Did you know that up to 80% of plant water requirements are met by groundwater? Without this water supply, how do you think mature trees would find the 100 to 300 liters (25-75 gallons) of water they need every day in midsummer? This is known as capillary action. It’s the ability of water to seep through the tiny spaces between grains of soil and reach the surface, like oil rising to the wick of an oil lamp. This is thanks to the surface tension possessed by water, just like any other liquid! This water source is constant, and the amount of water that rises depends on the size of the particles that make up the soil.
A sandy soil brings up less water than a clay soil. This explains why sandy soils dry out faster and require more watering than silty or clayey soils, because the spaces between the grains of sand are larger and make it more difficult for water to rise by capillary action.
When Does Soil Drought Occur?
This is when the amount of water consumed by plants, in addition to that which evaporates from the soil, exceeds that which the soil can bring up by capillary action. As less water rises from sandy soils, this type of soil dries out more quickly than clay soils, which have a greater water upwelling capacity.
Examples of Capillary Action
What happens when you water your houseplants by putting the water in the saucer? Capillary action causes the water to rise up into the soil, just as oil gradually soaks the wick of an oil lamp.
Houseplants
Similarly, why do white or yellowish salts form on top of the potting soil of your houseplants, especially in the pots of those you water through the saucer? This is because the water that rises to the surface of the potting soil carries with it salts from the fertilizers, and when it reaches the surface, it evaporates and the salts crystallize. If capillarity didn’t exist, this wouldn’t happen!
Trees, Lawns and Soil
Why is it hard to get a nice lawn under a big tree? Given that the main source of water is underground, who drinks the water first? The tree roots or the surface lawn?
Why is heavy, loamy or clayey soil harder to warm up in spring? It’s this phenomenon again! More water rises in the soil by capillary action, and since the water that rises is cold because it comes from melting snow, the sun has a hard time warming it.
Watering Containers
Why is watering pots and raised planters so difficult to regulate? Because they’re completely cut off from the natural water source. No upwelling of water by capillary action. This means you have to water a lot more, and watering is never as regular as watering by capillary action.
Another factor to consider is the distance between the water source (groundwater) and the need (the roots of your plants). Imagine if the oil from a lamp had to climb up a wick 1 metre (3 feet) long, would it make it to the top? The answer is no. Why not? Because the greater the distance between the source and the need, the less the liquid can travel. So, even if you’re growing in trays on the ground, by raising the bed you’ve reduced the natural supply of water by capillary action.
Groundwater is free and effortless. It’s to your advantage to take use it, just as plants do in nature, because nobody waters them in times of drought.
A Few Suggestions and Eco-Responsible Tips
Prioritize a watering system that uses less or no drinking water.
Control evaporation losses
- Never leave soil bare. Apply organic mulch around all your plantings. Mulch limits surface runoff, keeps roots cool and slows soil evaporation, significantly reducing the need for watering.
- Try to avoid surface watering, as any moisture on the surface will evaporate.
- Some watering systems are better designed to reduce evaporation, such as drip systems or soaker hoses.
- For gardens and landscaping, avoid sprinklers, whether manual or automatic, because in addition to unnecessarily wetting the foliage, the vast majority of the water you put in will evaporate. Watering in the evening to avoid this will only delay evaporation, which will still occur the following day. This delay won’t water the plant any more, but it will allow slugs, fungal diseases and weeds to take hold.
Restore Soil Capillarity
The drier the soil, the less capillary action can bring water to the roots, and the more your plants suffer. Wetting the surface doesn’t water the roots. Prioritize a type of watering that will bring the water to the right place, under the soil, without wetting the surface, but at root level, and above all that will re-moisten the “wick” so that natural capillarity resumes. Watering systems that take this logic into account are rare. A system like Logissol-O has been designed with this in mind. It’s the only type of watering system that reactivates capillarity.
Underground water is like wood in a wood stove. You light the fire with a match, but you don’t rely on the heat of the match to heat the house. Watering should be like the match. The water you supply should not be used to meet the needs of the plants, but rather to reactivate natural capillary action so that it can water your plants. This is what a proper watering system can do, with much less water and much less effort. This is more than laziness, it’s wisdom!
Great article. Many thanks. I do have one disagreement though. Many “experts” push mulching everywhere. However this makes it difficult and added work for our insectivore birds to get at the insects. Dead leaves are easier for birds to “shuffle” aside. In the areas where I do not mulch, the wrens are regularly at the ground finding insects which I presume they eat and take back to their nestlings. I never see them at the mulched areas.
Thank you for a better explanation than just ‘don’t use overhead waterers.’