On-the-go soil sensors for precision agriculture
Introduction
Soil testing results are important inputs to the profitable application of fertilizer, lime, and other soil amendments. When soil test results are combined with information about the nutrients that are available to the various crops, a reliable basis for planning the fertility program can be established (Hoeft et al., 1996). An appropriate test may be based on local soil and crop conditions as well as personal preference. A standard test usually includes determination of available phosphorus (P), exchangeable potassium (K), calcium (Ca), and magnesium (Mg), their saturation percentages, the cation exchange capacity (CEC), pH, and lime requirement. Some laboratories may also test for organic matter (OM) content, salinity, nitrate, sulfate, certain micronutrients, and heavy metals (Foth and Ellis, 1988). In addition, the crop growth environment is affected by soil texture (sand, silt and clay content), level of soil compaction, moisture content, and other mechanical and physical soil properties.
One of the most critical aspects of soil testing is actually obtaining representative soil samples (i.e. collected with adequate spatial density at the proper depth and during the appropriate time). Practical advice related to the collecting and handling of soil samples was given by Vitosh et al. (1995), Hoeft et al. (1996), and Gelderman and Mallarino (1998). However, the location and number of soil samples depends on the approach used to manage soil fertility (Havlin et al., 1999). Currently, random, adaptive, and grid sampling techniques are often used. In random sampling, soil cores are obtained from random locations within the field. In adaptive sampling, selected locations depend on prior information. Grid sampling, on the other hand, involves systematically collecting samples from predetermined points in the field. None of the existing soil sampling practices has been recognized as the most effective (Wollenhaupt et al., 1997).
Numerous attempts to develop on-the-go soil sensors have been previously reported and reviewed (Hummel et al., 1996, Sudduth et al., 1997). The development of sensors is expected to increase the effectiveness of precision agriculture (Pierce and Nowak, 1999). In particular, sensors developed for on-the-go measurement of soil properties have the potential to provide benefits from the increased density of measurements at a relatively low cost (Sonka et al., 1997). Although only a few soil sensors are commercially available, there is an on-going effort to develop new prototypes. The purpose of this publication is to review recently reported concepts for on-the-go measurement of soil mechanical, physical and chemical characteristics, and to discuss potential applications of such measurement methods.
Section snippets
Instrumentation and methods
The global positioning system (GPS) receivers, used to locate and navigate agricultural vehicles within a field, have become the most common sensor in precision agriculture. In addition to having the capability to determine geographic coordinates (latitude and longitude), high-accuracy GPS receivers allow measurement of altitude (elevation) and the data can be used to calculate slope, aspect and other parameters relevant to the landscape.
When a GPS receiver and a data logger are used to record
Conclusions
Although various on-the-go soil sensors are under development, only electrical and electromagnetic sensors have been widely used in precision agriculture. Producers prefer sensors that provide direct inputs for existing prescription algorithms. Instead, commercially available sensors provide measurements, such as electrical resistivity/conductivity that cannot be used directly since the absolute value depends on a number of physical and chemical soil properties, such as texture, organic matter,
Acknowledgments
This publication is a contribution of the University of Nebraska Agricultural Research Division, Lincoln, NE, Journal Series No. 14275.
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