By Janice Mathews, UC Master Gardener of Napa County.
Healthy soil is not just dirt under our feet. It is a living ecosystem that sustains plants, animals and humans.
Healthy soil has four components. About 25 percent is air, allowing soil organisms to return essential nutrients to the soil while providing pathways for water. Another 25 percent is water, which soil organisms need, like all living things. Roughly 45 percent is mineral, a mixture of sand, silt and clay.
The remaining 5 percent, and arguably the most important part, is fresh and decomposed organic matter, microbes and humus. This component supplies nutrients for all the soil microorganisms, which feed our plants and all the other organisms in the soil food web.
There are more microorganisms in a spoonful of soil than there are people on earth. What’s more, soil contains a tremendous diversity of organisms, only about 5 percent of which have been identified.
Through the interactions of mycorrhizal fungi, soil forms aggregates. These clumps of soil are bound together with a tough, sticky substance called glomalin. This substance gives soil a strong, stable texture, allowing air and water to move through it and creating smaller passageways for roots and for water retention. Glomalin also helps sequester carbon.
Plants use photosynthesis to feed soil microbes which in turn feed animals, including us humans, while cycling nutrients back to the plants. That small 5 percent of soil plays an important role in slowing climate change while supporting life.
We can all help build soil and keep it healthy. Here are a few things home gardeners can do:
Minimize soil disturbance. Don’t break up those soil clumps. They make your soil hold together when it rains, minimizing erosion. If you till or double dig, the soil must restart the building process. Plus, tilling uses fossil fuels and releases carbon into the atmosphere.
Build organic matter. Adding compost attracts the organisms that loosen soil and create air pockets for water and roots. Compost improves soil structure and health and increases moisture retention. It delivers nutrients to the plants in a form they can use, reducing the need for pesticides and fertilizers. No need to dig it in. Just cover your soil surface with compost to help it retain moisture.
Leave roots in the ground. Living roots keep the soil system running, feeding soil organisms. Try cutting off just the tops of vegetables at the end of their season and leave the roots to decompose in place. This practice works for cover crops as well.
Cover bare soil with mulch. An organic material such as straw, bark, leaves or lawn clippings will help retain water, keep soil cool and minimize compaction and weeds.
Aim for diversity. Diverse planting above the ground means diversity below the surface. Soil organisms thrive with an abundance of different nutrients. Diversity also helps reduce disease and pests.
Rotate crops. This practice helps minimize plant pathogens, reducing the need for pesticides. Planting the same plant in the same place every year also depletes soil nutrients. Try rotating your annual flowers as well to prevent diseases from getting established.
Reduce energy use. Use electric or battery-powered equipment instead of gas-powered equipment to reduce fossil fuel use. Hand tools are an even better choice, where possible, and may be healthier for you, too.
Minimize the use of synthetic fertilizers, herbicides and pesticides. Replace with organic products derived from natural sources such as plants, animals or the ocean.
Inorganic fertilizers (sometimes called synthetic or chemical fertilizers) come from mined minerals or lab-created chemical compounds. The processing itself contributes to global warming. Inorganic fertilizers may be faster acting and less expensive, but they don’t improve soil health and may harm it.
Inorganic pesticides and herbicides can kill the soil organisms that feed and protect your plants. They can also harm beneficial insects and may harm pets and people. Turn to them only as a last resort and use as little as possible.
Finally, use water judiciously. With proper irrigation at the right time, you can keep soil healthy and plants happy. Healthy soil absorbs and retains rain, minimizing runoff and contributing to a beautiful and productive garden.
Workshop: Join UC Master Gardeners of Napa County for a workshop on “Winter Rose Care” on Saturday, January 10, from 10 am to noon, via Zoom. The workshop will highlight pruning techniques for all rose types, along with everything needed to make your roses a success. Reserve here to receive the Zoom link.
Workshop: Join UC Master Gardeners of Napa County for a workshop on “Starting Your Own Vegetable Seedlings” on Saturday, January 10, from 2 pm to 4 pm, at UC Cooperative Extension, 1710 Soscol Avenue, Napa. We’ll cover all the basics, including proper moisture, temperature and light for successful seed starting, and care of seedlings until they are ready for transplant. We provide some seeds, containers and soil; bring your own seeds if you like. Reserve here.
Help Desk: The Master Gardener Help Desk is available to answer your garden questions. Use our online Plant Problem Help Form or email us at mastergardeners@countyofnapa.org. Include your name, address, phone number and a brief description of the problem. You can also visit us in person on Mondays and Fridays from 10 am until 1 pm at the University of California Cooperative Extension Office, 1710 Soscol Avenue, Suite 4, Napa.