rate could be increased. Competition from weed pressure may be reduced by increasing the seeding rate at planting (Mortensen et al., in press). Increasing the seeding rate in small-grain crops in California can help control Johnson grass and smart weed. Plant population data could also be used to check on the effectiveness of precision planters that drop seeds at a set spacing up the row, which is an important manageable factor when expensive hybrid seed is used. Improved knowledge of field conditions and pest pressure can help a producer make planting decisions.

Although seeding rates can be adjusted at planting, many factors can affect the plants throughout the season. Varying seeding rates may not necessarily result in an expected plant population distribution. Knowing the actual plant population at harvest time is important in interpreting yield maps and for management decisions made throughout the growing season. Technology currently under development for corn in the Midwest will measure variability of a plant population and plant spacing (Birrell and Sudduth, 1995; Easton, 1996; Plattner and Hummel, 1996). Information gathered across a field will generate a data layer that can be compared with yield maps, desired seeding rates, or weed maps. These devices are still in the developmental stage but illustrate the potential for using sensing techniques to gather useful information as a part of normal field operations such as cultivation or harvesting.

Soil Variability

Soils vary significantly as a result of regional geological origins and past and present cultural practices. At the highest level of resolution, soil physical, biological, and chemical properties vary vertically, horizontally, with treatment, and with time. For example, variable distributions of soil nutrients in fields may result from improperly adjusted mechanical application equipment (Bashford et al., 1996; Olieslagers et al., 1995). In other cases, past practices, such as an old feedlot, can generate local pockets of higher organic matter producing healthier plants than surrounding areas. Thus, natural variability patterns and management practices need to be considered in assessments of soil spatial variability.

Soil layers that restrict rooting depth are a major concern in many areas. Electromagnetic induction techniques have been used to assess the presence of and depth to claypan layers (Doolittle et al., 1994). Limited work on assessing soil compaction has indicated a potential profitable return to site-specific tillage operations instead of whole-field subsoiling (Fulton et al., 1996). Soil physical, chemical, and biological properties have dramatic effects on crop production. However, only a few commercially available sensors can assess these properties in the field. Practitioners are limited to sampling and laboratory analysis for determination of in-field variability, which is costly and time consuming. The number of commercially available sensors will be a limiting factor for precision agriculture in the immediate future.

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