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sponsive categories (91-m grid) and a different recommendation for fields in the responsive range (61-m grid).The efficiency of the management zone approach to improve fertil-Other problems with grid-point sampling are that imizer recommendations relies on accurately locating zone boundaries. portant information may be missed when grid distancesThe objective of this study was to determine the impact of different techniques of identifying management zones on soil NO Ϫ 3 -N and are too large, and many farmers perceive that grid-point Olsen-P (sodium bicarbonate extractable-P) sampling variability. Soil sampling is not profitable. samples were collected on a 60 by 60 m or denser grid, in three fields Grid-cell sampling is an approach where a composite (65, 53, and 40 ha). These samples were analyzed for NO Ϫ 3 -N and sample is collected from a block with a specified size Olsen-P. Soil nutrient data was used to simulate the effect of different (Wollenhaupt et al., 1994). The sample from each block techniques to identify P and N management zone boundaries. Apis analyzed and the resulting value represents the average proaches evaluated for locating management zone sampling boundvalue of the cell. Many current traditional soil sampling aries included: (i) sampling areas impacted by old homesteads or strategies contain some aspect of grid-cell sampling. Feranimals separately from the rest of the field; (ii) sampling different guson et al. (1998) and Buchholz (1999) recommended grid cells; (iii) use of geographic information systems (GIS) or cluster that in Nebraska and Missouri, respectively, the largest analysis to identify zones based on apparent electrical conductivity (EC a ), elevation, aspect, and distance (connectedness); and (iv) sam-sampling area in a field should be 8 ha or less. In Monpling each soil series separately. An F statistic was used to determine if tana, Jacobsen (1998) recommended that the largest the sampling approach reduced nutrient sampling variability. Results sampling area should be 40 ha. suggested that: (i) old homesteads or areas impacted by animals should The management zone approach is based on the hybe sampled separately from the rest of the field; (ii) grid-cell sampling pothesis that a field is a mosaic of different habitats was more consistent in reducing within zone soil-test variability than with each having unique characteristics that influence the other techniques tested; and (iii) zones that are not continuous soil properties and management (Doerge, 1999; Fleming should be sampled and managed separately. et al. , 2000). Information that can be used to identify the different management zones include (i) prior experience, (ii) yield maps, (iii) soil survey maps, (iv) topogra-

Wheat yields in the Great Plains are frequently improved by potash (0‐0‐60 N‐P‐K) application on soils with seemingly abundant supplies of available K. The CI component in this fertilizer may cause many of the observed yield responses. Field research was undertaken to determine the effects of CI on grain yield, plant CI, kernel weight and growth rate, and foliar disease and/or disease‐like symptoms of winter wheat cultivars. Two sites with five CI levels (0, 22, 45, 90, and 135 kg ha−1) and two cultivars (Redwin, Cree) were used in 1988. Seven sites with two CI levels (0 and 45 kg ha−1) and six cultivars (Redwin, Cree, Weston, Manning, QT542, Neeley) were used from 1991 to 1992. Chloride increased yield by an average of 267 kg ha−1 (7%) at seven responsive sites. Size of yield response to CI was generally not affected by cultivar selection, but differed greatly with location. Whole‐plant CI analyses were useful in discriminating between yield responsive (≤ 3.0 g kg−1) and nonresponsive (>3.0 g kg−1) sites to CI fertilization. Improved CI nutrition enhanced plant development, increase immature kernel weights, and accelerated kernel growth during the grain‐fill period. Fertilizer CI increased mature kernel weights, up to 17%, at eight sites. Kernel weight was the most important yield component affected by CI. Physiological leaf spot in Redwin and Manning, flag leaf senescence in Weston and QT542, powdery mildew (Erysiphe graminis DC. f. sp. tritici Em. Marchal), and leaf rust (Puccinia recondita Roberge ex Desmaz. f. sp. tritici) were suppressed by fertilizer CI. These phenomena may explain in part the yield responses to CI; however, yield response to CI occurred even without foliar disease and disease‐like suppression.

Competitive indigenous strains of Bradyrhizobium japonicum occupy many soils of the Midwest, but they are generally considered to be less efficient in fixing N2 than many strains added as inoculants. Inoculant strains will not improve N2 fixation in soybean [Glycine max (L.) Merr.], however, unless they are able to form nodules on the host plant. This research evaluated whether strains selected for competitiveness in the greenhouse could compete against indigenous bradyrhizobia in the field in forming nodules when added at rates used for commercial inoculants, and whether higher nodule occupancy resulted in improved plant growth. Plots were established in north‐central Iowa at seven sites on four representative soil series. Seven to 12 strains were introduced in two or three soils each year from 1983 to 1985 at approximately 5.8 log10 viable cells per centimeter of row as granular‐peat inoculants. Results showed that average strain recoveries ranged from 0 to 46%. Some introduced strains differed in their competitive ability among the sites tested. The most competitive strains were AN8, AN10, AN11, AN23, AN27, and AN30, which typically occupied at least 20% of the nodules at a given site. When considering all members of each serogroup tested, strains from serogroup 123 were the best competitors. Increased nodule occupancy by the introduced strains did not result in increased plant growth or yield. Nodulating and nonnodulating ‘Clark’ isolines and urea fertilizer treatments were used to evaluate contributions from N2 fixation and plant growth with added N fertilizer.

We compared infestation levels of cereal aphids (Homoptera: Aphididae) in spring-seeded wheat and barley grown with and without preplant tillage for 8 site yr in eastern South Dakota. Crop residue covered approximately 25% of the soil surface with preplant tillage, whereas without preplant tillage 50% or more of surface residue was conserved. Rhopalosiphum padi (L.) comprised nearly 90% of all cereal aphids sampled, and R. maidis (Fitch), Schizaphis graminum (Rondani), and Sitobion avenae (F.) collectively comprised the remainder. R. padi routinely infested lower parts of tillers and were generally concealed by surface residue in plots with no preplant tillage. Across 7 site yr, R. padi were more abundant in plots with no preplant tillage than with preplant tillage (272.6 +/- 54.4 versus 170.1 +/- 37.2 aphid days per 25 tillers). However, in comparisons at individual site years, R. padi were greater in no-preplant tillage plots only once. For all cereal-aphid species combined, infestations were greater in plots with no preplant tillage for 1 of 8 site yr, but did not differ with tillage when compared across all site years. Cereal aphids were never more abundant in plots with preplant tillage. Our results show that conservation tillage leads to greater infestations of R. padi in spring small grains, as increased surface residue provides a favorable microhabitat for this aphid.