On-the-go soil sensors for precision agriculture

Abstract

The basic objectives of site-specific management of agricultural inputs are to increase profitability of crop production, improve product quality, and protect the environment. Information about the variability of different soil attributes within a field is essential for the decision-making process. The inability to obtain soil characteristics rapidly and inexpensively remains one of the biggest limitations of precision agriculture. Numerous researchers and manufacturers have attempted to develop on-the-go soil sensors to measure mechanical, physical and chemical soil properties. The sensors have been based on electrical and electromagnetic, optical and radiometric, mechanical, acoustic, pneumatic, and electrochemical measurement concepts. While only electric and electromagnetic sensors are widely used at this time, other technologies presented in this review may also be suitable to improve the quality of soil-related information in the near future.

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.