Soil moisture is the amount of water present in the soil.

Gaps between soil particles are called pore spaces or voids. These voids contain various amounts of either water or air. Soil moisture content can be expressed in different basis: Soil is the material on the surface of a lithosphere subject to weathering, and especially the earthy portion of that material. ... Water (from the Old English waeter; c. ... Look up air in Wiktionary, the free dictionary. ...

• Gravimetric: the mass of water/mass of solid material
• Volumetric: the volume of soil/total porosity

The amount of void space within a soil depends on the distribution of particle sizes, and is quantified by soil porosity. Used in geology, building science and hydrogeology, the porosity of a porous medium (such as rock or sediment) is the proportion of the non-solid volume to the total volume of material, and is defined by the ratio: where Vp is the non-solid volume (pores and liquid) and Vm...

Soil moisture may be measured in situ with different instrument, such as Time Domain Reflectometry (TDR), neutron probe, capacitance probe, etc. In the laboratory, it is measured gravimetrically; by weighing the moist volume of soil, drying it, and then weighing it again. The difference in mass corresponds to the mass of water which was in the soil (water is of a known density, therefore the volume of water can be determined). Time Domain Reflectometry is a form of one-dimensional radar. ... Density (symbol: ρ - Greek: rho) is a measure of mass per unit of volume. ...

When the soil gets too dry, plant transpiration drops because the water is becoming increasingly bound to the soil particles by suction. Below about a certain point, called the wilting point in agricultural settings, plants are no longer able to extract water. At this point they wilt and cease transpiring altogether. Conditions where soil is too dry to maintain reliable plant growth is referred to as agricultural drought, and is a particular focus of irrigation management. Such conditions are common in arid and semi-arid environments. Transpiration is a process caused by the evaporation of water from leaves of plants and its corresponding uptake from roots in the soil. ... A drought or an extreme dry periodic climate is an extended period where water availability falls below the statistical requirements for a region. ... High-altitude aerial view of irrigation in the Heart of the Sahara (, ) Irrigation (in agriculture) is the replacement or supplementation of rainfall with water from another source in order to grow crops. ... An arid environment has a high precipitation deficit, receiving much less precipitation annually than would satisfy the climatological demand for evaporation and transpiration. ... Semi-arid generally describes regions that receive low annual rainfall (25 to 50 cm /10 to 20 in) and generally have scrub or grass vegetation. ...

Soil moisture is more generally considered within the context of hydrology, where it represents the immediate store of infiltrating rainfall, before it either evapotranspires or contributes to groundwater recharge. Water covers 70% of the Earths surface. ... See: espionage, urban exploration, entryism, heating, ventilation, and air conditioning. ... In meteorology, precipitation is any kind of water that falls from the sky as part of the weather. ... Evapotranspiration (ET) is the sum of evaporation and plant transpiration. ... Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of geologic formations. ...

See also

• Water resources
• Water content