LABORATORY EVALUATION OF A PROXIMAL SENSING TECHNIQUE FOR SIMULTANEOUS MEASUREMENT OF SOIL CLAY AND WATER-CONTENT

Precision agriculture, largely the application of information and comm unications' technologies to in-field data gathering and management, ma y be regarded as 'best practice' far crop growth in the future because of its twin goals of maximising economic returns whilst concurrently minimising environmental impact. The practice of precision agriculture , whether it be to differentially apply fertiliser, seed, pesticide, i rrigation or tillage requires detailed knowledge of the spatial and te mporal variation of crop yield components, weeds, soil-borne pests and attributes of physical, chemical and biological soil fertility. Howev er, a detailed description of fine or even coarse scale variation in s oil properties has always been difficult and costly to perform. Sensin g and scanning technologies are currently being developed to more effi ciently and economically describe and obtain precise information on th e extent and variability of soil attributes which affect crop growth a nd yield. Combining these technologies with vastly improved ground pos itioning systems allows detailed mapping of soil resource and crop yie ld variability which may therefore be an important input for site-spec ific decision making. Experiments were conducted to design an invasive sensor for real-time, simultaneous measurements of clay, organic matt er and soil water content from reflectance of a suitable wavelength or combination of wavelengths in the near infra-red (NIR) portion of the electromagnetic spectrum. Soil materials were prepared with varying a mounts of clay, soil water and organic matter according to a response- surface design, and the reflectance spectra measured at 2-nm intervals from 1300 nm to 2500 nm. Response-surface models were fitted to the r eflectance data at specified wavelengths. Reflectance showed significa nt response to clay content and soil water but not to organic matter. A thorough selection procedure using non-linear modelling and root-mea n-square-error of prediction was used to derive the four most suitable wavelengths (1600, 1800, 2000 and 2100 nm) for simultaneously measuri ng clay and soil water content. In a simulation experiment clay conten t was more accurately predicted than water content. (C) 1998 Elsevier Science B.V. All rights reserved.