As Gardeners we all understand that at times there is a need to use chemicals in our garden to protect and improve our crops.
The most common chemicals we use are chemical based fertilizers and soil conditioners like lime that improve the health of our plants. Often these chemicals are safer for the home or small gardener and cheaper for larger farms then the alternative of organic products.
When it comes to pesticides, fungicides and anti bacterial products that protect our crops from bugs and disease we are often a little more careful because these products can make us sick or even kill us if used improperly.
Most chemicals that are applied to vegetable gardens come with recommendations of how much, how often and how long to wait after application before harvesting.
You must also be extremely careful about how you mulch any contaminated fruit or plant material because it can cause problems in the next years crop. You should discard any plants and fruit that have disease and not return that material to your soil.
Solarizing Soil to reduce Pests and Disease
Fortunately there are options that you can take in your vegetable garden to reduce the need of chemicals. This is not to say that you can eliminate all need of chemical products but by following simple methods you can reduce your need of chemicals and improve the quality of your soil.
Solarization is a method of using the sun to increase the heat of your soil to about 150F or somewhat higher to kill bacteria and other diseases along with some pests.
The method is rather simple but it does require some work.
For solarization to work your soil must be moist and to contain the heat you must use plastic just like putting a lid on a pot of boiling water.
Begin by tilling your soil and piling it into a row that is no more then 5 feet wide and at least 2 feet deep.
If you are planting in raised gardens you will need to prepare a bed of soil deep enough to raise and keep the temperature for a number of weeks and if you only have 6 inches of topsoil it will not work well enough to do the job. You can combine all your soil or double over your soil in each bed as you see fit.
Once the soil is tilled and piled you will need to saturate it with water. This can be more difficult then you think and professionals will normally embed a irrigation pipe in the center of the pile. home gardeners will just need to be diligent and also turn the soil by hand so water penetrates deep.
Around the edge of the pile you want to dig a trench that will accept the edges of the plastic and then be back filled to hold it in place.
Should you use Clear, White or Black Plastic in your Garden?
Although your first instinct might be that black plastic would attract more sun and better heat the soil you will actually get the best results with clear plastic.
Because color plastic is opaque it won’t let sun through to the soil. White Plastic would actually reduce sunlight heat transfer. Black Plastic is often used as a mulch in large gardens but it still will not allow enough light / heat in to raise the temperature of the soil.
If you must use black plastic then you should extend the time by 30%.
Clear plastic will also allow you to view the soil and quickly spot areas where moisture levels are different.
Understanding why temperature is important when sterilizing garden soil
In our kitchen we know that the danger zone for bacteria to grow is between 40F and 140F.
If food is kept cooler then 40F it will slow the growth of bacteria. If it is frozen like a sub-zero freezer it will reduce it to the point where even products like meat and fish can be kept safe for extended periods.
If the food is above 140F bacteria will be killed and this is why when we make soups or prepare meat we cook it for a minimum period of time until its internal temperature reaches 140F . Yes there are people that eat sushi and steak tartar.. but they are idiots. Well lets put it this way they are risking getting sick for a meal that is probably no more satisfying then one that is cooked correctly.
So, now we understand what we need to do to kill the disease causing bacteria and other problems. We must heat the soil to about 150F completely through just like it was a steak. The hotter the better and one thing you might find is that pests may work their way out of the soil but if there are eggs or grubs in the soil they will probably be killed.
When is the best time to Solarize or Sterilize Garden Soil?
Unfortunately the best time to treat your soil is during the hottest part of the growing season.
This may work for some gardeners that plant an early and late crop but for others that grow items that need 2 months of grow time before any harvesting can begin and longer for the main harvest you may find yourself working at the end of the season.
For results to happen the air temperature should remain over 70F so this isn’t something you can do after your last harvest.
You will also need 6 to 8 weeks of constant temperatures to kill the bacteria.
Remember this is just like cooking a steak. If the temperature is lower like a roast that you cook at 250F for 6hrs then you must apply that same idea to sterilizing your soil.
The temperature must always be over 140F but if it is just at that point it can take 30% longer just like a roast or in a smoker. If the temperature is hotter in the 160F range a standard 6 to 8 weeks will be fine.
Final Notes about Solarizing your Garden Soil to Sterilize it of Pests and Disease
Remember that this method although practiced and studied by professionals is not a be all end all to pest and disease mitigation in your garden.
Solarization can improve your chances of disease removal but you should also follow the standard methods of crop rotation and use chemical products when necessary.
Crop rotation can be difficult for small gardens. If you have a tomato disease it can limit your ability to grow tomatoes for 2 years and sterilizing your soil is only one part of making sure you do not have future problems.
In the worst conditions a small gardener may find that purchasing 10 to 20 yards of new soil is a better way to remove disease over a shorter time. You can then sterilize your soil and use it as clean fill outside of your vegetable garden.