I often get questions from people about how to choose good soils for vegetable gardens. These people don’t have a lot of soil because of where they live or are building raised beds or wicking beds for fruit and vegetable production.
If you are thinking of buying soil from a supplier, you can ask them some questions before you buy it. These questions should help you understand whether or not it is fit for purpose for growing vegetables, fruit and other non-native plants.
Question 1: Where does the soil come from?
If the supplier can’t answer this question, or gives you a range of options, you probably don’t want to buy it. If they don’t know where it comes from, then there is no guarantee that the soil will be appropriate for your use. Last time I purchased soil, the supplier said ‘the soil is from new development around X and Y locations’. This gave me some reassurance that the soil would be fit-for-purpose in raised vegetable beds because I knew it was from ex-agricultural areas.
Question 2: Can I see your EPA compliance documents for the soil you have for sale?
In Victoria and NSW, and similarly in other states, any soil that is moved from one location to another should meet the definition of ‘clean fill’ or virgin excavated natural material (VENM) – that the soil does not pose a hazard to human health and environment when it is reused. The exact requirements and terminology depend on the state you are in. Ask to see the clean soil certificate or laboratory testing to make sure that it meets your states legislated requirements (e.g. EPA Victoria, NSW EPA).
Question 3: What does it look like?
It is never a good idea to buy soil without having a look at it. You want to make sure it has some organic matter in it and that it is not just a hard clay. If you are using the soil in a garden bed you probably want some organic matter, sand and clay in the soil mix. Have a feel of the soil – is it gritty? Can you see bits of decomposed plant matter? Does it leave a fine powder on your hands? If yes to all of these, it probably has all three of those soil components. In some cases the soil may not have a lot of organic matter in it, such as VENM. If that is the case, see Question 4 below.
Question 4: How long have you had it for?
The longer soil sits in a pile the longer it will take to recover. Stockpiling soil creates ideal conditions for leaching of essential nutrients, for nitrogen to be converted into forms that plants can’t access to grow, for carbon to be converted to gas or break down and become water repellent. Therefore, soil that has been stripped, moved and sitting in a big pile will not have optimal nutrient cycling, soil biology or organic matter. The longer it has been in a big pile (>2m tall) the longer it will take to recover. Asking how long the supplier has had it for gives you an indication of how long it will need to recover. I always suggest buying ½ soil and ½ mushroom compost* to help kick-start health recovery of your soil especially for vegetable gardens or non-native plants. When using the soil and compost mix, I suggest adding some of your own compost, watering in well and using a green manure or cover crop, knowing it will take three to six months for the soil to recover and for plants to really thrive again.
Asking a few questions about the soil you are going to buy can avoid purchasing soil that is not fit for your use. Asking these questions is not a guarantee that the soils will be safe to use and fit for purpose, but should help you to make an evidence based decision on whether or not it will be good for your use.
In our next blog we will write about what do to when you buy soil and it just doesn’t seem to be growing plants!
* I prefer mushroom compost as it had to be food safe for growing the mushrooms, so it is the most clean option if you are purchasing compost where the source is unclear.