Sterilize soil in oven

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.

Ask an Expert: Is sterile soil necessary?

Why Sterilize Soil?
Soil is full of living things: bacteria, fungi, worms and insects. Many of the organisms in soil help break down larger pieces of organic matter like grass, twigs and bark into small particles of humus, which is the best substance you can have in the soil. These organisms also break down nutrients into forms that plants can more easily take up through their roots. That’s the good part about the living things in soil.
The bad part about living things in soil is that there can be detrimental fungi, bacteria and insects in the soil along with the beneficials. In most garden soil, there is also an abundance of weed seeds. If you are gardening organically, from start to finish, and need to create a new flower bed, one way to rid the bed of weeds is to sterilize the soil. Doing this can save you lots of time and money in the long run. If you are starting your own seeds, you need sterile potting mix so that seedlings do not succumb to damping off, and other life-ending diseases that plague seedlings grown in un-sterile potting mix.
How to Sterilize Potting Soil
Sterilizing soil for seed starting is one of the easiest things you can do to make seed sprouting a success. Here’s how you do it:
1. Put your desired amount of soil in an oven roasting bag
2. Add some water to the soil
3. Tie the bag
4. Poke a hole for a meat thermometer
5. Put the soil bag on the pan
6. Heat the soil to between 160-170 degrees. (Set the oven on 200, and lower if temp. of soil goes above 170).
7. Use the meat thermometer to check the soil temperature.
8. Leave it in for about 30 minutes
9. Remove soil and let it cool thoroughly before planting anything in it!
Solarization Sterilizing Garden Soil
Garden soil is a bit more difficult to sterilize, but it can be done using a process called solarization. This is a method of using heat from the sun to kill disease organisms that cause plant problems like verticillium wilt, root rot, damping off and others. In order for your garden to have the full benefit of solarization, the soil needs to reach a temperature of 114 degrees F (46 degrees C) for at least four to six weeks. Here’s how to solarize your soil:
1. Rototill the soil in the area you want to solarize.
2. Water the area so that the top foot is moist.
3. Cover the area with clear plastic, held down with bricks or tacks.
4. Leave in place for at least a month. In cooler areas, leave in place for two months.
5. Check the soil every two weeks to make sure it is still moist.
6. Remove plastic and plant the garden!
A few notes about sterilizing garden soil: For the most part, beneficial organisms will survive the solarization. You can add humic acid or compost to the soil to put back in the good stuff after you’ve solarized. It is not a bad idea to test the compost for weed seeds by watering a little and seeing if it grows. You can water with compost tea to add beneficials back as well.

Easy Oven Pasteurization 123

Here is a pasteurization method for all you beginners out there. I didn’t invent it, actually got the method off of Major Millet’s tek. Works like a charm. I just hear so many problems with ppl pasteurizing that I decided to post this for those of you who have trouble.
It is tried and tested and works with less contamination factor than most methods out there imho. until you get pasteurization down.
Now, we won’t get into arguments about anything here. I know there are bigger and better ways of doing pasteurization. Such as steam with a steam/temp gauge or in a cooler. Many many methods. This is just one method.
I feel this is a fail safe way to pasteurize for even the easiest beginner. It worked so well I have never tried another method since then. This might be a different story if your doing about 20 bulk tubs worth of substrate. You might want a barrel then.
And I know many have the problems with doing it in the oven with smell and such. As long as your dung is dry and composted. The smell will not be anything. The mess is also minimal bc your using a mixing box to mix your substrate. you just rinse out the box when your done. The subs smell like dirt.
Easy Pasteurization using turkey tins
1) a turkey tin from walmart or grocery store
2) some medium grade foil
3) your substrate and some water
4) a sterilite plastic bin for mixing or anything you can use to mix in. a big bucket
1) preheat oven to 170. some ovens lowest setting are 200. this is ok. set it to 200. ive done it for years at that temp with no problems.
2)take your big bucket and mix up your substrate to just above field capacity. this means, instead of having one drip between your finger when you squeeze the sub in your hand, you have about two drips or a steady flow of water. you want just slightly above field capacity to account for water loss due to evaporation.
3) once you achieve moisture content. stick your sub into the turkey tin. you can compress it as much as you like to fit more subs into the tin. although light and fluffy is how i do it. i actually use these turkey tins as my measuring volume for substrates. its easy to judge that two turkey tins full is one of my single tubs filled to about 3″ sub depth with 2 to 3 quarts spawn used. it may vary depending on the size of your fruiting chamber. now, the standard oven will fit 4 turkey tins stacked on top of each other. you may have to pull the metal racks out and leave the bottom rack in place to stack them that high. either way. you can do 2 tubs a day with this. if you want a rotation. get 8 turkey tins. when the first 4 are done. you can easily take those out to cool. then put in your 4 newly mixed subs into the oven for another pasteurizion. allowing you to total 4 bins per day at spawning.

you can hear arguments about this being a small amount of substrate. for personal use its fine. and a 4 tub a week rotation is plenty for yield.
4) cover the turkey tin in foil. then place it in the oven for 2.5 hours minimum. the 30 mins extra is to account for allowing the substrate inner temp to get to pasteurization temps. once that happens. then you want to pasteurize for 2 hours solid. you can easily fit 4 in most ovens.

5) once the 2.5 hours is up. allow your subs to cool. take your turkey tin out to your clean area for spawning.

Edited by eatyualive, 17 August 2014 – 03:02 PM.

Preparation of Mushroom Growing Substrates

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So you’ve got your bag/jar of spawn, and you’re ready to turn it into many bags (or buckets) of mushrooms. Here’s a simple + effective way to go about that process.

Firstly, take your bag or jar of spawn that you’ve either cultivated yourself, or bought from a mushroom supplier.

Break up the spawn (easier to do without touching the spawn if done in a bag, which is one reason why spawn is generally grown in bags).

Then it’s time to prepare the bulk substrate. This is the material that the mycelium (ie your spawn) will eat.

Different types of mycelium like to eat different types of things. Here we’re using oyster mushroom mycelium, which grows well on straw and other high-carbon, grassy materials.

Here we’re using organic sugarcane bagasse – because it’s a waste product, and easy to replicate for our students at home.

Pasteurising the substrate

First step is to pasteurise your substrate, to give your mycelium a head start over the rest of the organisms in the bagasse.

In this case we are using pasteurisation over sterilisation, which removes all life. Pasteurisation involves killing some, but not all of the organisms present, and it’s easier than sterilisation.

Why pasteurise?

  • Pasteurisation is used for fruiting substrates, because the spawn is already growing vigorously and can out-compete most organisms
  • Pasteurisation is usually cheaper than sterilisation – it requires less equipment and energy.
  • Pasteurisation leaves some thermo-tolerant organisms behind, which can offer protection from competitors to the mycelium

There’s a bunch of ways you can go about this process, but here’s how we’ve whittled ours down over the years to the most time, energy + cost efficient way for home-scale mushroom growing that we’ve figured so far.

You will need:

  • About 3 litres (3 quarts) of grain spawn of your choice of mushroom variety
  • 30 litres (8 US gal) of substrate – anything high in cellulose and low in sugars and nitrogen should work, we used organic sugar cane bagasse
  • 250 ml (1/2 cup) hydrated lime (also called builders lime)
  • 250 ml (1/2 cup) rice bran (you could also use wheat or oat bran, this is just to give our fungi a little extra nutrient, if you get mould omit the rice bran next time)
  • eye and hand protection and a face mask
  • a tub to mix and hydrate the substrate in
  • a clean brick (or similar inert + heavy thing)
  • A 50 litre (14 US gal) bucket (or plastic garbage bin) with lid
  • 3 pillowcases (funky 70s patterned ones like ours optional)
  • rope
  • 40 litres (10 US gal) of 80ºC (175ºF) water – a large urn is good for this job
  • A clean table or tarp
  • Clean mushroom bags or buckets to put your inoculated substrate in.

First, put on some gloves, a mask and eye protection before you start. Hydrated lime is highly caustic and can burn your throat and cause serious damage to your eyes. It does not produce any toxic byproducts, but it does burn.

Alrighty, let’s go. Put your substrate in the tub and add the hydrated lime. The lime is great for shifting the pH of the substrate suddenly (which kills a bunch of competition).

Add the rice bran then add enough warm water to ensure all the mixture is damp. Then mix it all up and around.

You want to make sure the mixture is nice and damp and well mixed so the heat in the next step can get right into the middle of the bags, dry substrate is a very good insulator.

Fill your pillowcases with the wetted mixture, and tie them up well with rope – leave a fair play of rope attached, as you will hang these up while hot to drain.

Put your pillowcases in your clean 50L bucket and, carefully, pour your 80ºC (175ºF) water over them until the bucket is full. Add a brick on top of the bags to submerge them, and put the lid on. Then go wash your hands.

Wait 2 hours. To pasteurise the substrate, it needs to remain above 60ºc (140ºF) for two hours.

In our climate, this method is enough to do the job – if you’re somewhere colder, consider insulating your bucket somehow for this period, or use a metal barrel on some bricks instead and make a small fire underneath to maintain temperature.

After two hours has passed, test water temp with a thermometer (ours came out at 65ºc).

Before you go on to the next step you want to increase the cleanliness level a little. We don’t want our nice contaminant free substrate to come in contact with any nasties.

Make sure you clean your hands with alcohol gel hand sanitizer or a 70% alcohol solution (30% water ) to kill off any potential contaminants. If you don’t want to get alcohol gel on your hands wear rubber or latex gloves and clean them with the alcohol.

Wipe down any surface that the substrate will be in contact with using alcohol solution.

If all is well, take out your soggy substrate filled pillowcases and hang ’em up to drain for at least 15 minutes. We want all the water to drain out of the substrate, you might need to give them a squeeze to remove any excess water.

Now you’re ready for the next step!

Inoculating the Substrate

Once the pasteurised substrate has stopped dripping and you can’t squeeze any water out of it and dump it out onto a clean surface to cool.

Once it is down to below 35ºC (95ºF) it’s time to add your grain spawn.

Take your bag of broken-up spawn and sprinkle it along the length of the substrate.

Then, with clean hands, mix! You want that spawn distributed throughout the substrate as evenly as possible.

Once everything is all mixed, it’s time to bag up. Take handfuls of inoculated substrate and pack them into your mushroom bags, jars or buckets, whichever you are using.

Seal things up so that nothing else can get in there, and put your future mushrooms in a warm dark place – a stable temperature of about 24ºC (75ºF) is best for most species.

After a few weeks (or months, depending on your spawn-to-substrate ratio and mushroom variety) – your creations should be ready to fruit! When the substrate in the bag or bucket is white and fully colonised with mycelium, you know it’s about ready.

When you think it is ready open your fruiting bag / bucket / basket and put it in a place with the some airflow, cool temperature and high humidity. We’ve used our bathroom in the past.

You can often trigger a stronger fruiting by soaking your mycelium in cool clean water overnight.

Happy eating!


  • All our mushroom cultivation articles
  • Growing Gourmet & Medicinal Mushrooms by Paul Stamets (the text book on this subject)
  • Organic Mushroom Farming and Mycoremediation by Tradd Cotter
  • Spawn: our buddy Marita at Milton Mushrooms or also Aussi Mushrooms have heaps.
  • Mushroom bags: AU reseller or get together some friends and bulk order from here

We run Gourmet Mushroom Cultivation Courses, as you can probably tell from the above photos! They’re excellent and have spawned many happy home + enterprise-level mushroom growers. Join us sometime.

Big thanks to Nat McComas for the beautiful photos above. Love your work, lady!

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Garden Myths – Learn the truth about gardening

There is lots of talk about using sterile soil. Some people even sterilize their soil before they use it. Others buy bags of sterile soil. All of this is done in an effort to reduce pests, diseases and sprouting seeds. This all seems to make a lot of sense but there are a couple of basic questions that one should ask;

  • Does sterile soil really exist?
  • Does sterile soil make a difference to your plants?

I’ll deal with the second question in my next post. Today I want to discuss the existence of sterile soil.

Sterile soil – does it exist?

Sterile Soil – What is It?

It is always good to start by defining the terms you will use. What is sterile soil?

One definition I found on line is, “Sterile soil is garden or potting soil that has undergone heat or chemical processing to kill any pathogens and seeds that are in it. ” I think most people would accept this definition, but it is only part of the story. It is also important that the product, once sterilized is kept sterilized until it is packaged, and the package must be such that the product remains sterile until you open it.

A better definition of sterile soil would be “soil that contains no life forms, including seeds, which has been packaged in sterile conditions.”

Sterilizing Soil

You can buy potting soil that is labeled sterile. Most of these products have been treated with chemicals to kill pathogens and seeds. In some cases they have been treated with high temperature steam.

According to Colorado State University (ref 1) “temperatures routinely used to heat soil will not result in completely sterile soil / potting media. The goal is to heat the potting mix to a point that kills the plant pathogens of concern.”

It seems as if the sterilization treatments are not likely to be 100% effective.

Packaging Sterile Soil

If a company does sterilize the soil, they then need to package it in sterile conditions. This would be similar to packaging processed food, or medical supplies. Sterile packaging requires a very high level of cleanliness in order to keep microbes from contaminating the product. It just does not seem like the kind of facility a manufacturer of soil would have?

I contacted some suppliers of so called sterile soil. When I asked them about their sterile packaging facilities they had no idea what I was talking about.

I doubt that any company supplying soil will have a sterile facility to package it. If you know of one, let me know in the comments below – I’d like to talk to them.

Homemade Sterile Soil

You can’t buy the stuff, but you can sterile it yourself. There are many described processes on the internet. Colorado State University ( ref 1) provides information and temperatures for sterilizing soil. You will need a temperature of 212F (100C) for 30 minutes in order to sterilize it (ref 2).

But ……”Excessive soil heating may also increase chance of phytotoxicity due to soluble salts, manganese toxicity, and toxic organic compounds. Soil mixtures high in readily decompostable organic matter (manure, leaf mold, compost) are most likely to give injury when exposed to excessively high temperatures” (ref 2).

Once your soil is sterile you will need to keep it sterile until you use it. This means storing it in sterile containers, and filling pots in sterile conditions – something home owners can’t easily do.

If you are not going to store it in sterile conditions – what is the point of sterilization?

Killing the seeds would be one benefit, but this is not sterile soil.

Is Peat Moss Sterile?

Sphagnum peat moss gets special credit for a variety of features including sterility. Even Pro-Mix, one the the largest producers of sphagnum peat moss in North America clearly states that peat moss is not sterile.

The Sterile Soil Myth

For the home gardener sterile soil is a myth – it does not exist.

At best you can buy products that have been treated to kill most pathogens and most seeds. If it was not contaminated during packaging, it will start getting contaminated as soon as you open the bag.

Don’t be concerned about this. I’ll explain why next week.

(1) Start Seed and Transplants in Sterilized Soil;

(2) Using Heat to Eradicate Soil-borne Plant Pathogens;

(3) Photo source; Doran

If you like this post, please share …….

How To Sterilize Soil & Organic Potting Mix Recipes

Potting soil is sterilized to give your plants the best environment possible by killing weed seeds or disease organisms that might be lingering in the soil. Most commercial products have already done this but if you want to use that rich dark gold right from your garden or reuse what you have, here are a few different ways you can do it yourself.

You’ll also find a few different recipes for making your own organic potting soil (found at the bottom of this page).

Oven Method: (small batches)

  • Fill an ovenproof container about 3 inches deep with soil, mix in a generous amount of water (not enough to make it runny or soupy but thoroughly wet) then cover with aluminum foil.
  • Bake in a preheated oven (200°F) until the temperature of the center reaches 180°F (use a meat thermometer to measure). Once the temperature reaches 180°F, bake for 30 minutes.
  • Do not overheat or overbake since it can release toxins harmful to plants as well as kill beneficial organisms.
  • It can smell quite foul when baking, this is normal.

Microwave Method: (small batch)

  • To use the microwave, put about 2 pounds of moist soil in a thick, plastic bag. Leave the top open and place it in the center of the microwave.
  • Heat it for two to five minutes on full power, checking the temperature in the middle of the dirt with a thermometer. When the target is reached (180°F to 200°F), close the bag carefully and put in a cooler to hold the heat in.
  • Allow to cool. Source:

Sun Method: (large batches)

  • Choose a spot in the yard that receives at least 6 hours of sun during the day (8 hours a day is best).
  • Lay out clear plastic sheeting and cover with a layer of dirt about 4 inches deep. Spray generously with water (not so much that it becomes runny muck).
  • Cover with another sheet of clear plastic and secure the plastic in place by laying a border of rocks all along the edges of the plastic.
  • Bake in the sun for at least 4 weeks in hot, sunny weather and up to 6 or 8 weeks in cooler weather (this technique is only good for summer).
  • Tip: Rake up the dirt each week to make sure the heat reaches all of it.


  • Reusing potting soil is fine when using with mature plants, though new seedlings or bedding plants require sterilized to have the best chance to thrive.
  • Don’t use dirt straight from the garden for a potting medium (alone), mix it with other ingredients to make it lighter and more beneficial for your plants (see below for a few recipes).
  • After completing one of the methods above, the dirt will likely be hard and clumpy. Break it down first before mixing with other ingredients.
  • Make sure that potting containers themselves are clean since they can also harbor disease organisms. You can wash the containers in a bleach and water solution or see this page for more suggestions.
  • You can use these same methods for used potting soil and sand.

If you want to make your own organic potting mix, you still have to avoid using any prohibited ingredients and that means checking out all the individual ingredients for their organic acceptability.

It may surprise you to learn that products like peat moss or limestone are sometimes treated with prohibited materials such as wetting or anti-caking agents, so don’t rely on assumptions about purity.

In addition to meeting certification requirements, your final product will also need to provide plant roots with the right amount of air, water and nutrients.

(Source: Potting Mixes For Organic Growers).

Classic Soil-Based Recipe

1/3 mature compost or leaf mold, screened
1/3 garden topsoil
1/3 sharp sand

For Styrofoam Seedling Flats

2 parts compost
2 parts peat moss
1 part vermiculite, pre-wet

For Sowing

5 parts compost
4 parts soil
1-2 parts sand
1-2 parts leaf mold, if available
1 part peat moss, pre-wet and sifted

Note: All ingredients are sifted through a 1/4-inch screen. For every shovelful of peat, add two tablespoons of lime to offset the acidity.

For Pots and Baskets

30 percent topsoil
60 percent peat
10 percent perlite
5 pounds lime per cubic yard
3 pounds dolomitic lime per cubic yard

Note: The handling of this pot mix is the same as for pack mix.

Bedding Plant Recipe

25% compost
50% peat moss
25% perlite or vermiculite

*Source: National Sustainable Agriculture Information Services (first published February 28, 2007 and moved to this page for better organization)

Looking for a diy seed-starting mix? Try this recipe (Source:

4 parts screened compost
1 part perlite
1 part vermiculite
2 pars sphagnum peat moss (or coir)

Controlled Release Recipe:

  • Add 1 TBS blood meal, 1 TBS kelp, 1 TBS greensand, 2 TBS bone meal to 1 gallon of potting mix. Mix well. Source:

Q. I’ve had problems with damping off when I start seedlings in my home-made potting soil. How can I sterilize it to prevent my seedlings from rotting? I don’t want to use chemicals, if at all possible.

A. Damping off is caused by any one of several soil fungi. To prevent seedling losses from these pathogens, you should treat your home-made soil mixes. The preferred treatment is really not soil “sterilization” but is actually “pasteurization” because not all organisms are killed.

Small batches of potting soil can be sterilized in your home oven. Place the damp soil mix in a container in the oven. I find that covering the container with foil helps to contain the odor, which may be objectionable, depending upon the potting soil ingredients. If you have an outdoor gas barbecue that allows you to control the temperature accurately, you could move the process and any possible odors outdoors.

You can even use solarization to pasteurize your potting soil if the weather is warm and sunny. Simply seal the damp soil in a clear plastic bag and leave it in the sun to heat up. This method is certainly easier but it is difficult to control the temperature. If you have a compost bin thermometer, you can monitor the temperature with it.

We used to recommend heating the soil for one hour at 200 degrees, but this kills too many beneficial organisms. Instead, we now recommend 140 degrees for 30 minutes. This still kills the pathogens but allows many of the beneficial organisms to survive.

These beneficial organisms that survive will help fill the gap left by the killed pathogens. As my plant pathology professors often reminded us, “nature hates a vacuum.” If the beneficial organisms are not there, harmful pathogens, including those for damping off, will find it easy to recontaminate the soil. That is why we want to preserve as many as possible.

Q. All spring and summer I conserved water and abstained from planting my usual flower border. Now I’m hearing weather predictions for a possibly wet winter and want to take a chance on planting a border of ranunculus for spring color and let Mother Nature do the watering. I’ve always grown them from potted plants but this year I want to start my own plants. Would it be difficult?

A. In mild winter areas such as ours, the Persian Ranunculus, Ranunculus asiaticus, is one of the most popular members of the fall-planted bulb, tuber and corm groups. They are easy to grow and produce dozens of colorful flowers to enhance the garden or fill a vase. The flower colors are white, yellow, orange, and rose in varying shades, either solid or with contrasting edges to the petals. The individual plants are up to 12 inches in diameter with leaves that resemble flat leaf parsley. Each plant can produce dozens of flowers with straight stems 12 to 18 inches long.

You can plant your ranunculus tubers as soon as the tubers become available at local nurseries, but tubers are reported to sprout more quickly if they are planted a little later in the fall, after the soil is cooled. The ranunculus tuber should be planted one to two inches deep with the little “fingers” pointed downward. Be careful not to break the little fingers as they are rather brittle.

There is no need to soak the tubers before planting, but they will be less brittle if you do soak them for an hour or two. Water the soil thoroughly after planting, but do not water again until sprouts appear unless the soil becomes very dry. Tubers will rot if they are overwatered.

The sprouts will emerge in 14 to 21 days and will develop quickly into leafy green mounds. Be on the lookout for hungry birds and snails; they can destroy young plants overnight. When spring approaches, the first flowers will appear. Give the plants good garden care. When the heat of summer arrives, the plants will begin to die back. At this time you can lift the corms, dry them, and store in a cool dry place until replanting next fall. Corms left in the ground over the summer usually don’t survive.

Ottillia “Toots” Bier has been a UC Cooperative Extension master gardener since 1980. Send comments and questions to [email protected]

Contact the writer: [email protected]

Everything You Need to Know About Potting Soil for Indoor Plants

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To many beginner gardeners, dirt is dirt is dirt. It can’t matter what kind you use, right? Wrong. Soil is what houses and nourishes your plants. Just as you wouldn’t move into any old apartment, you shouldn’t plop your houseplants into any old soil. While most plants will make do with what they’re given, they won’t truly thrive unless provided with the proper environment.

“A quality potting mix is important for any plant,” says Summer Rayne Oakes, founder of Homestead Brooklyn and author of the upcoming book How to Make a Plant Love You. “You want to start off on the right foot.” Here, everything you need to know about potting soil for indoor plants.


What is potting soil?

Commercially available potting soil is generally composed of three ingredients: peat moss (the dense dirt base), pine bark (which creates space and allows for airflow), and perlite or vermiculite (fluffy, volcanic materials that lighten the soil). It’s possible to make your own custom blends-which Oakes does on occasion for the more than 600 plants that occupy her Brooklyn home. “Depending on the plant and what’s available, I’ll add a little extra perlite or pumice for a lighter soil,” she says. However, commercially available blends will suit the needs of the vast majority of plants.

Where can you buy potting soil?

Any nursery should have a quality assortment of potting soils, says Oakes, who buys most of hers pre-mixed at a local garden center. “There are a lot of good mixes on the market,” she says. “Sometimes I’ll mix in extra perlite, but you should be able to buy something off the shelf.” One thing to keep in mind: the brand your mother swears by in California will likely be different if you buy it in New York. Potting soils are mixed regionally, says Oakes, so formulas tend to vary. If you relocate, expect your go-to brand to have a slightly different consistency.


What kind of potting soil do you need for succulents?

While all-purpose potting soil suits most houseplants just fine, some-such as succulents-have more delicate roots that don’t tolerate excess moisture. These plants require lightweight, well-aerated blends that drain quickly and prevent soil compaction. “Succulents are resilient, but you do have to be careful about moisture,” says Marianne Hugo, director at Coastkeeper Garden, a nonprofit conservation garden in Orange County, California. Hugo hosts succulent classes, where she provides students with an easy, DIY succulent mix recipe: 2/3 all-purpose potting mix plus 1/3 perlite. Commercial succulent mixes vary, but generally contain a lightweight mix of peat moss, perlite and mycorrhizae, a fungus that promotes root health.

What is organic potting soil?

You’re probably familiar with the term “organic” in the supermarket. But when it comes to soil, the label has a different meaning. Conventional, non-organic potting soils are sterilized to kill pests, eggs, bacteria and other microorganisms that have taken up residence. Hence, they are free of “organic” matter. Key nutrients and minerals are then added in after the sterilization process. Organic potting soils contain compost, manure, bone and blood meal, worm casings and other organic materials, which naturally supply the nutrients and minerals. Organic soils are free of pesticides and genetically engineered chemicals. Although considerably more expensive, organic soils are more nutrient-dense and environmentally conscious than conventional blends. If you have room in your budget, they’re worth the splurge. “Ideally, you want to keep everything as organic as possible,” says Hugo.

Tips To Sterilize Potting Soil, Garden Soil And Soil For Seeds

Since soil can harbor pests, diseases, and weed seeds, it’s always a good idea to sterilize garden soil before planting to ensure the most optimal growth and health of your plants. While you can go out and purchase sterile potting mixes to meet your needs, you can also learn how to sterilize soil at home quickly and efficiently.

Methods for Sterilizing Soil for Seeds and Plants

There are several ways to sterilize garden soil at home. They include steaming (with or without a pressure cooker) and heating the soil in the oven or microwave.

Sterilizing Soil with Steam

Steaming is considered one of the best ways to sterilize potting soil and should be done for at least 30 minutes or until the temperature reaches 180 F. (82 C.). Steaming can be done with or without a pressure cooker.

If you’re using a pressure cooker, pour several cups of water into the cooker and place shallow pans of level soil (no more than 4 inches/10 cm. deep) over top of the rack. Cover each pan with foil. Close the lid but leave the steam valve should be left open just enough to allow the steam to escape, at which time it can be closed and heated at ten pounds pressure for 15 to 30 minutes.

Note: You should always practice extreme caution when using a pressure for sterilization of nitrate-rich soil, or manure, which has the potential of creating an explosive mix.

For those not using a pressure cooker, pour about an inch (2.5 cm.) or so of water into the sterilizing container, placing the soil-filled pans (covered with foil) on a rack over the water. Close the lid and bring to a boil, leaving it open just enough to prevent pressure from building up. Once the steam escapes, allow it to remain boiling for 30 minutes. Allow the soil to cool and then remove (for both methods). Keep foil on until ready to use.

Sterilizing Soil with an Oven

You can also use the oven to sterilize soil. For the oven, put some soil (about 4 inches deep) in an oven-safe container, like a glass or metal baking pan, covered with foil. Place a meat (or candy) thermometer into the center and bake at 180-200 F. (82-93 C.) for at least 30 minutes, or when soil temp reaches 180 F. (82 C.). Anything higher than that can produce toxins. Remove from oven and allow to cool, leaving the foil in place until ready to use.

Sterilizing Soil with a Microwave

Another option to sterilize soil is to use the microwave. For the microwave, fill clean microwave-safe containers with moist soil — quart size with lids are preferable (no foil). Add a few ventilation holes in the lid. Heat the soil for about 90 seconds per every couple pounds on full power. Note: Larger microwaves can generally accommodate several containers. Allow these to cool, placing tape over the vent holes, and leave until ready to use.

Alternatively, you can place two pounds (1 kg) of moist soil in a polypropylene bag. Put this in the microwave with the top left open for ventilation. Heat the soil for 2 to 2 1/2 minutes on full power (650 watt oven). Close the bag and allow it to cool before removing.

How Hobby Growers Sterilize Potting Soil by Marci Degman, Landscape Designer

Hi, I’m Marci Degman, the aspiring gardener, and today we’re going to talk about how to sterilize your potting soil. The reason you would want to do that is because seedlings tend to be very, very sensitive to pathogens, insects, or anything that would get into your potting soil. Most of the time your everyday potting soil is OK for house plants and other plants, but seedlings need every bit of help that they can get. So, even though you buy this and it says that it’s sterile, sometimes while it’s sitting stored at the store, garden center, maybe in your potting shed, pathogens and things can get in the tiniest little holes. So, it might look good, you want to inspect it of course for insects or anything, but even if you don’t see anything, there can be pathogens. So what you want to do, the first method that I’m going to talk about is baking. And what you want to do is get a good baking dish. I like to use a shallow one because that way I can stir it and I can keep an eye on it. You don’t want to leave it in too long or have hot areas that are burning that you don’t notice. You don’t want to change the soil, you just want to heat it up. The best temperature is 180 to 200. Most of our ovens don’t want to go to 180, so if you have 200, set your oven there. And you want to bake your potting soil for 30 minutes. But don’t walk away and ignore it. Fill in maybe just a little bit more. Put it in and about every five to ten minutes check it and take a spatula and move it around so it doesn’t get too hot. If you start to notice any changes in the potting soil, you don’t want it to burn or turn black. Take it out. It’s done. The other way that you can heat it up is in the microwave. Th bad part is you can’t put a very big pan in there. But the good part is you only have to put in the microwave at high for two and a half minutes and if you’re doing just seedlings, all you need is a little bit at a time. So those are the two main ways that you can sterilize potting soil. If you have a really big amount, say for outdoor projects, things that you really need sterile soil for, you can pile up a mound of soil, cover it in black plastic and let it sit in the sun. That’s called solar sterilization, and today we’ve learned how to sterilize our potting soil.”

Note: for information on professional grade soil sterilizers, including a Control and improve the quality of your important growing mixes.

Professional Soil Sterilizers are an excellent alternative to other labor intensive methods, including the use of ovens that result in not only a mess, but long-lasting earthy odors which develop in ovens. (click on link)

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