Soil moisture is the water content of the soil. It can be expressed in terms of volume or weight. Soil moisture measurement can be based on in situ probes (e.g., capacitance probes, neutron probes) or remote sensing methods.

Water that enters a field is removed from it by runoff, drainage, evaporation or transpiration. Runoff is the water that flows on the surface to the edge of the field; drainage is the water that flows through the soil downward or toward the edge of the field underground; evaporative water loss from a field is that part of the water that evaporates into the atmosphere directly from the field's surface; transpiration is the loss of water from the field by its evaporation from the plant itself.

Water affects soil formation, structure, stability and erosion but is of primary concern with respect to plant growth. Water is essential to plants for four reasons:

In addition, water alters the soil profile by dissolving and re-depositing mineral and organic solutes and colloids, often at lower levels, a process called leaching. In a loam soil, solids constitute half the volume, gas one-quarter of the volume, and water one-quarter of the volume of which only half will be available to most plants, with a strong variation according to matric potential.

Water moves in soil under the influence of gravity, osmosis and capillarity. When water enters the soil, it displaces air from interconnected macropores by buoyancy, and breaks aggregates into which air is entrapped, a process called slaking.

The rate at which a soil can absorb water depends on the soil and its other conditions. As a plant grows, its roots remove water from the largest pores (macropores) first. Soon the larger pores hold only air, and the remaining water is found only in the intermediate- and smallest-sized pores (micropores). The water in the smallest pores is so strongly held to particle surfaces that plant roots cannot pull it away. Consequently, not all soil water is available to plants, with a strong dependence on texture. When saturated, the soil may lose nutrients as the water drains. Water moves in a draining field under the influence of pressure where the soil is locally saturated and by capillarity pull to drier parts of the soil. Most plant water needs are supplied from the suction caused by evaporation from plant leaves (transpiration) and a lower fraction is supplied by suction created by osmotic pressure differences between the plant interior and the soil solution. Plant roots must seek out water and grow preferentially in moister soil microsites, but some parts of the root system are also able to remoisten dry parts of the soil. Insufficient water will damage the yield of a crop. Most of the available water is used in transpiration to pull nutrients into the plant.

Soil water is also important for climate modeling and numerical weather prediction. The Global Climate Observing System specified soil water as one of the 50 Essential Climate Variables (ECVs). Soil water can be measured in situ with soil moisture sensors or can be estimated at various scales and resolution: from local or wifi measures via sensors in the soil to satellite imagery that combines data capture and hydrological models. Each method exhibits pros and cons, and hence, the integration of different techniques may decrease the drawbacks of a single given method.

Water is retained in a soil when the adhesive force of attraction that water's hydrogen atoms have for the oxygen of soil particles is stronger than the cohesive forces that water's hydrogen feels for water oxygen atoms. When a field is flooded, the soil pore space is completely filled by water. The field will drain under the force of gravity until it reaches what is called field capacity, at which point the smallest pores are filled with water and the largest with water and gases. The total amount of water held when field capacity is reached is a function of the specific surface area of the soil particles. As a result, high clay and high organic soils have higher field capacities. The potential energy of water per unit volume relative to pure water in reference conditions is called water potential. Total water potential is a sum of matric potential which results from capillary action, osmotic potential for saline soil, and gravitational potential when dealing with downward water movement. Water potential in soil usually has negative values, and therefore it is also expressed in suction, which is defined as the minus of water potential. Suction has a positive value and can be regarded as the total force required to pull or push water out of soil. Water potential or suction is expressed in units of kPa (10³ pascal), bar (100 kPa), or cm H₂O (approximately 0.098 kPa). Common logarithm of suction in cm H₂O is called pF. Therefore, pF 3 = 1000 cm = 98 kPa = 0.98 bar.