Soil production function refers to the rate of

bedrock In geology, bedrock is solid rock that lies under loose material ( regolith) within the crust of Earth or another terrestrial planet. Definition Bedrock is the solid rock that underlies looser surface material. An exposed portion of bed ...

weathering Weathering is the deterioration of rocks, soils and minerals (as well as wood and artificial materials) through contact with water, atmospheric gases, sunlight, and biological organisms. It occurs '' in situ'' (on-site, with little or no move ...

into

soil Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...

as a function of soil thickness. A general model suggests that the rate of physical weathering of bedrock (de/dt) can be represented as an exponential decline with soil thickness:

de/dt = P_0 \exp

where ''h'' is soil thickness ''P₀'' m/yearis the potential (or maximum) weathering rate of bedrock and ''k'' ⁻¹is an empirical constant. 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 (for freely-drained soils). Parameters ''P₀'' and ''k'' are related to the climate and type of

parent material Parent material is the underlying geological material (generally bedrock or a superficial deposits, superficial or drift (geology), drift deposit) in which soil horizons form. Soils typically inherit a great deal of structure and minerals from th ...

. The value of ''P₀'' was found to range from 0.08 to 2.0 mm/yr for sites in

northern California Northern California (commonly shortened to NorCal) is a geocultural region that comprises the northern portion of the U.S. state of California, spanning the northernmost 48 of the state's List of counties in California, 58 counties. Northern Ca ...

, and 0.05–0.14 mm/yr for sites in southeastern

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

. Meanwhile values of ''k'' do not vary significantly, ranging from 2 to 4 m⁻¹. Several

landscape evolution model A landscape evolution model is a physically-based numerical model that simulates changing terrain over the course of time. The change in, or evolution of, terrain, can be due to: glacial or fluvial erosion, sediment transport and deposition, rego ...

s have adopted the so-called ''humped'' model.The humped soil production function
Arjun Heimsath. Arizona State University, Earth and Space Exploration This model dates back to G.K. Gilbert's ''Report on the Geology of the Henry Mountains'' (1877). Gilbert reasoned that the weathering of bedrock was fastest under an intermediate thickness of soil and slower under exposed bedrock or under thick mantled soil. This is because chemical weathering requires the presence of water. Under thin soil or exposed bedrock water tends to run off, reducing the chance of the decomposition of bedrock.

See also

Notes and references

Further reading