Soil moisture

Newly dug swales filling with water Photo: DanMcTiernan

The right soil moisture is needed for optimum plant growth. It also regulates the exchange of energy and carbon between the soil and the atmosphere.

All soil water which can be taken up by plants is called 'plant available water' and its amount differs from site to site depending on soil texture, soil organic content, stone content, activity of small burying animals, plant rooting depth and root density, and soil management by humans.

Soil moisture characteristics

Following heavy rain, full saturation occurs when all the soil pore spaces are filled with water, preventing air entering the soil. If this situation exists for a long period, roots cannot breathe and plants die. As the water drains, air (including oxygen) can enter the pore spaces, which is vital for roots.

After the drainage has stopped, the large soil pores are filled with both air and water while the medium and smaller pores are still full of water. At this stage, the soil is said to be at field capacity, ideal for crop growth. This is usually 2-3 days after a major wetting event.

When the soil reaches permanent wilting point, the remaining water is no longer available to the plant, and plants will quickly wilt, and then die.

Ways to maintain soil moisture

There are two ways to measure how much moisture is in your soil and how much is available to your plants: a) Soil moisture content - the amount of water in the soil, usually described as a percentage based on mass or volume; b) Soil moisture tension - how hard the plant root has to work to extract water from the soil.

Keep moisture levels right by:

  • Hand watering with a hose or a watering can
  • Drip irrigation using a network of hoses with small holes
  • Wicking from a porous reservoir under the soil
  • Sprinkler hooked to the end of a hose that sprays water through the air
  • Soaker hose that leaks water along its length

This text is taken from the GROW Observatory document 'Making sense of sensors'. 

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This content is derived from work created within the GROW Observatory, which received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 690199.


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