Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Soil production function refers to the rate of bedrock weathering into soil as a function of soil thickness. The reduction of weathering rate with thickening of soil is related to the exponential decrease of temperature amplitude with increasing depth below the soil surface, and also the exponential decrease in average water penetration. Parameters P0 and k are related to the climate and type of parent materials. Arjun Heimsath found the value of P0 ranges from 0.08 to 2.0 mm/yr for sites in Northern California, and 0.05–0.14 mm/yr for sites in Southeastern Australia. Meanwhile values of k do not vary significantly, ranging from 2 to 4 m-1.