Plant Physiology - Lecture Notes - O. M. Tarnopilska 2019

3. Water metabolism in plants. Diffusion, osmosis, chemical and water potentials, osmotic pressure
3.2 Forms of soil moisture

Depending on their availability to plants, forms of soil moisture are classified as follows. Gravitational Water fills the spaces between soil particles and is readily available to plants. However, it evaporates quickly and easily drains down into lower soil horizons under the force of gravity, which is why it is present in the soil only after rainfall. Capillary water fills the capillaries within soil aggregates and is highly accessible to plants. It is retained in the capillaries by surface tension forces; therefore, not only does it not drain downward, but it also moves upward from the groundwater table. Film water envelops colloidal soil particles. Water from the peripheral layers of Hydration shells can be absorbed by roots. Hygroscopic water is adsorbed by dry soil when placed in an atmosphere with 95% relative humidity. This thin layer of water molecules is held with such high force that its water potential reaches 1,000 bar, making it unavailable to plants.

The percentage of soil water content at which a plant enters permanent wilting is referred to as the wilting coefficient or wilting percentage. While wilting in different plant species may begin at the exact same moisture level, the time span from the onset of wilting to plant death (the wilting interval) can vary significantly. For instance, in bean plants, it lasts a few days, whereas in millet plants, it spans several weeks. Wilting begins later in plants with a more negative osmotic potential and a lower Transpiration rate.

«Dead storage» of soil moisture refers to The amount of water completely unavailable to plants. It depends on the mechanical COMPOSITION OF THE soil: the higher the clay content, the greater the «dead storage» of moisture. The amount of water available to a plant represents the difference between field capacity (the maximum amount of water retained by the soil) and the «dead storage».



Last update: 07/08/2026

Editorial and Educational Adaptation: This material has been compiled based on the primary/original source text. The project team performed an editorial review, corrected technical inaccuracies, structured sections, and adapted the content for an educational format.

What was processed:

  • elimination of formatting defects (OCR errors, structural breaks, corrupted characters);
  • editorial organization of content;
  • standardization of terminology in accordance with academic sources;
  • verification of factual statements against the original source text.

All mentions of the author, publication year, and origin of the primary text have been preserved in accordance with the source.