Soil moisture

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:

• 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.

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).

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.