Wilting Point

Did you know that soil has a wilting point?

It’s not that soil wilts, instead, wilting point is reached when the water needed by a plant to stay upright has been used up.

A plant rooted in soil that is beyond the wilting point (Pewebe) CC BY-SA 3.0

Soil moisture

Water, plants, and soil perform an intricate dance. The soil has spaces, called macropores and micropores, that allow air, roots, and water to move through. The water molecules in the soil are strongly attracted to each other, using surface tension. This is how soil holds onto water, despite the pull of gravity. The amount of water a soil profile can hang onto is called its water holding capacity, or field capacity. When a soil is holding all the water it can, and becomes saturated, any additional water is pulled into the ground water by gravity, or runs off as urban drool, where it eventually is discharged into rivers, lakes, and oceans. At the opposite end of the soil moisture spectrum, a soil can be so dry that it becomes hydrophobic. Hydrophobic soil actively repels water.

​Sponges act the same way. If you have a completely dry sponge, water will tend to run across the top, rather than be absorbed. Once a little water is absorbed, a lot more is pulled into the spaces that make up the majority of a sponge. Finally, if even more water is added, it will simply flow through the sponge.

​Why the wilt?

Plants wilt for several different reasons. Bacteria or fungi may be blocking the xylem, there may be too much salt in the soil, ice damage may have occurred, or because the soil has reached the wilting point. When plants do not contain enough water, plant cells cannot remain plumped up, or turgid. As water becomes less available, cells shrink and become floppy. [Tree trunks do not get floppy because they contain lignin. Unlike cellulose, which is a sugar-based material, lignin is alcohol-based, but we will discuss lignin another day.] The problem with wilting is that there is a point of no return. This is a soil’s permanent wilting point.

Permanent wilting point

Permanent wilting point is death for plants. If a soil moisture rating reaches or surpasses the permanent wilting point, it doesn’t matter how much water you add later, the plant will die. This occurs more often with containerized plants, but it can happen anywhere.

Soil texture plays a big role in how much soil moisture is not enough to keep a plant alive. This is due to the soil’s ability to hold the water so tightly that plant roots cannot suck it in. The permanent wilting point occurs at 15 to 20% for clay soil, 10 to 15% in loamy soil, and at 5 to 10% in sandy soil. 

You can monitor soil moisture using an inexpensive moisture meter.