Gardening presents many challenges, natural and human-made. Eliminating as many of these challenges as possible is a constant goal. An excellent first step is to create a sterilization area in your garden to sanitize soils, tools, and other items prior to their use.
One of the problems associated with purchasing manufactured soils and composts in garden centers is that you have no idea how they have been processed and handled or whether they are what they say they are. For example, biomass from water filtration systems is often sold as "top soil" or "mushroom compost" in those 40-lb bags. Biomass is not necessarily a problem if it has been properly treated but should be labeled as such. Also, the packaging on landscape products often is perforated or damaged during handling, thereby permitting moisture, insects, mold, mildew, bacteria, fungus, and disease to penetrate the contents, any one of which could be detrimental to your plants. Furthermore, each season, the retail soils, packaged composts and manures seem to be getting worse, whether sold by a Big Box retailer or a local garden center. Unfortunately, this situation is likely to worsen before it gets better. As the cost of food in the supermarket and its risks encourages more people to grow their own, the demand for soil products will only increase, causing some retailers to sell whatever they can get their hands on without forethought as to what damage poor quality control of these products will do to the good intentions of their customers.
The good news is that there are several economical means a gardener can use for managing these problems, depending upon budget and scale of operation. The first, of course, is cultural: don't introduce items into your growing environment without first determining their relative safety. If possible, set up an area to disinfect and clean items before you introduce them in your garden.
For most items, a benign disinfectant, such as household bleach (5.45 percent sodium hypochlorate) mixed with nine parts of water is adequate to thoroughly wash containers and tools used in the growing environment. Allow items to dry completely before re-introduction to the garden. Some containers and tools can be disinfected in an automatic dishwasher. Fabrics, such as row covers and pots, along with greenhouse plastics can be sanitized in washing machines. Metal items (pipes, posts, watering cans, tools, etc.) can be treated with heat: direct sun, boiling water, vapor steam, a flame weeder, torch, or heated over hot wood ashes in a fire pit depending upon the type of item.
Plant materials can be stabilized with hydrogen peroxide, the type available at local pharmacies, dollar stores, or supermarkets. Hydrogen peroxide is a liquid oxygenating product used widely as a treatment for wounds, dental problems, pollution control, and detergents. More recently, hydrogen peroxide has been introduced to control viral diseases, such as mosaic virus on squash. To treat plants, mix 8 oz. per gallon of water to spray as an organic disease fighter on plants. It is quite effective on bacterial diseases. Hydrogen peroxide is not a stable compound and can be toxic when not properly handled, but it represents another tool in the organic gardener's arsenal to control agents of microscopic diseases and pathogens.
Gardeners can also replace soil with soilless alternatives, especially in raised beds and containers. Materials like forest fines, rockwool, wood chips, sawdust, sand, rock, clay and pumice granules, flexible polyurethane foam blocks, and coconut fibers are great for growing all manner of crops: flowers, shrubs, vegetables, and small trees. In Holland, growers reported, on average, a 10 to 20-percent increase in cash income when using non-organic substrates.
Nonetheless, despite a gardener's best practices and efforts, there will always be some things that just "blow" in from elsewhere. In treating soils for planting, three inexpensive methods are available to the prudent gardener: bokashi inoculation, pasteurization, and solarization. Testing the soil is always a best practice to determine the best possible strategy. These three controls can be used for general problems associated with weed seeds, insects and pathogens.
Inoculating Soils with Bokashi
The easiest way for most gardeners to inoculate retail soils and other materials, which is also the simplest, is to use a bokashi inoculant to treat the packaged soils or solarization. Sometimes, a combination of bokashi, cornmeal, and other organics also are required. Although the cornmeal and bokashi are fungicides, bokashi works best in an anaerobic environment. Use of natural inoculants, such as bokashi and cornmeal in combination with vermicomposting or lactobacilli, are highly effective against many soil pathogens, but takes time to produce a complete cleansing.
Used widely today to compost kitchen waste indoors, there are two main components of the Bokashi system: an airtight container and a microbial inoculant that is made from wheat bran, rice bran, blackstrap molasses, mineral rock salt, and microorganisms (lactobacilli, fungi/yeast, and phototropic bacilli). These products can be purchased in a kit or you can make your own.
To sanitize bags of soils, my method is to load five 40-lb bags of these "soils" in a 32 gallon plastic trash container, putting a generous portion of inoculant combined with some horticultural cornmeal between each bag. The container is closed and allowed to overwinter. Prior to using, place the bokashi-laden material in a large wheelbarrow and add any amendments that you want (perlite, vermiculite, sphagnum peat, coarse sand, hardwood fines, cocopeat, fertilizers, etc.). After thoroughly mixing everything, you can begin using the revitalized media with confidence. The media can be overwintered in beds as well where earthworms will find the material a welcome feed. Lightly till in spring.
Pasteurizing Packaged Soils
Pasteurization is the process of heating soils to a temperature between 115° F and 215° F, which usually kills most diseases, fungus, weeds and insects in the soil. The standard processing temperature is usually 180° F (see Table 1). Of course, to use this method, a good thermometer is a must. I recommend that gardeners use this method for small quantities of soil, such as when recycling potting soils.
Small-scale pasteurization can be accomplished in a kitchen oven, but is best accomplished outside because of the potential for highly unpleasant odors generated when the soil is heated. Remember, you do not know where they came from or how they got to your store. Soils may be treated in different type ovens: electric oven, microwave oven, or over charcoal in a backyard cooker. I have found the microwave to be the most efficient and recommend it with a qualified WARNING: First, make sure the integrity of the microwave door seal has not been compromised and, second, prior to using a microwave oven, make sure you check for any metals (paperclips, safety pins, nails, screws, aluminum foil, etc.) in the soil by screening each and every batch you process as these can cause an explosion.
Best practice: Use these pasteurization techniques as a last resort. Microwave pasteurization is quicker for small quantities of soil. If you do employ pasteurization in your gardening hygiene system, always screen the media for metal and other objects each and every time prior to processing and, certainly, do so before putting any batch in a microwave oven. To guide you, use the carpenter's rule of thumb for sawing wood: measure twice, cut once. In microwave pasteurization, the rule is screen twice, process once. Also, any kitchen implements used in pasteurizing (thermometers and containers) should be kept for use only for other soil pasteurization projects.
Table 1. Temperatures necessary to kill soil pests
Water molds (pythium and phytophthora)
Worms, slugs, centipedes
Most plant pathogenic bacteria
Most weed seeds
Few resistant weed seeds & plant viruses
Pasteurizing by Microwave Oven
The primary benefit of using a microwave oven (MW) for soil sterilization is that it is both a convenient as well as a faster method (2 to 5 minutes per kg of soil compared to 30 minutes in a conventional oven) of eliminating soil borne pathogens without excessive detrimental effects on some key soil life, such as prokaryotes, organisms important to cell respiration. Used MW units can be acquired for free or inexpensively making it a very cost-effective solution for many gardeners. Microwave soil treatment effectively eliminates Pythium spp., a common plant disease that causes root rot, fusarium spp. and non-cyst-forming nematodes. They infect a large range of hosts and can live a long time in the soil and neither crop rotation nor field fallowing are effective controls. Not so susceptible to MW treatment are Rhizoctonia, a pathogen responsible for "damping off", a condition that kills many seedlings, and Vesicular-arbuscular (VA) mycorrhizal fungi, which were reduced, but not eliminated in the experiments carried out by Ferriss. Apparently, regular spraying with biological teas (such as chamomile tea, cloves, bat guano, and seaweed) is just as effective a control for Rhizoctonia. Research shows also that there is less post-processing recolonization by fusarium and other fungal pathogens when using MW as opposed to aerated steam.
When processing, the best containers are a microwavable dish, 1.5 mil polypropylene bags, or a clean, durable cardboard container. There should be some moisture in the soil, but not as much as in a conventional oven. Having water in the media is important because when it's heated by the MW radiation the heat is transferred to other soil components by conduction (Ferris, 1984) thereby increasing the efficiency of heat transfer throughout the media.
From Ferriss' experiments, we learn that 4 kg (about 9 pounds) seems to be the maximum amount of material that can be treated effectively in standard microwaves at a time of about 7 minutes. One approach is to treat 2 kg of media for 300 seconds (5 minutes), followed by a re-mix and microwave again for another two minutes or more to complete the sterilization cycle. The temperature check is repeated and then the soil is set aside to cool. Expect the rate of cooling to be greater after treatment for drier media than relatively moist media. If the media is too moist, however, it is best to let it dry out to be treated later.
Finally, be careful not to recontaminate the processed media with unclean tools, unprocessed soil, amendments, or seeds. After all your effort, it is equally important to clean old or used plant containers, pots, and tools with a 10% chlorine bleach disinfectant.
Soil solarization is a process of moistening and heating the soil with the help of the sun. It is an effective way to kill or suppress many fungi, nematodes, pathogens, weeds, and insects in the top six inches of the soil in locations with high summer air temperatures (greater than 85°F during the mid-afternoon is preferred, but not essential) and plenty of sun. Research has shown that solarization is often as effective as herbicides, fumigants, and other hazardous and expensive chemical pest control methods. It is most immediately effective in the top 2 inches of the soil, with varying results below that.
Solarization involves placing a transparent clear plastic or a transparent tarp on the soil surface. The plastic uses energy from the sun to raise the soil temperature. The tarp should be transparent because black or colored tarps will not heat the soil sufficiently. A single layer of 1-ml clear plastic laid on the soil surface can greatly increase solar soil heating and provide good control of weeds and pathogens. Recent studies at San Diego State University (California) and Tuskegee University (Alabama) have demonstrated that increased temperatures can be reached with a second layer of plastic; bubble packs also worked well.
Ideally, the best time to solarize is in the high heat of summer for four to six weeks. Soil solarization is not effective when a crop is growing, but solarizing separate portions of a garden at different times is an option – either in late spring after early crops have been harvested or in mid-summer to late summer in areas of the garden where a spring-planted crop was harvested. Still, the best time, is during the dog days of summer. Most soil biota, such as earthworms, will begin vacating an area at the first sign of intense heat in the early stages of the process.
To solarize soil in vegetable gardens or planting beds, begin by leveling the soil and removing any plants, including any large weeds and crop debris. Then, break up large clumps of soil and moderately irrigate the soil. Moisture helps the heat to penetrate more deeply and makes pests, primarily fungi and bacteria, come closer to the surface where they are more sensitive to high temperatures. Next, anchor clear plastic or a tarp on the soil and burying the edges of the tarp or plastic under soil or boards. Leave the covering in place for four to six weeks in full summer sun.
The influence of these high temperatures on weeds, seeds, and pathogens is complex, but the results are clear, solarization has been found to reduce or eliminate many common plant pathogens like fusarium, verticillium, and root rots. This effect has led to most of the research on soil solarization. Control of some insect pests through the destruction of their eggs or larvae has also been noted, especially those in the top six inches of the soil. In other cases, solarization can promote a growth response on top of weed and disease control. Deep rooted, heat tolerant weeds with rhizomes are usually suppressed but may not be killed by a single treatment and some old seeds still may germinate after solarization. Feeding the soil in the autumn to encourage seed germination and then aggressively removing any plants with rhizomes that germinate gives increased control.
Deep tillage should be avoided at all times. Follow up strategies to soil solarization include: (1) apply horticultural cornmeal and organic teas in the fall to increase microbiological activity in the solarized soil over the winter and as a control over more persistent fungi pathogens; or, (2) sow a winter cover crop with weed suppression attributes, like winter tares, winter hardy to Zone 5, to improve control of the more resistant weed species thereby preventing seed germination in the first place. Sow winter tares in August in well-irrigated pH balanced soil; till in before flowering in the spring. Winter tares is the perfect crop to sow prior to growing a soil cleaning, nitrogen demanding crop in the brassica family and as the final stage in controlling problems in the affected growing bed.
John W. Bartok, “Steam Sterilization of Growing Media.” In Landis, T.D.; Dumroese, R.K., technical coordinators. Proceedings, Forest and Conservation Nursery Associations. 1994, July 11-14; Williamsburg, VA. Gen. Tech. Rep. RM-GTR-257. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station: 163-165. http://www.fcnanet.org/proceedings/1994/bartok1.pdf.
R. S. Ferriss, “Effects of Microwave Oven Treatment on Microorganisms in Soil.” The Phytopathological Society, 74:1, 1984, p. 124. [Internet: http://www.apsnet.org/phyto/PDFS/1984/Phyto74n01_121.PDF]