For daily wit & wisdom, sign up for the Almanac newsletter.
Have you noticed the price of potting mix these days? Filling up containers isn’t cheap, and it can cost a small fortune to fill raised beds. Slash your biggest garden expense! See five ways to make your potting mix go much, much further and fill beds for a fraction of the price.
What is Potting Mix?
“Potting mix” is “soilless” and is the standard medium to fill up pots and raised beds, both indoors and outdoor. We also use soilless potting mix a the growing medium for seed-starting in pots and trays
Soilless potting mix is lightweight and fluffy because it has ingredients that help with aeration and moisture retention (which young roots need). Potting mix may also contain a slow-release starter fertilizer.
Difference Between Potting Mix and Potting Soil
Don’t get “potting mix” confused with “potting soil” Potting soil or garden soil is heavy and includes sand, clay, and other cheaper additives that won’t work for containers. Some people will purchase bagged garden soil for in-ground use because their own soil is too compact or lacks fertility.
Potting mix is one of the biggest expenses in the garden. If you want to be more economical in the garden this year, learning how to use less potting mix for better results is essential.
1. Fill Pots for Less
It never ceases to amaze us how quickly we can go through bags of potting mix, especially early on in the growing season. But ask yourself: Do you really need to fill all of your containers with pricy potting mix?
Many shallow-rooted crops like lettuce, radish and strawberries can cope in soil depths as shallow as 6 inches (or 15 cm), so there’s little point in having expensive potting mix below this.
However, they do still benefit from having moisture stored at those lower levels, so we can start filling containers with free materials to bulk things out: chopped up sticks and twiggy material, part-decomposed leaves or leaf mold, or straw, or even old potting mix salvaged from elsewhere.
Use this table for an idea of the minimum depths of quality potting mix you should be aiming for, then cut corners on anything beneath these depths. Stop wasting mixes and save up to 50% or more!
Minimum soil depths for common crops
Min. 6in/15cm deep Min. 8in/20cm deep Min. 12in/30cm deep
Basil Beets/beetroot Broccoli
Cilantro/coriander Cabbage Carrots
Endive Chard Cauliflower
Lettuce Cucumber Chinese cabbage
Mustard greens Garlic Corn
Radishes Peas and beans Eggplant/aubergine
Scallions/spring onions Peppers Kale
Spinach Summer squash Onions
Strawberries Turnip Pumpkin and winter squash
There are some crops that do need plenty of deep fertile soil. For heavy feeders such as squash and cucumber a great option is to fill the deeper layers with especially nutrient-dense materials such as kitchen scraps or the freshly-cut leaves of nutrient accumulators like comfrey. Over the growing season, dead leaves and organic matter releases nutrients, making them available to searching roots.
2. Reuse Potting Mix
If you have old potting mix, it can be used to part-fill other containers, or we can rejuvenate it. There’s life in the old mix yet! Start by dumping out old potting mixes in to a wheelbarrow or onto a tarp or other
surface to dry out a bit. Once it has, go through, and pick out any obvious bits of larger root and other plant debris then store in lidded containers or bags until you’re ready to use it.
To revive it, simply take your old mix and add the same volume of new mix. Then go in with a handful or two of organic fertilizer such as blood, fish and bone. Give it all a thorough mix and you’re good to
Reusing old potting mixes does carry the risk of passing on soil-borne diseases, but you can minimize this risk by using potting mixes you grew flowers in for growing edibles the second time round, and vice versa. And, of course, don’t reuse mixes if they contained heavily-diseased plants.
3. Stretch Potting Mixes
Both new and rejuvenated potting mixes can be made to go further using either garden-made compost, leaf mold or sand, depending on what you plan to grow in it.
Rich, dark compost is a fantastic source of goodness for our heavy feeders, so simply mix sieved
compost with your potting mix to create an extra-rich growing medium for these hungry plants. Potatoes, squash-family crops, tomatoes, and brassicas such as cabbage are just a few examples of crops that will relish the moist, nutrient-heavy environment this should create.
Conversely, some other vegetables grow better in a less rich soil. Carrots, for example, will happily grow in potting mix cut with sand to create a very free-draining growing medium. You might be able to pick up sand for cheap, or perhaps you have an old sand pit that can be plundered. If you have to buy sand, horticultural/gardening sand is great, but builder’s sand is much cheaper and will work well too. Just mix the sand with equal parts potting mix for a fantastic root crop mix.
Compost pits or trenches are a superb way to save on bagged composts, or to take the pressure off your own garden compost so it goes further. For example, in a zucchini bed, rather than add valuable compost on top of the soil surface, dig out and set aside some of the soil. Add a good layer of kitchen
scraps into the bed, and then return the soil to cover it. The kitchen scraps will rot down over the course of the winter, leaving a deliciously rich, moist layer for the roots to grow down into. Same goes in any other veggie bed such as a green bean bed; dig out a trench, fill that with more kitchen waste,
then return the soil on top. Then plant the beans into the top layer of soil and they’ll race away!
5. Fill Beds for Free
And now for the best tip to save your hard-earned money: filling raised beds or building up new planting areas for free.
Instead of digging down, you can create instant raised beds that you can plant straight into in spring.
Much like making compost trenches or pits, this method composts materials where they lie to produce a growing medium into which you can plant.
See the above video of the lasagna technique, using layers of materials to gradually build things up. Start by dropping a pallet collar beds into position here to help keep it neat and tidy but also you could make a new bed without any sides.
Line bottom of the bed with a few layers of cardboard, which will help to kill off the grass. Give the cardboard a thorough water to wet it if it’s dry.
And then layer up the ingredients. You want alternating layers of carbon-rich brown materials and nitrogen-laden green materials, starting with a 2 to 4 inch (or 5 to 10 cm) layer of browns such old straw. Then it’s in with a similar depth layer of greens such as old, spent crops and garden trimmings. Now another layer of browns and then more greens… and so on till we reach a depth of at least 18 inches (or 45 cm) and, ideally, deeper than that if you have enough materials to hand. If it’s dry, water regularly to help all that organic matter begin rotting down.
This might look a bit too piled right now, but it will slump down within a matter of weeks. Autumn or early winter is a great time to start off new beds this way, as the repeated freezing and thawing of winter should help break things down ready for planting in spring. If it hasn’t completely broken down by then, you can just finish off with a couple of inches of garden compost to cap everything then plant into that.
An alternative to the lasagna method is to build up a bed by progressively moving from larger pieces of organic matter to smaller pieces before finishing with a cap of compost or soil. So, for example, the bottom of the bed might be larger logs or bits of branches, then sticks, then twiggy material, before finishing off with plant waste such as dead leaves, and then compost or potting mix, part-filling the beds with sticks and prunings to save on bagged compost. The other bonus of this is that as the woodier material slowly breaks down it releases back the nutrients it contains to feed your plants, much like a slow-release organic fertilizer.