How does soil store moisture?
Every year when the long, hot, dry days of summer are upon us, often with accompanying water restrictions, the question of how to increase the ability of soil to store water becomes top of mind.
Before diving into soil moisture retention strategies, it's useful to review how soils store water in the first place.
The importance of soil structure
The capacity of any given soil to store water depends mainly on the structure of the soil and the organic matter content. Soil is composed of tiny rock fragments in the form of sand, silt and clay, which are bound together with organic matter into clumps of varying sizes known as aggregates. The space in between these aggregates is called the pore space, where the air-water exchange takes place. The amount of water a soil can store in its pores depends on the amount of pores and their size.
How soil holds water
Water in large pore spaces is pulled quickly through the soil profile by the force of gravity, while water in small pore spaces is able to resist the downward gravitational pull. This is because the clay and organic matter particles that the soil aggregates are composed of carry a negative charge, which the positive ends of water molecules can bind to. This enables the water to be held against the force of gravity but not so strongly the roots cannot absorb it. Most soil water plants absorb is from these smaller pore spaces, as water is held there longer than it is in the larger pores.
How to increase moisture holding capacity
The principles behind increasing water storage capacity in your soil are the same as for creating a healthy garden soil in general. Support the formation of aggregates in your soil by feeding your soil microorganisms with compost or other forms of organic matter, as they produce the glue that holds aggregates together. Avoid walking on soil while it is wet as this compresses aggregates and the pore spaces. Avoid over-enthusiastic tilling unless the soil is compacted. Mulch the top 1-2 inches of your garden beds with compost, bark, straw, or other materials to stop the evaporation of water from the surface of the soil, as water evaporating from the soil surface will pull more water along behind.
On irrigation
When irrigating plants in the ground, less frequent deeper drinks is more effective than frequent small drinks, as a longer irrigation time enables all the small pores to fill up for the plant to access later. Plants in pots will dry out much faster than in the ground, and most potted plants will need to be checked every day during heat spells to see if they need water.