This function calculates the permeability of the top soil
calc_permeability(A_CLAY_MI, A_SAND_MI, A_SILT_MI, A_SOM_LOI)
(numeric) The clay content of the soil (%)
(numeric) The sand content of the soil (%)
(numeric) The silt content of the soil (%)
(numeric) The organic matter content of the soil (%)