data. The comparisons are limited to soil test P and K data. Literature Summary The profitability of VRF practices depends on the accuracy of soil test maps used to make fertilizer rate maps. Various techniques and sources of data are being used to create nutrient management maps, including soil surveys, infrared photographs, satellite imagery, and a variety of grid‐ and cell‐sampling schemes. Results from two studies show that sampling by soil type is not acceptable for mapping soil test levels. Another study reports that a 200‐ft soil sampling g...

Economic and environmental incentives to accurately predict corn (Zea mays L.) N requirements emphasize the need to assign appropriate N credits for soybean (Glycine max L.) in soybean‐corn crop sequences. This study was conducted to determine corn response to N and N credits for soybean in crop sequences. The effects of corn‐corn (CC), soybean‐corn (SbC), and soybean‐corn‐corn (SbCC) crop sequences and applied N (0 to 225 kg N ha−1) on corn grain yield, N uptake, and soil NO3 and NH4 concentrations were determined for 4 yr at three sites differing in climatic and soil characteristics. Four‐year mean yields in SbC were 1.4 and 2.2 Mg ha−1 higher than in CC at two sites with nonirrigated silt loam soils, but yield effects due to crop sequence were small on an irrigated sandy soil. Corn response to applied N varied markedly among the three sites and between years at the sites on silt loams soils. Mean corn N uptake in SbC was higher (51 kg N ha−1) than in CC on silt loam soils but not on the sandy soil. Soybean N credits estimated with a fertilizer replacement value (FRV) approach and from the difference in N rates at maximum yield in SbC and CC sequences (DNM) differed markedly among locations and years and ranged from − 22 to 210 kg N ha−1. Soybean provided little N to subsequent crops on sandy soils due to probable loss of residue N through leaching prior to use by the following crop. On silt loam soils, crop sequence effects on yield and N uptake indicate soybean N contributions to subsequent crops; however, fixed value N credits or N credits based on N response data combined over years will seldom accurately predict actual soybean N contributions. Site‐specific diagnostic tests are needed to improve crediting of N supplied by soybean in crop sequences.

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Studies were conducted to examine the effect of potassium (K) on soybean aphid, Aphis glycines Matsumura, population growth. A laboratory feeding assay examined the effect of K-deficient foliage on life table parameters of soybean aphids, and field experiments were designed to determine the effect of three soil K treatment levels on aphid populations and their impact on soybean yields. The feeding assay found that life table parameters differed between aphids feeding on the K-deficient and nondeficient soybean leaves. Soybean aphids in the K-deficient treatment exhibited significantly greater intrinsic rate of increase (r(m)), finite rate of increase (lambda), and net reproductive rate (Ro) relative to aphids feeding on nondeficient leaves. No significant difference was observed in mean generation time (T) between the two treatments. However, the field experiment repeated over 2 yr showed no effect of K on soybean aphid populations. Soybean aphid populations were high in unsprayed plots and feeding resulted in significant yield losses in 2002 at all three K treatment levels: when averaged across 2001 and 2002, unsprayed treatments yielded 22, 18, and 19.5% less than the sprayed plots in the low, medium, and high K treatments, respectively. No significant interaction was observed between aphid abundance and K level on soybean yields in either year. This study therefore suggests that although aphids can perform better on K-deficient plants, aphid abundance in the field may be dependent on additional factors, such as dispersal, that may affect final densities within plots.

Many municipalities have examined composting as an alternative to landfilling for the management of organic solid waste materials. Ultimately these materials will be land-applied and therefore some knowledge of nutrient availability will be necessary to optimize crop yield and minimize environmental risk. Field studies were conducted in 1993 and 1994 on a silt loam and a loamy sand soil in Wisconsin to determine the effect of municipal solid waste compost (MSWC) on corn (Zea mays L.) yield, plant nutrient concentration, and soil nitrate N content. Municipal solid waste composts with ages of 7, 36, and 270 d were applied at rates of 22.5, 45, and 90 Mg ha(-1) to small plots. Rates of commercial nitrogen (N) fertilizer, ranging from 0 to 179 kg N ha(-1), were applied to separate plots to determine the N availability from the MSWC. Treatments were applied in the spring and incorporated before planting corn. The 270-d MSWC increased corn whole-plant dry matter and grain yield at each location in both years above the 7- and 36-d MSWC. Rate of MSWC only affected grain yield at the loamy sand site in 1994. Municipal solid waste compost had minimal effect on the levels of plant nutrients in the whole-plant tissue measured at physiological maturity. Nitrate N measured in the top 90 cm of soil was higher throughout the growing season in treatments receiving recommended N fertilizer when compared with any of the MSWC treatments. It was estimated that 6 to 17% of the total N in the 270-d MSWC became available in the first year. The land-application of mature MSWC at the tested rates would be an agronomically and environmentally admissible practice.