A semi–quantitative approach to soil suitability
A semi-quantitative approach combines the presumed effects of each soil factor in a multiplicative function. Alternatively the rating for the most limiting soil factor is used to determine an overall crop suitability.
The overall formula takes the form of Suitability Index = (Rating Texture * Rating Soil Depth * Rating Salinity * etc…) or Suitability Index = Lowest Rating (all soil factors).
Global Agro-ecological Zoning
FAO’s agro-edaphic suitability classification used in AEZ is to a large extent based on experience documented by Prof. C. Sys and others. In this system, a suitability rating is proposed for each soil unit (FAO units in the Soil Map of the World), by individual crops at three defined levels of inputs and management circumstances. In an extension of the system using the Harmonized World Soil Database, soil qualities were used to rate individual crops. This soil assessment system has been expanded for larger regions and countries by FAO and others.
Visual Soil Assessment and Soil Health
For site specific soil assessment Hewitt and Des McGarry developed a methodology that was both cheap and fast and determined the main soil qualities with a minimum of laboratory measurements. The method was used widely by the LADA project