The water in 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.
Field Capacity, Wilting Point, Available and Unavailable water vs. Soil Texture
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.
Permanent Wilting Point
As soil is subject to evaporation and withdrawals from plants, water content decreases and tension increases to a point where plants can no longer extract water. Maintaining soil at this level for any length of time can cause permanent damage to plants. Tension at PWP can be as great as -15 bar (-1,500 centibar). VWC ranges from 2% for sandy soils to 30% for high clay-content soils.
Soil Moisture Terms
Root Zone Moisture
In practice, the soil-water relationship within the root zone is complicated and continually changing. A root system can extend well below the soil surface through several soil horizons. Each horizon can have a different soil type or structure, so water holding capacity and the relationship between VWC and tension can vary throughout the root zone. Furthermore, since the root system grows throughout the season, the set of soil conditions it is subject to continually changes. This presents challenges when using root zone soil moisture information for making decisions.
For historical readings of Soil Moisture and Soil tempertaure view data on the Harvest.com page