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Topsoil is composed of mineral particles and organic matter and usually extends to a depth of 5-10 inches (13–25 cm). Together these make a substrate capable of holding water and air which encourages biological activity. There are generally a high concentration of roots in topsoil since this is where plants obtain most of their vital nutrients.
Water balance components in agricultural land. The water balance components can be grouped into components corresponding to zones in a vertical cross-section in the soil forming reservoirs with inflow, outflow and storage of water: [2] the surface reservoir (S) the root zone or unsaturated (vadose zone) (R) with mainly vertical flows
Soil bulk density is equal to the dry mass of the soil divided by the volume of the soil; i.e., it includes air space and organic materials of the soil volume. Thereby soil bulk density is always less than soil particle density and is a good indicator of soil compaction. [ 47 ]
Extensive water balances are discussed in agricultural hydrology. A water balance can be used to help manage water supply and predict where there may be water shortages. It is also used in irrigation, runoff assessment (e.g. through the RainOff model [6]), flood control and pollution control.
Pores (the spaces that exist between soil particles) provide for the passage and/or retention of gasses and moisture within the soil profile.The soil's ability to retain water is strongly related to particle size; water molecules hold more tightly to the fine particles of a clay soil than to coarser particles of a sandy soil, so clays generally retain more water. [2]
Soil also functions by maintaining the quantity and quality of air by allowing CO 2 to escape and fresh O 2 to enter the root zone. [7] Pore spaces within soil can also absorb water and hold it until plant roots need it. The soil also moderates temperature fluctuation, providing a suitable temperature for the roots to function normally.
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The air/water balance is critical to maintaining high temperatures 130–160 °F (54–71 °C) until the materials are broken down. [ 9 ] Composting is most efficient with a carbon-to-nitrogen ratio of about 25:1. [ 10 ]