Keep your soil covered. This is only one of the decrees from a recent presentation by the UC Master Gardeners of Napa County. But what does covering the soil mean? Why do we want to cover it and with what?
The answers are pretty simple. We want to cover the soil with plants, and it turns out that weeds are better than nothing at all.
An area covered with plants compared to bare, tilled soil provides a stark illustration of one of the causes of global warming. Bare soil is literally being cooked by the sun.
A short video during the Master Gardener presentation showed a temperature gun aimed at a green leaf. The temperature registered 59.7°F. A green piece of paper registered 89.6°F, and bare soil registered a whopping 138.9°F.
Large areas of bare soil produce a heat island effect, causing living microorganisms in the soil to die. Unfortunately, the damage doesn't end with lifeless soil. On a grand scale, these conditions change weather patterns and affect water supplies. No wonder bare soil contributes to global warming.
I remember my first-grade teacher discussing an article the class had read about farming. The article described the practice of plowing deep furrows in a field and leaving the field fallow for a season or two to rebuild fertility. We were all indoctrinated with such thinking in the 1950s. I remember thinking this notion sounded perfectly reasonable.
Science now shows us that nothing could be further from the truth. When soil is left bare, soil microorganisms and mycorrhizal networks are broken up. With nothing to sustain it, unplanted bare soil degenerates and the carbon cycle stops working.
In contrast, the process of photosynthesis is a cooling, or endothermic, activity as plants make the carbohydrates they need for energy. There is a net water gain when land is continuously covered with plants.
The conventional practice of tilling produces a net water loss. If the soil is planted with a cover crop, water can keep infiltrating the soil. Life beneath the surface keeps functioning and stays healthy.
That's just one of several reasons to plant cover crops. Legumes like fava beans and clover are good choices because they return nitrogen to the soil when they're chopped down and left on the soil. Fast-growing buckwheat feeds pollinators. Mustard and brassicas are natural soil fumigants, combatting nematodes. Grass roots hold soil in place and help keep the ground cool. All of these cover crops attract beneficial organisms, pull carbon from the atmosphere and help retain water.
Mulching is another good practice to keep the ground cool. We usually think of plant material when we think of landscape mulches, but inorganic materials like rock or stone can function as mulches, too. Organic mulch mimics nature, recycling nutrients back into living plants.
To mulch your garden beds, first lay down two inches of compost. Then top with three inches of mulch. Straw is an excellent organic mulch for garden beds. Leaves make great mulch, too. We just need to banish the unproductive habit of raking our leaves and filling up big plastic bags with them.
Perennial plants evolved to form compost-like layers and often rely on the decaying plant materials they generate. Keeping our gardens in a more natural state is preferable to having them perfectly manicured.
It's always a good idea to spread compost, but especially before you mulch. Some mulches, such as wood chips, can leach nitrogen out of the soil. If you apply compost first, the effect will be buffered.
If you follow these practices, you will improve soil tilth, build organic matter, retain soil moisture, slow mineral leaching, attract beneficial soil organisms and improve the yield and nutritional value of your fruits and vegetables.
Our takeaways are these:
Bare, tilled soil contributes to global warming.
Cover the soil with living plants.
Use cover crops.
When this isn't practical, add compost and mulch.
The presentation this information is gleaned from is “Soil is the Solution, healing the earth one yard at a time,” https://www.youtube.com/watch?v=JqA8DqBtRuo . It describes practices we can learn and implement about soil to help slow and ultimately reverse the damage humankind has done and slow global warming.
Become a Master Gardener Volunteer! UC Master Gardeners of Napa County is now accepting applications for the Class of 2023. Click on the ”Join Us” button at napamg.ucanr.edu to read the informational brochure and register to attend a mandatory information session for applicants. Applications are due before 5 p.m. on September 30.
Gardening with the Masters: Join UC Master Gardeners of Napa County and Ole Health on Saturday, August 20, from 10 am to 1 pm at Ole Health South Campus, 300 Hartle Court, Napa, for a workshop on gardening. Children five years old or older, accompanied by an adult, are welcome. Attendance is limited. Register here: https://www.olehealth.org/our-services/community-outreach-resources
Got Garden Questions? Contact our Help Desk. The team is working remotely so please submit your questions through our diagnosis form, sending any photos to mastergardeners@countyofnapa.org or leave a detailed message at 707- 253-4143. A Master Gardener will get back to you by phone or email. For more information visit https://napamg.ucanr.edu or find us on Facebook or Instagram, UC Master Gardeners of Napa County.