The water in your soil is stored on the surface of the soil particles, as well as in the pores, which are holes or gaps between individual soil particles. Pores will contain both water and air/oxygen. The amount of moisture in your soil will depend on weather (rain, heat, sun exposure, wind), runoff/drainage, and soil type (which dictates the size of the pores).
There are 2 important characteristic values for soil moisture:
- Field Capacity is the amount (%) of soil moisture or water that a soil can hold after excess water has drained away. It also represents the ideal balance of air and water in the soil pores. Above field capacity, the pores are oversaturated with water and oxygen levels are restricted.
- Wilting point is the moisture level, below which, plants will wilt and eventually die. Water in the soil below wilting point is not available to the plant.
Soil type will determine field capacity and wilting point, as the texture and structure of the soil will dictate how much and how fast water can infiltrate the soil. For example, sandy soil has large pores which allow water to move freely, and therefore also has a low field capacity. Conversely, the pores in clay are small, causing water to move slowly. Clay also forms a sticky mass when it gets wet, which slows drainage and increases water-holding capacity.