b) Nutrient Availability Index (NAI

b) Nutrient Availability Index (NAI): The NAI, is based on the method developed by Radcliffe
et al. (1982) and is given by NAI= N x P x K x pH. The soil map provides information of N, P, K and
pH, those of which were used in the overlay process to create the spatial layer of NAI. The values of the rating factor of the NAI component were given by Mongkolsawat et al. (2009a), Mongkolsawat et
al.(2009b), Putklang et al. (2010) and Mongkolsawat and Putklang (2010).
c) Water retention (I): Water retention was compiled from soil texture on which the factor
rating values for crops were based. The values assigned to the crops are referred to a number of studies (Mongkolsawat et al., 2009a, Mongkolsawat et al., 2009b , Putklang et al., 2010 and Mongkolsawat and Putklang, 2010) by which the ‘I’ layer was established.