How To Sterilize Potting Soil?

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Potting soil or even general garden soil usually has plenty of harmful pathogens, larvae, bacteria, and microorganisms. These elements can lead to crop diseases and restrict nutrient breakdown in the soil for even distribution. Sterilizing your potting soil will kill and clear living organisms that harm the soil quality.

Sadly, there are a bunch of insects, worms, and bacteria present in the soil that promote nutrient breakdown. Sterilization can kill such beneficial elements and curb the growth of your plants altogether. Although, sterilizing your potting soil can do more good than harm. The negative impact can be reduced by choosing the right method.

how to sterilize potting soil

Several people prefer to go out and purchase readymade sterilized soil. However, purchasing sterilized soil has the following drawbacks:

  • No idea about its shelf life
  • Wear and tear of packaging
  • Mishandling and fewer maintenance precautions
  • Unaware of the method of sterilization 

There are a bunch of ways one can sterilize their soil at home and achieve sure-shot results.

What is Potting Soil Sterilization?

Potting soil sterilization is a method to create the ideal soil-based environment for plants by boiling off the weed seeds or toxic organisms. Gardeners prefer this method when they want to protect their expensive crops from possible pest-attacks. 

sterilize potting soil

This solution is cheaper than facing the damage of an entirely spoilt batch of crops. Three situations that can benefit from soil sterilization are:

  • Seed germination
  • Spreading of stem and shoot-cuttings
  • Changing soil environments for juvenile plants

This guide will help you gain some knowledge of the difference in potting soil and garden soil.

2  Types of Potting Soil Sterilization Methods

Potting soil sterilization methods generally costing of heating and chemical treatments. 

  • Heat treatments include placing the soil in a microwave, oven, pressure cooker, or performing solarization (sterilization due to heat).
  • Chemical treatments include the application of different agents, fertilizers, compost, etc. 

When large-scale sterilization needs to be done for commercial tasks, then chemical treatments are preferred. This is due to the lower costs, lesser time-consumption, and reduced efforts required.

sterilize potting soil

Drawbacks of Chemical Treatment Methods for Sterilizing Potting Soil

Gardeners disagree with chemical-based potting soil sterilization methods for the following reasons:

  • Present-day chemicals have a more single disease-centric approach. Thus some chemicals only combat one type of disease and insects. This leads to a significant amount of harmful soil constituents remaining unaffected. 
  • When chemicals are not applied in an ideal manner, its ingredients can remain in the soil. This increases the probability of the plant roots absorbing these chemicals, leading to an undesired chemical concentration in plant tissues. 

strelizing potting soil

 

  • Gardeners who follow chemical treatment solutions can suffer from application burns or internal inflammation. In fact, even nearby people can experience sensory disturbances. 
  • Unlike heating methods, chemical application requires the soil to be kept away for a significant period of time. This is done to let the chemicals degrade and avoid negative effects on the plant.

easily sterilize potting soil

 

  • Consistently fuelling your potting soil with chemicals could increase the tolerance of diseases/insects in the soil. This could make your efforts invalid in the long run.

Bonus tip: These gardening gloves could reduce the chances of bodily harm due to chemicals used in soil sterilization.

Potting Soil Sterilization: Heat Treatments

Gardeners who create potting soil at home raise the temperature of the soil via direct dry heat or steam. Once the temperature reaches a threshold, plenty of fungal spores and harmful bacteria die out. Although the longer the soil is exposed to heat, the more sterilized it turns out to be.

When the following temperatures are consistently maintained for 30 minutes, the corresponding attributes are eradicated:

 

Temperature

120

145

160

180

Harmful Bacteria Removed

Oomycetes: Water molds

75 % plant pathogenic attributes: fungi, worms, slugs, centipedes

Remaining plant pathogenic attributes: bacteria and other insects/Weed Seeds

Heat-resistant fungi, plant viruses, and other diseases

strelizing potting soil

 

Steaming or Boiling Water

Fill up a pressure cooker with 70% percent water and place a rack inside the cooker. Place 4-inches of soil in separate containers above the rack and make sure these containers are heat-proof. Cover up every container with a foolproof layer of foil.

Up next you can position the lid onto the pressure cooker and leave the steam valve slightly open. This permits the steam to keep venting until and unless sufficient pressure begins to build. You have to regulate the heat source and navigate the pressure levels to ensure steam is building.

how to sterilize potting soil

 

Once the steam valve automatically closes, process the soil at 10 pounds of pressure for 20 to 30 minutes. Finally, shut down the heat source and let the pressure subside before you decide to remove the lid.

No need to remove the layer of foil until you decide to actually use the sterilized soil.

Microwave

This microwave method works best for small batches of soil. 2 things to ensure are that your soil must be moist enough and that there are no evident metal traces in the soil mixtures.

Start by placing approximately 2 pounds of soil inside a zipper plastic bag. Don’t let the soil be too moist, but moist enough to crumble when the heat pressure is cut off. 

sterilize potting soil

 

Simply place it in the microwave with the top zipper left open and set the temperature on high. Once you suspect that the soil in the middle of the bag has hit 180 to 200 you can switch it off.

This generally takes anything from 4 to 10 minutes, depending on the power ratings of your microwave. Finally, remove the plastic bag, close the top, and store it in a cooler temperature until the bag/soil temperature comes back to normal temperature conditions.

Oven

The oven method works best for small and medium batches of soil. Even better, it is safer than the 2 methods spoken about above. Although the oven method has one major drawback. The unpleasant smell will spread like wildfire in your kitchen. It is advised you leave the windows and doors open for increased ventilation.

sterilize potting soil

 

Mix your soil with water to make it moist. The soil should be easily compacted into a ball and breakdown when the pressure is removed. Ensure you do not add extra water and increase saturation to high levels. This will make the process much slower and lead to incomplete sterilization due to tightness. Mix just enough water to create a substantial amount of steam.

how to sterilize potting soil

 

Use an over-proof container and load it up with 3-inches of soil. Use foil to cover up the container from the top. You have to pre-heat your oven to 200 before placing the container inside. 

Use a thermometer to keep a track of the soil temperature. Once it hits the 180 mark, let it bake for an additional 30 minutes. Finally, power down the oven and let the soil cool down.

Solarization

This method relies on natural sunlight and is fit for large batches of soil. If you are looking to sterilize your entire backyard then this could be the perfect technique. The process mainly depends on choosing the right type of plastic and spreading it above your lawn. This traps the heat rays from the sun and raises soil temperature, in turn killing all pathogens and weed seeds. 

Small batches of potting soil need to be wrapped in a plastic bag and exposed to the sun for about 24 48 to 72 hours. Do not use black plastic as it doesn’t let the heat seep through and deflects over 60% of heat. On the other hand, clear plastic will trap and allow the seepage of over 90% of solar energy.

how to sterilize potting soil

 

3 types of plastic that can be used are:

  • 4mm plastic for small areas
  • 1.5 to 2mm plastic for soil in windy regions
  • 1 mm plastic offers exceptional soil heating but can easily get damaged by animals or harsh weather conditions

Ensure that all plant waste, thatch, and litter is cleared before placing the plastic. Up next, first place the plastic around your lawn and then pour potting soil mixture evenly around the plastic, except for within 6-inches of the edges. 

sterilize potting soil

 

Use a watering can to moisten the soil. Do not use a garden hose as this would over-irrigate the soil. Finally, you can cover this soil with another layer of plastic on the top. Make sure you secure the edges with rocks or heavy materials to prevent wind from spoiling your set up.

Around 4 to 6 weeks of summertime would lead to spot-on solarization and thus sterilization. 

Tips and Tricks

  • Make sure you do not overheat your soil. This could lead to the development of toxic manganese followed by soil reactions to give birth to additional toxic compounds.  Even worse, phytotoxicity could restrict your plant growth altogether.
  • If you have made your customized potting mixture, ensure the organic matter (compost, manure, etc.) is broken down to its grain. Clusters of decomposable organic matter would lead to increased internal soil damage due to heating. 

how to sterilize potting soil

 

  • Performing indoor potting soil sterilization will release an unpleasant odor with a combination of petrichor, an earthy smell. Nature lovers usually can bare the smell, but it could prove unbearable for some. 
  • If you notice insects in the soil after one/two months of solarization, do not take additional precautions to kill them. This is because the insects that survive post-sterilization are usually the ones that promote greater soil health.

how to sterilize potting soil

 

  • Most importantly, sterilize the container in which you plan to use potting soil. You don’t want to infect your newly sterilized soil right after going through the sterilization grind.

Verdict

The time-invested in soil sterilization could be well worth it if a gardener uses the same soil for multiple seed germination cycles. Besides, sowing seeds in a purified growing environment leads to healthier plant growth and a better harvest. 

Not a potting soil fanatic? This guide should teach you how to prepare the garden soil.

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