Soil fertility assessment

The time-consuming collection of detailed objective soil content measures is justified when biophysical analysis is warranted. The classification criteria for the enrichment and the paucity of the eight soil chemical properties were assigned according to the second soil survey in Shaanxi province [48], as shown in Table 4.

In order to eliminate the dimensional differences among the soil properties, the eight soil chemical properties, comprising, AN, AP, AK, TN, TP, TK, OM, and pH, were tested and nondimensionalized according to the following computational formulae, Eq 11:

where F_i is the nondimensionalized fertility index of i soil properties, C_i are the measured values of i soil properties estimated by ordinary kriging or regression kriging at a spatial resolution of 5 m, and x_a, x_c, and x_p are the indices for the eight soil chemical properties, as shown in Table 5.

The comprehensive soil fertility index (F), was calculated as follows:

where F¯i is the average value of each fertility index, F_iminis the minimum value of each fertility index, and n is the index number [49,50]. i.e., n = 8 in this study. Soil fertility grading was based on the method of Kan and Wu (1994), as shown in Table 6 [50]. We improved Nemerow index (F) and replaced the maximum value for a single factor in the original Nemerow index (F0), which is mainly used for soil pollution assessment, with the minimum value of single factor based on the Liebig's law of the minimum. In the improved Nemerow index, additional amendments (n –1)/n are used to increase the credibility of the evaluation.