Water Holding Capacity

​Some soils can hold a lot of water and other soils hold very little. Since many plant nutrients are held in suspension (float around in water), and nearly all plants need water to stay upright and alive, the amount of water a soil can hold has a huge impact on plant health.

The water holding capacity of a soil also has a big impact on how often you need to water and feed your plants. Let’s find out what your soil’s water holding capacity is, and how it impacts your garden.

Water for plants

Not all the water held by a soil can be reached by plants. Sometimes it is too deep. In other cases, the water may be protected by a layer of rock. The amount of water in the soil that plants can reach and use is called plant available water. If a plant cannot reach enough soil water, it will hit a wilting point. At the opposite end of the moisture spectrum, field capacity (or drained upper limit) refers to the total amount of water a soil profile can hold. Anything beyond that saturation amount leaches into groundwater, or runs off, as urban drool.

Stages of water holding (USDA Bulletin 462, 1960) Public Domain

Soil texture

The first step to calculating your soil’s water holding capacity is to identify its texture. Soil texture refers to the percentage of sand, silt, and clay that makes up your soil. You may be surprised to learn that those words only refer to particle size, and not to any chemical properties. Very large particles (sand) have very large spaces, called macropores and micropores, between the particles, while clay particles have very tiny spaces.

These spaces allow air, water, nutrients, and roots to move through the soil. In the case of sand, the large macropores and micropores allow water to slip away quickly, carrying nutrients, pesticides, and other materials away with it. Loam, having smaller macropores and micropores than sand, slows the leaching of water, which increases that soil’s water holding capacity. Clay, with the tiniest macropores and micropores of all, has the highest water holding capacity, being able to hold up to six times the amount of water held by sand, but that ability comes at a price. Roots, air, and nutrients also have a difficult time moving through clay soil.

​Soil structure

The way the soil particles clump together into aggregates is called soil structure. There are eight types of soil structure, based on size, shape, and stability. Different soil structures can hold on to different quantities of water (inches per foot):

• coarse sand - 0.50
• fine sand - 0.75
• loamy sand -  1.00
• fine loamy sand - 1.25-2.00
• silt loam - 1.75-2.00
• silty clay loam - 2.00
• clay loam - 2.00
• heavy clay - 1.75-2.00

If you look closely at your soil with a hand lens or a simple microscope, you will be able to see these different shapes. The way the shapes create spaces within the soil impacts its water holding capacity. Soil structure also helps explain soil compaction. Some shapes and sizes, clay in particular, are far more likely to become compacted or have drainage issues

What is your soil’s water holding capacity?

You can conduct an experiment to see what your soil’s water holding capacity is, using simple household items. First, collect a soil sample from your garden. You may want to collect several different samples, for comparison. [Just be sure to label everything clearly!] Each sample should be the same size - 1/2 cup should work well. Then, place a paper coffee filter in a funnel, and put your sample in the coffee filter. Place the funnel into a large cup. Next, very, very slowly, pour 2 cups of water into the soil. Be sure to move the stream around, wetting all of the soil. Wait five minutes. Then, measure the water that flowed through the soil, into the cup. The difference between what you started with (2 cups) and what you ended up with will tell you how much water that amount of soil can hold.

Improving water holding capacity

Too much water and too little water are both bad for plants. Nutrient loss, leaching, and water pollution are also part of the equation. Adding aged compost to your soil improves its water holding capacity, along with its structure, texture, and microbe biodiversity. As organic material decomposes, a wide variety of spaces, minerals, and plant nutrients become available, allowing roots to reach the food and water they need, without struggling in a pit of mud.