Field Capacity

After a soil is saturated, water first drains rapidly from the large pores under gravity, then progressively more slowly. Field capacity is the water content reached when this rapid drainage has largely ended and the soil holds about as much water as it can retain against gravity. Above field capacity, excess water occupies pores that would otherwise hold air, so soils wetter than field capacity are poorly aerated.

Saturation

The state in which all pore space is filled with water and aeration is minimal.

Field capacity

Water content after rapid gravitational drainage has largely stopped; the practical upper storage limit.

Permanent wilting point

The water content at which plants can no longer extract water and wilt irreversibly.

The water held at field capacity depends mainly on texture, structure, and organic matter. Fine-textured soils with abundant small pores retain much more water than coarse, sandy soils, and organic matter increases water retention. Because field capacity reflects the balance of pore sizes, structural changes such as compaction also alter it.

• Texture: clays and loams hold much more water at field capacity than sands
• Organic matter increases water retention and raises field capacity
• Structure and pore size distribution shape how much water is held against gravity
• Field capacity is a soil property; the actual water content fluctuates with weather and crop use

Field capacity is the upper reference point for how much water the soil can usefully store. Together with the permanent wilting point, it defines the plant-available water in the root zone. In irrigation scheduling, field capacity is commonly the target to which the root zone is refilled, and irrigating well beyond it wastes water to drainage and can waterlog the soil.

Growers and irrigation planners use field capacity as the full point of the soil water store. Monitoring how far soil water has fallen below field capacity, and refilling before the crop is stressed, is central to efficient irrigation. Because the value depends on texture and structure, it is best estimated for the specific soil rather than assumed.