The Dirt on Saving Water in the Garden: Good Soil

Gardeners can take many steps to save water, such as saving rain water and using drip irrigation. But one of the most crucial steps lies beneath our feet, in the soil that protects and nourishes roots and controls water drainage and aeration.

black sand beach Maui
Black sand in Maui along Hana Highway. The black sand is made from lava rock.

What you can’t see: soil microbes

Tiny microbes feed soil and plants, relying largely on humus, the rich organic matter that results from decay of natural materials. When conditions are right, insects, earthworms and microorganisms in healthy soil create humus. Organic matter in the soil ensures that microbes thrive and the soil drains as it should. And when a plant is otherwise healthy, it’s less vulnerable to diseases and can likely survive with less water. In Master Gardener training, I learned that New Mexico soils have organic matter of 20 percent or lower on average. At our place, rocks take up 90 percent of the soil (slight exaggeration). We located our vegetable garden in an area that used to be part of an apple orchard and that’s near the river. The soil is much better than near the house, likely because of organic matter working its way into the soil over the years.

xeric rock garden
Got rocks? We’re certain the rock garden walls and borders were made with rocks excavated on our land! Many of the native and xeric plants have adapted to soil conditions.

Xeric plants and soil drainage

The thing is, many xeric plants are more affected by soil that remains wet than by lack of water. Lavender comes to mind; wet feet can cause root rot. Add a sudden drop in temperature to the dampness and lavender plants are at risk of dying. I’m seriously worried about ours after 18 inches of snow. Plant care instructions for nearly all xeric plants read “place in well-draining soil” because too much water in the soil suffocates a plant’s roots. Sand dries too quickly, but clay and compacted soils fail to drain.

Lavender does better with well-draining soil and less water.
Lavender does better with well-draining soil and less water.

Plants use valuable energy to pull moisture from soil. When they have to work harder to access water in soil, plants become stressed and wilt. If your soil drains quickly, you have to water more often and less deeply. Loam is the name used for balanced soil that has a fairly even mix of sand, clay and silt. As mentioned, clay soils hold too much water. Amending either extreme (sand or clay) with organic matter helps plants access the appropriate amount of water needed with less stress. Instead of adding sand or clay to balance soil, gardeners should use organic matter to gradually improve soil health and function.

This soil at the entrance to our vegetable garden is compacted foot traffic. Notice how water from snow melt stands in the soil.
This soil at the entrance to our vegetable garden is compacted from foot traffic. Notice how water from snow melt stands in the soil.
Only a few feet away is this raised bed with organic matter added. No standing water under the same conditions; the water is working its way down through the soil.
Only a few feet away is this raised bed with organic matter added. No standing water under the same conditions; the water is working its way down through the soil (and rocks).

Here are a few tips for ensuring your plants’ soil is healthy, providing nutrients, water drainage and oxygen to roots:

Add organic matter. The type of matter you add is more specific to your soil’s pH and other factors. But most natural organic amendments, such as manure or green manure, can help.  Also called cover crops, green manure is the purposeful planting in fall and winter of crops that restore soil nitrogen and organic matter used up by summer plants. When using animal manure, add it to compost or make sure it’s cooked completely; don’t add fresh manure to growing plants.

You can see the difference in the color and texture of the mushroom compost that sits on top of the summer vegetable bed. The soil needs some help.
You can see the difference in the color and texture of the mushroom compost that sits on top of the summer vegetable bed. The soil needs organic matter from the compost and some broken-up plant residue.

Properly handle plant residue. Although some schools of thought are in favor of letting spent plant material stay on the ground to decay and provide organic matter, gardeners should use caution. Insects overwinter in plant residue on the ground (good if you want monarchs, but you can’t really go around selecting which insects get to take up residence). And it’s never a good idea to use diseased plant material. We kept some healthy plant material, but anything that looks funky goes into compost bins or garbage. We don’t mess with “wilder” areas around the property.

An unripe pear and leaf material remain under a large, old pear tree. They'll compost and improve soil, but also attract ants and other insects.
An unripe pear and leaf material remain under a large, old pear tree. They’ll compost and improve soil, but also attract ants and other insects.

Use organic mulches. Adding mulch around (but not against) the base of the plant helps slow water evaporation from the soil. Organic mulches eventually break down, improving soil make-up.

Here at the Los Angeles County Arboretum and Botanic Garden, landscapers have added mulch around many beds, including blue fescue.
At the Los Angeles County Arboretum and Botanic Garden, landscapers have added mulch around around many plants, including blue fescue.
Cacti at the Arizona-Sonora Desert Museum in Tucson, Ariz. need no mulch!
Cacti at the Arizona-Sonora Desert Museum in Tucson, Ariz. need no mulch!

Rotate crops and use cover crops. In vegetable and herb gardening, move annual crops around so that the soil nutrients get a break. Families of plants differ in how they use soil nutrients.

Stop tilling. Rototilling, the traditional farm method, burns off carbon and adds to weed emergence. It also breaks soil up into smaller particles, which makes it less permeable for air and water. Repeated tilling leads to loose top soil. Heard about the Dust Bowl? Work added organic matter into the top few inches only, or let it sit on top of soil.

When in doubt, test your soil to determine how much of it is sand, clay or loam. You can perform your own home test pretty easily using a mason jar, as described on page 11 of this publication from the San Diego County Water Authority.