(a) The soil productivity index system shall be used to rank and classify soil map units, representing mineral soils or subdivisions thereof, and classified pursuant to section 370.8 of this Part into mineral soil groups 1 through 10. The commissioner shall assign a specific soil productivity index to each such soil map unit as follows:

1. (1) A soil map unit that represents the mineral soil that is determined to be capable of producing the most tons of TDN per acre per year shall be assigned a soil productivity index of 100. For the purpose of this paragraph, such soil map unit shall be known as the base soil map unit.

2. (2) In the case of all other soil map units representing mineral soils, the soil productivity index to be assigned to an individual soil map unit shall be determined by:

   1. (i) dividing the number of tons of TDN per acre per year that the soil map unit is determined to be capable of producing by;
   2. (ii) the number of tons of TDN per acre per year that the base soil map unit is determined to be capable of producing; and
   3. (iii) by multiplying the resulting quotient by 100.

(b) The number of tons of TDN per acre per year that a mineral soil's soil map unit is capable of producing shall be determined as follows:

1. (1) One or more crops shall be selected on the basis of their significance to agriculture in New York State.
2. (2) A maximum rotation for the selected crops shall be derived by using the universal soil loss equation, considering prudent management necessary to maintain soil productivity and capability.
3. (3) Yields, expressed in tons or other appropriate unit of measurement per acre per year, shall be estimated for each of the selected crops and shall be weighted in accordance with the relative importance of each of the selected crops in the rotation.
4. (4) TDN factors shall be developed for each of the selected crops.
5. (5) The number of tons of TDN per acre per year shall be calculated by multiplying the weighted yield for each selected crop by the TDN factor developed for that crop and by summing the results.