Soil moisture
From Biocrawler, the free encyclopedia.
Soil moisture is a ratio which quantifies the amount of water contained within a soil column. Commonly this is used as a measure of the amount of water in the vadose zone (above the water table).
Gaps between soil particles are called pore spaces or voids. These voids contain various amounts of either water or air. Soil moisture is the ratio of the volume of the voids to the volume of water, and thus ranges from zero, when there is no water in the soil, to one, when all the voids are filled with water and there is no air. Soil in the latter condition is saturated. The amount of void space within a soil depends on the distribution of particle sizes, and is quantified by soil porosity.
Soil moisture may be measured in situ with a tensiometer. In the laboratory, it is measured gravimetrically; by weighing the moist volume of soil, drying it, and then weighing it again. The difference in weight 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).
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 refered to as agricultural drought, and is a particular focus of irrigation management. Such conditions are common in arid and semi-arid environments.
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.
