Assessing soil quality for sustainable agricultural systems in tropical countries using spectroscopic methods

Soil quality assessment is a process by which soil resources are evaluated on the basis of soil function. The need for an effective, low-cost method to evaluate soil quality is important in developing countries because soil degradation is a major impediment to sustainable crop growth. Soil organic matter (SOM) or soil organic C (SOC) is an important indicator of soil quality (Gregorich et al., 1994) because it affects many plant growth factors, including water-holding capacity and long-term nutrient availability. In general, SOC varies across landscapes, soil types and climatic zones and is characterized by both labile and recalcitrant or humified forms. There are many techniques that measure the size and turnover time of SOC pools to evaluate soil quality in the laboratory or the field to help guide sustainability of agricultural management practices. Among these methods are several spectroscopic procedures which are rapid and relatively low-cost. The KMnO4 method developed by Weil (2003) has been adapted for field use and measures a labile C fraction. Near infrared (NIR) spectroscopy has also been adapted for field use and could provide a rapid method to measure soil C fractions (Shepherd et al., 2007). Another technique which has been studied is the use of diffuse reflectance infrared Fourier-transformed (DRIFT) mid-infrared spectroscopy which can identify labile and recalcitrant C in soil (Ding et al., 2002). However, many of these techniques have not been assessed under a wide range of soil types and cropping systems.