Beginner’s Guide to Autoclaving Soil

Autoclaving soil is a laboratory technique used to eliminate microorganisms from soil and is often an important step in any agricultural research process. Here’s how it’s done.

How to Sterilize Soil Using an Autoclave

When sterilizing soil in an autoclave, the most important factor to consider is steam’s ability to fully penetrate the soil and thus achieve proper sterilization. In other words, it’s best to run a vacuum cycle when sterilizing soil.

A vacuum cycle consists of both pre-cycle and post-cycle vacuum phases. Pre-cycle vacuum functionality is the most critical factor to consider when selecting an autoclave for soil processing. Air can become trapped in soil, so a series of pre-cycle vacuum pulses will ensure that steam is able to properly penetrate the load.

A post-cycle vacuum phase is also recommended for soil sterilization, as soil will inevitably be soaked in condensed steam during the cycle. Post-cycle vacuuming (i.e. the drying phase) reduces the weight of goods that need to be unloaded from the autoclave.

When loading an autoclave with soil, it’s important to properly load and space out the soil being sterilized. A layer of soil greater that 4” in depth or soil packed extremely tightly into containers can retain air and inhibit proper sterilization, even with pre-cycle vacuuming. If the trays in which the soil is to be sterilized have holes for drainage in the bottom, secondary containment is also critical, otherwise soil may flow out of the tray and clog the autoclave drain and/or damage the internal piping of the sterilizer.

A typical autoclave cycle for soil sterilization is 30–60 minutes at 121° C or 20–30 minutes at 132° C. You should validate these cycles using biological indicators immersed in your particular soil load.

Soil Sterilization Methods

Outside the lab, soil sterilization is a farming technique used to eliminate harmful microorganisms, pathogens, spores and plant culture pests from mineral soil without destroying the soil or drastically altering its chemical composition. Soil sterilization can help promote plant health by releasing essential nutrients and killing off pestilent microbial populations. It can also increase crop yields, especially in fields that are replanted following a regular crop rotation cycle.

Though there are many ways to sterilize soil, there are three primary methods used in laboratory settings: chemical treatments, gamma radiation and heat.

There are many different sterilants you can use for chemical soil sterilization, including formaldehyde, phenols, cresylic acid, ethylene oxide and methyl bromide, though the two most common are sodium azide and propylene oxide. Sodium azide is proven to inhibit bacteria and actinomycetes and reduce fungal populations; however, it can also contribute to erroneous CO2 evolution readings, and shifts in inhibitor concentration pose an explosion hazard. Propylene oxide is a more effective sterilant than sodium azide, though propylene oxide-treated soil has been proven to inhibit the growth of certain crops, making it ill-suited for agricultural research. In addition to these challenges, sodium azide, propylene oxide and other chemical treatments alter soil pH and chemistry with no guarantee of fully sterilizing the sample.

Gamma radiation is another common soil sterilization technique. Y-irradiation at 10 kGy is powerful enough to eliminate actinomycetes, fungi and invertebrates in most soils, and most soil bacteria can be eliminated by 20 kGy. Certain contaminants — namely radio-resistant bacteria — require y-irradiation at 70 kGy.

There are two methods of heat sterilization for soil: dry-heat sterilization and steam sterilization. Though it is suitable for agrarian purposes, dry-heat sterilization — which typically entails exposing a thin layer of soil to the sun for an extended period of time or baking a soil sample in a dry-heat sterilization oven — is insufficient in laboratory settings. The reason for this is that, although it significantly reduces the population of bacteria and other microbes in soil, the dry-heat approach does not eliminate them completely. That leaves us with steam sterilization.

As its name implies, soil steam sterilization employs steam heat to sterilize soil samples. In agrarian settings, soil steam sterilization is typically achieved by exposing soil to superheated steam either in an open field or a greenhouse; the temperature of the steam kills off viruses, pathogens, bacteria, fungi, pests and more.

For laboratory applications, where only a small volume of soil is sterilized as a given time, autoclaving is generally the soil steam sterilization method of choice. Conceptually, it works the same as open field or greenhouse sterilization, in that it uses heat to kill off microbials and other contaminants; however, autoclaves can achieve higher temperatures and at a faster rate, making this approach more efficient. Autoclaving is also easier to validate than most other forms of soil sterilization, heat-based or otherwise, which is key because microbes in soil can be incredibly resilient.

Sterilize Soil with a CSS Autoclave

With over 75 years of experience designing, manufacturing and installing autoclaves for research laboratories, universities, hospitals and more, Consolidated Sterilizer Systems has sterilization down to a literal science. Regardless of what you need to sterilize, whether it’s soil, laboratory equipment, surgical equipment or something else entirely, CSS has the right autoclave to get the job done. Best of all, we offer a wide variety of customization options, so that your steam sterilizer meets your exact specifications.

Simple. Reliable. Sterile. That’s the Consolidated way. Contact us today to get started.