The soil production function: A brief history and its rediscovery

In 1877, G.K. Gilbert reasoned that the rate at which bedrock is converted to soil reaches a maximum under an optimal soil depth that facilitates contact between bedrock and water such that freeze-thaw and chemical weathering are maximised. In doing so, he outlined the functional dependence of soil production (bedrock weathering) on local soil depth. However, the concept of a soil production function does not appear to have been utilised until well into the following century when Carson and Kirkby (Carson, M.A., Kirkby, M.J., 1972. Hillslope Form and Process. Cambridge University Press, Cambridge, 475 pp) expressed it as a notional relationship. They also noted that at depths less than optimum, instability exists that will either drive soil depth to the optimal weathering depth or to zero depth. More recent work has also described a declining exponential soil production function in which the highest rate is at zero soil depth. Despite dealing with a fundamental issue in soil science, viz. soil formation, the soil production concept has been applied mostly to landscape evolution studies. The situation is ripe for its use in soil science and developments in techniques such as cosmogenic nuclides will assist this.