Solarizing the Soil - cooking the garden soil with summer's heat.

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Solarizing the Soil
This past spring, 1999 was the first time this desert gardener didn't plant a productive vegetable garden. The previous dry winter made food scarce for the local wildlife including birds, gophers, ground squirrels, rabbits and javelina. With no intention of feeding the animals by supplying them crops of bush beans, squash, tomatoes, and corn, the birds, rodents, and rabbits would have to fend for themselves and find their food source in the surrounding desert as nature intended. About the only things I stuck in the ground were a few summer squash and pots of herbs which could easily be watered by the drip irrigation system.

Thoughts of a successful late summer and fall garden crept into my head. I hoped to rebuild some of the garden's raised beds and decided to solarize the soil to cut down on the nematode populations that had increased over the past few years. Other plant damaging insects found in the soil would also be killed. The many earthworms in the compost amended soil would have to migrate to deeper depths away from the killing heat.

The idea for soil solarization is simple. Let the hot sun of summer cook the soil and what's in it using heat and moisture. What better place than Tucson, with summer temperatures above 100 degrees, to practice soil solarization.

Soil Prep & Clear Plastic Covering
Organic matter from the compost bin was added to the soil and thoroughly worked in with a garden fork. The bed was raked smooth with a slight raise in the center so moisture could roll off the plastic that would eventually cover the bed. The drip irrigation system was put back in place and then the raised bed was thoroughly watered so the heat could easily move through the soil.

I opted for clear, inexpensive plastic paint drop cloths for my plastic covering knowing they'd probably deteriorate rapidly under Tucson's hot sun and possibly need to be replaced after a few weeks. (Editor's note: the thin plastic drop clothes started to deteriorate about the middle of June after being on the beds for about six weeks.) I anchored the drop clothes to the edge of the raised beds with staples then nailed wood strips to hold the plastic firmly in place. It was a good thing because we had an awful lot of heavy wind this spring.

Clear plastic spread tightly
over moist soil, plus Tucson's hot desert sun, equals good soil solarization.

If I didn't have raised beds, I'd have covered the plastic with soil heaped around the edges to hold the covering in place.

Next I stuck a long soil thermometer into one of the beds so I could keep track of the change in temperature as the solarization took place. The soil reading at a four inch depth was about 72 degrees when the solarization started. After a few warm days in May the soil near the plastic covering was as high as 130 degrees but fell off rapidly into the eighties and nineties the deeper the thermometer was pushed into the soil.

I thought I heard the nematodes scrambling to go deeper into the soil to find cooler temperatures as the heat, like slow moving lava, crept over them. Nematodes are supposed to be very slow moving in the soil so here's hoping the heat got to them before they had a chance to migrate to safety.

When the solarization process is complete (anywhere from four to six weeks or longer if the clear plastic covering holds up), I want to be able to plant directly into the raised beds without disturbing the soil. That way no new unwanted organisms will be introduced or brought up from the depths of the soil.

Now when I plant in August I'll have to see if I notice any change in my crop productivity. I'll be able to tell about my success with the nematodes by checking the roots when I pull up the plants. With Tucson suffering somewhat of a drought due to La Niña this year and all the talk of conserving water this was the perfect time not to grow my customary vegetables. I saved water and improved my vegetable growing soil as well.

Soil Solarization Update (November, 1999)
Once the soil was solarized I planted a late summer crop of tomatoes for a fall (till the first hard frost) harvest. As the picture below shows I didn't get rid of my nematode problem. Two of the plant's roots were so badly infested neither tomato vine produced fruit. The tomato varieties planted were not nematode resistant so I invited the poor production.

I'm guessing the sun solarization of the soil doesn't penetrate deeply enough to rid the planting beds of nematodes or the soil wasn't wet enough prior to covering with the clear plastic to transfer the sun's heat deep into the soil. Fortunately there is another summer ahead, and another chance to try soil solarization.

Healthy roots from a tomato grown in a container using sterilized potting soil.
Nematode invested roots from a tomato grown in solarized soil.