Enhancing soil with mulch, compost gives garden a better chance to thrive

BY MARY BERUBE
(This article originally appeared in the November 12, 2022 edition of the San Diego Union-Tribune)

Did you ever wonder how your neighbor gets those beautiful tomatoes every summer? Or how that one house has landscaping that just looks so healthy? One secret is what makes a really good home for those plants — the soil.

As gardeners (and everyone can be a gardener), we can’t control a lot of what happens to our plants. The amount of sunlight, the humidity, even the availability of water, might be variables that are difficult to manage. But we can work with our soil to give the roots of a plant the best chance of delivering water and nutrients to the leaves and fruit above.

What is soil? Your soil is a mix of 25 percent water, 25 percent air and 45 percent minerals. Those minerals are derived from years and years of San Diego rocks weathering away and depositing over time to become our landscape. This process results in the different amounts of sand, clay and silt, three of the components of soil. The ratio of these three to one another determines your soil texture, which in turn results in how well soil holds water and provides for plant growth.

Air creates space for nutrients to move to the roots, water brings those nutrients up to the leaves, and the minerals supply the nutrients such as nitrogen, phosphorus and potassium, as well as micronutrients such as calcium and iron.

• Not enough air? You’ll get compacted soil and the roots will have difficulty getting through to take up what they need.

• Too much water? You’ll see a failure to thrive and an increase in disease as the roots fail to take up the flood of water.

• Not enough minerals? You’ll see unhealthy-looking plants and vegetables that are probably not what you desire.

If you’ve paid attention, you’ve realized that the above components total only 95 percent. That last 5 percent of your soil is organic humus, roots and organisms — and the key to better soil. And, more importantly, organic matter and all of its benefits is a variable you can control.

Just like the food web in which a plant is eaten by an insect and an insect eaten by a bird, there is a soil food web. In fact, in a teaspoon of soil there are more microorganisms than people on Earth! Algae, microscopic insects, earthworms, beetles, ants and fungi are all in the best soil. Some act as decomposers, breaking down organic matter into “bite-size” pieces for plants. Some are artists gluing individual grains of clay, silt and sand together to produce even larger aggregates and a better soil structure. Some even act as partners with the plants, helping to supply needed nutrients.

To build up the soil and increase the garden’s ability to grow better plants, you can enhance your soil with amendments such as compost and mulch. Fertilizer is an amendment but is the one most overused by home gardeners. Building up the soil with compost and mulch is preferred and lasts longer.

Consult the California Master Gardener Tip Sheet, “Composting Is Good for Your Garden and the Environment,” which explains that compost contains nutrients and beneficial microbes, helps your soil hold water, improves plant growth and provides a supplemental amount of slow-release nutrients.

Mulch is complementary to compost, in that a layer of mulch overlies the soil, preventing water from evaporating and reducing weeds in vegetable, flower and raised-bed gardens. California native plants prefer little to no mulch once established and, generally, no soil amendments.

Soil is a fascinating topic. Did you know that California has a state soil? A middle school in Madera, along with federal, state and professional soil scientists, studied soil conservation. They drafted legislative bills as well as wrote poems about soil, made soil displays in science and art classes, and wrote an official state soil song. On Aug. 20, 1997, the San Joaquin Soil Series in the Central Valley became the official Soil of California.

To learn more about soil science, you can explore these resources: “Home & Garden — Healthy Soils for a Healthy California” from the University of California and “Soil Health | NRCS” by the U.S. Department of Agriculture’s Natural Resources Conservation Service.

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Berube has been a UC Master Gardener of San Diego County since 2020; before that, she was a Master Gardener in Tennessee and Virginia. She enjoys cycling and training her Australian shepherd. Her interest in soil was piqued by her degree in geology.