Soil Solarization


It seems that most gardeners are wrapped up with their spring gardens. It has been a tough year of heat and dry weather.

With the recent rains that some have been getting in my part of the state, many have moved on to a fall garden. Other gardeners have had enough of the summer heat and will be taking a break to start their fall/winter gardens much later.

For those who are taking a break and want to eliminate some of the weed seeds, disease, or nematodes, consider “solarizing” your garden.

I learned how home gardeners can solarize their soil several years back from my state vegetable specialist, Dr. Joe Masabni. Masabni states that soil solarization is a simple, safe, and effective means to control parasitic nematodes, soil borne plant pathogens (diseases), and some weed seeds.  It offers an alternative to some organic and non-organic pesticides as well as the lengthy crop rotations now needed to control many damaging soil pests. In addition, this procedure should give good weed control in many situations.

Radiant heat from the sun is the lethal agent involved in soil solarization. A clear polyethylene mulch is used to trap solar heat in the soil. Over a period of several weeks to a few months, soil temperatures become high enough to kill many of the damaging soil pests and weed seed to a depth of nearly 8 inches.

Just how hot can it get? Results from studies in 2011 and 2012 indicate that when the ambient air temperature reached 99 degrees F, the solarized soil reached over 170 F. That’s hot enough to cook chicken.

To be clear, none of these pests will be completely eradicated from the treated area, but their numbers in the top layer (say 6 to 8 inches) will be greatly reduced, allowing fewer problems in following crops.

The soil to be solarized must be worked up to seed-bed condition–that is, cultivated until it’s loose and friable with no large clods or other debris on the soil surface. A garden tiller will eliminate clods or other debris that create air pockets that reduce heating of the soil and keep the tarp from fitting tightly over the soil surface. A clean, flat surface will also prevent the accidental puncturing of the thin plastic mulch by debris.

Make sure moisture levels are adequate for working the soil before laying the plastic. If the soil is dry, water the areas to be solarized before laying the tarp.  Getting the soil wet at the beginning is crucial because most soil pests are more sensitive to high temperatures in wet soil than in dry soil. When possible, lay a soaker hose or drip irrigation lines under the tarp to maintain moisture levels during soil solarization.

You can use any cheap plastic painting “drop cloth”.  Using two layers of thin plastic sheeting separated by a thin insulating layer of air will increase soil temperatures and the overall effectiveness of a solarization treatment. The edges of the sheets must be buried or otherwise secured to prevent blowing or tearing of the tarp by the wind.

For effective solarization, the edges of tarps laid over raised beds must be buried in the adjoining furrows. Due to the lower temperatures reached in those areas, expect some increase in pest and weed problems along the edge of the stripped mulches.

Long, hot, sunny days are needed to reach the soil temperatures required to kill soil borne pests and weed seed. The longer the soil is heated, the better and deeper the control of all soil pests and weeds will be. During our hot summers, a solarization period of 4 to 6 weeks should be all that’s needed to control nematodes and soil borne plant pathogens.

For those concerned about losing beneficial microbes, indeed you will lose them in addition to you unwanted pathogens. However, populations of beneficial, growth-promoting and pathogen-antagonistic bacteria and fungi quickly recolonize solarized soil with the addition of compost or adding a biological control component to soil.

If you have plans for a fall garden, this may give you a late start, but the results of letting our hot summer work for you can be fantastic.

Cary Sims
Cary Sims is the County Extension Agent for agriculture and natural resources for Angelina County. His email address is Educational programs of the Texas AgriLife Extension Service are open to all people without regard to race, color, sex, disability, religion, age, or national origin.

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