Early in spring after snowmelt or heavy rain, soil can reach its maximum water holdingt capacity, which means that all the pores are filled with water. As the soil dries out and water drains off, either naturally or with the help of subsurface drains, the soil reaches its field capacity.
Large pores empty
In this state of field capacity, the slightly larger pores are emptied of water and replaced with air, while the finer pores remain filled with water. The higher up in the soil profile the pores are, the more likely they are to be filled with air. In a soil with 50% solid material and 50% pores, at field capacity approx. 10-20% of the soil volume is filled with air and 30-40% with water.
The plant-available water present in pores in the soil is the difference between field capacity and permanent wilting point.
It is the diameter of the water-filled pores in (see table) that determines how easy or difficult it is for plant roots to extract water from the soil. The pores in the soil are the result of its texture and structure.
|Water extraction force (‘root suction’) in metres of water column (mwc)||
Equivalent pore diameter (mm)
Kerstin Berglund, SLU
Root hair in pore
1) Root hair
It is pore diameter that determines how tightly water is bound in the pores. The smaller the diameter, the more tightly water is bound and the harder it is for the root to extract it. Finally the root hair reaches its limit and can no longer extract water from the narrow pores.
The water in larger pores is easily available, but with decreasing pore diameter it takes progressively more energy by the plant to take up water according to image "Root hair in pore" above. The limit is at permanent wilting point, when root suction is no longer enough to extract water from the soil pores and the plant wilts. However, in practice plants cannot utilise all the water down to permanent wilting point, but give up long before.
Root depth decides
The amount of water that a crop can take up is decided by a combination of:
- The amount of plant-available water in (see table below)
- Root depth in the soil profile
- Root interweaving in the soil
|Soil type||Plant-available water (mm) per 10cm soil layer|
|Clay brown||approx. 10–15|
Source; Kerstin Berglund, SLU
Together, these three factors impose a type of biological wilting point. In this context, it is important for the farmer to know that soil compaction can impair water supply for the crop. If wheel slip compresses large pores in the soil, this impedes drainage and lowers the ability of the soil to supply plants with plant-available water.
Maximum water-holding capacity = all the pores are filled with water – as is the case below groundwater level or after snowmelt or persistent rain for example
Field capacity = free water has drained off down to a drainage depth of around 1m. This state is often referred to as drainage equilibrium, since water stops flowing in drains/ditches. In the soil profile above, the larger pores are filled with air at field capacity, while the finer pores still contain water
Permanent wilting point = when the water in the soil is bound with a water extraction force that exceeds 150m metres of water column (1500 kPa), the roots can no longer extract it. This limit is called permanent wilting point and represents water held in pores with a diameter that is less than 0.002mm
Texture = soil texture refers to the proportions of mineral particles with different average diameter, i.e. the relative proportions of sand, silt and clay in particular according to table "Particle size distribution" in chapter The building blocks of soil