The amount of water available in our soils
With a hotter, drier summer predicted for much of the country under El Niño, it is timely to discuss how different soils store water, and how much of this water is available for plant growth.
Water moves through the soil and is held in gaps or voids called pores. The relative size and proportion of the pores determine vital soil physical characteristics such as drainage and aeration. During heavy rain, soils become saturated as all the pores fill with water. After the rain, the water drains from the largest pores, called macropores, unless there is an impediment to drainage. This is an important process, as once these macropores have emptied, air can get into the soil. At this stage, the water stored in the soil is at field capacity (FC), though not all the water is available to plants.
How much of the water is available to plants varies between soil types. Coarse-textured soils, or light sandy soils, have mainly large macropores that drain quickly, while fine-textured soils, or heavy clay soils, have mainly small micropores that drain slowly, if at all.
Neither of these pore sizes is ideal in terms of water storage for plants. The macropores drain quickly and therefore do not hold water for a long enough period for plant roots to access it. Micropores hold onto the water with so much suction that this water is extremely hard to access.
In between these two pore sizes are intermediate-sized pores. These pores hold onto water and are easily extracted by plant roots. They tend to be most numerous in intermediate textured soils such as sandy clay loams. However, all soils have a combination of macro, micro, and intermediate pores, and the proportion of sand, silt, and clay in soil plays a major role in determining the drainage, and most importantly, the amount of water available to plants. This is called the profile available water (PAW).
The PAW of soil is the difference between FC and the wilting point (WP). Simply put, the WP is when the uptake of water is insufficient to meet plant needs, and the plant starts to wilt, or worst case, dies. The PAW of soils tends to be larger in intermediate textured soils (loams) and smaller in coarse sandy soils.
The PAW of soil is expressed as mm of water. At the height of summer, it is not unusual for a soil to lose 5 mm of water per day while during winter it can be less than 1 mm per day. If a soil has a PAW of 100 mm, for example, then it has approximately 20 days (calculated as 100 mm divided by 5 mm per day) of water (in summer) before plant growth would be compromised, assuming no rain fell during that time. The benefit of understanding the PAW of your soils is to help anticipate where potential feed deficits may occur on-farm.
For help understanding the soil types you have on-farm, contact your local PGG Wrightson Technical Field Representative.