to non-saline soil methods in combination with electrochemical constituent sensing which separates components of direct contact conductivity (Colburn, 1997). Conductivity component analysis is employed for georeferenced data gathering and analysis by several commercial companies as well as for VRT in midwest crops. Apparent soil conductivity using electromagnetic methods is an indicator of clay content, depth to claypan, soil water content, hydraulic characteristics, productivity (Kitchen et al., 1996), and as a promising substitute for yield monitoring (Jaynes et al., 1995).

For immobile constituents (i.e., phosphorus and potassium), industry has not yet chosen to introduce real-time sensors. In some cases, phosphorus and potassium levels in the corn belt states where VRT was first used, are very high, and field availability has been found to exceed producer needs for the current crop year and the near future. In other regions, such as western states, lower availability of immobile nutrients is common. For these nutrients, discontinuous nutrient sensor mapping methods have the potential for gathering and analyzing soil samples in separate field operations. Three systems are under development by government and academia which automatically extract and analyze soil samples for phosphorus, potassium, and nitrates (Adsett and Zoerb, 1991; Birrell, 1995; Morgan and Ess, 1996).

There exists the potential for a vast increase in the timeliness and amount of information if additional means of data collection and analysis become available. Sensors will play an important role in supporting technology for precise applications of nutrients, pesticides, and other inputs. Only a few commercial sensors are available today. Efforts continue by both private companies and the public sector to develop real-time sensors for additional agricultural indexes. Basic research in the sensors arena is fundamental to an improved understanding of the variations in site-specific crop production in a wide variety of regional production systems.

Remote Sensing

Remote sensing—the acquisition of information from remote locations such as an airplane or satellite—is a potentially important source of data for precision agriculture. In the long term, remote sensing could provide numerous forms of information, both spatially and temporally. However, improvements are needed in the analytical products and delivery systems if remote sensing is to meet its promise for precision agriculture.

For more than 30 years remote sensing has been envisioned as a valuable source of information for crop management. The pioneering research of Colwell (1956) showed that infrared aerial photography could be used to detect loss of vigor of wheat and other small grains resulting from disease. Although much research and development was directed at large-area crop inventory applications of satellite data in the 1970s (MacDonald and Hall, 1980), much less attention

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