If you garden in containers, revitalising potting mix and doing an audit of your pots on a regular basis are essential routine maintenance activities for ongoing plant health. If you are starting to see problems in your plants, it’s probably time to check what they’re growing in! I’m regularly asked about potting mix so this is a trouble shooting post to help you revamp your mix and solve some common problems.
Should you re-use old potting mix and if so, how?
Now that I’ve got the recipe for your ‘perfect potting mix,’ I’m gradually re-potting my herbs and veges. What’s the best way to re-use the old soil? I’ve been feeding it regularly with the kelp/Epsom salts/molasses/water mix, so it’s probably not complete rubbish. I’ve added some to the compost bin, and used some to get a new no-dig garden going. Can you suggest any other uses?
One of the principles of Frugal Gardening involves RE-USING materials = less waste and reduces our footprint on the planet. It also has the added benefit of saving us money!
Bags of commercial potting mix can be quite expensive particularly if you have lots of pots and containers so making your own new mix and re-using the old definitely is more satisfying, sustainable and cost-effective.
First Step: Audit Your Pots!
Before discussing some ideas for re-using old potting mix, it’s a good time to do an ‘audit’ of your pots – the condition of your plants, soil and pot hygiene. It may not be necessary to re-pot completely or even right now. There are a number of factors to consider in each situation and I’ve developed a checklist to help you make the best decision.
Considering Re-potting? Use this Checklist First:
- How long has your plant been in the current pot? If you have been growing perennials, flowers, herbs or shrubs for 1-2 years, then it may be a good time to re-pot or renew at least some of the potting mix to keep the plant healthy. A lot depends on the quality of the potting mix you used in the first place, what you’re growing and the soil health. If it was a commercial mix, then you have invested money in it but like all growing mediums, plants use up the nutrients very quickly if they are not replaced on a regular basis.
It’s a bit like us losing energy if we don’t eat regularly! Bagged potting mixes are also primarily made up of pine bark which breaks down and you may notice the level dropping in the pot over time. You can top up the growing medium with additional ‘ingredients’ that won’t break down so quickly like vermiculite (aids drainage, helps aerate the mix, is a good thermal insulator and helps retain nutrients) and fluffy pre-soaked coir peat or coconut husk fibre (which is light, organic, a renewable resource and holds moisture extremely well). If the plant is a dwarf fruit tree for instance and is in a large pot for the long term, then other soil amendments can keep the potting mix healthy rather than re-potting just because you’ve passed the 1-2 year mark!
- Have you had any problems with pest or disease in the pot? Fungal and bacterial diseases in particular can be a problem e.g. different types of root rot, because plants are susceptible to further infection if the potting mix is re-used. Plant pathogens can stay in the soil potting mix for years continuing to reinfect plants. So, if this is the case, I would suggest putting potting mix from any contaminated plants and pots into the bin in a sealed bag rather than into your compost and starting again with fresh potting mix.
- Even serious composters respect the importance of good soil hygiene practices so occasionally this may be the best option rather than causing more ongoing serious problems. Cleaning the pot afterwards is also important. If your plants have been attacked by pests rather than disease, it’s very likely the soil needs attention to build up its health and a variety of integrated pest management (IPM) strategies can be used to resolve the pest issues.
- Does water quickly run out the bottom of the pot after you water your plant? If you see water spilling quickly out the drainage holes almost as fast as you are pouring it in, it may indicate your potting mix has become hydrophobic or water repellent. In this case, it is definitely time to re-pot because you are wasting water – a valuable resource and your plant is not getting the moisture it needs to be healthy and may soon become stressed. Even if the potting mix is not water repellent, the current mix may not have enough water holding capacity and needs to be replaced with long lasting moisture retentive ingredients. Alternatively, as an interim trial you can add Eco-Hydrate, an excellent soil wetting agent without the nasty chemicals in other polymer based wetting agents and water crystals.
- Have you been growing vegetable crops in the pot? If so, consider the importance of crop rotation.
This is the practice of rotating vegetable crops in different plant family groups to avoid a build up of soil borne diseases and pest problems. One of the simplest ways to practice crop rotation in pots is when one crop finishes, you replace it with a crop from a different family group. One system of crop rotation is to plant legumes, then leafy greens, flowering plants and finally root vegetables. For example, this just means if you have been growing beans (legumes) in the pot for a while, instead of replacing them with another legume crop such as snow peas, it is good hygiene practice to plant a different variety of vegetable (such as a leafy green crop of spinach or salad greens or a fruiting crop such as tomatoes, capsicum or cucumber instead).
‘Continually planting the same family of vegetables in the one pot can lead to an imbalance in potentially harmful soil microbes.’
- In a garden bed situation, beneficial bacteria and fungi and other organisms in the soil food web can maintain the balance in the food chain (it’s very much a microbe eat microbe world down there!) but in a pot or container, this doesn’t happen to the same extent. You can add beneficial ‘effective microbes’ (or E.M. as they are sometimes known) to your potting mix to maintain a healthy growing environment and ‘feed’ the microorganisms with diluted molasses and kelp. This quickly activates soil life and improves plant health.
- Check the soil pH – This is a maintenance activity I do regularly because the level of acidity sheds light on whether or not your plants are getting access to the nutrients in the pot. Using a low-cost soil pH meter or kit, now is a good time to check the pH balance.
- Most vegetables and herbs thrive in a pH range of 6.2-6.8 with 6.4 being optimum in my experience. This pH range allows the maximum uptake of the majority of minerals from the soil, accompanied by a healthy microbe population. If the soil is not within a healthy pH range for what you are growing, you can add dry organic supplements to help re-balance it. If the soil pH is where it should be, then don’t worry about re-potting just now. Top up with some fresh potting mix to help drainage, aeration and some organic fertilisers to boost the nutrients and continue to plant in this pot for another season.
- What is the soil level in your pot? If the growing medium has dropped well below the lip of the pot, then it is likely there was a reasonable percentage of compost or organic materials that have now been ‘used up’ by the current plant or the crop you have just finished.
The nutrients available to the plant or next crop have diminished and need replacing. Revitalising potting mix is an ongoing maintenance job for container gardeners! Now is a good time to rejuvenate the growing medium with additional organic fertilisers and some homemade potting mix to bring up the level again in the pot.
Based on the above checklist, you should be able to decide on the best option for you: revitalise your potting mix or replace it!
6 Ways to Reuse Old Potting Mix
Frugal gardeners avoid wasting valuable resources, so here are some ways to RE-USE ‘old’ potting mix:
- Refresh it with New Ingredients – Mix together well equal parts of pre-soaked coir(coconut husk fibre which helps retain moisture) – I soak these in HOT water to speed up hydration and add seaweed and molasses to double as a slow release fertiliser;
vermiculite (for drainage, nutrient retention, insulation and aeration). I prefer vermiculite to perlite which I stopped using after reading about the potential for serious health issues;
and compost and worm castings (vermicast) together in a container. These last two ingredients help add vital healthy microbes to your mix.
Then mix in slow release pelletised or powdered organic fertiliser (preferably one that has rock minerals in it). I only use certified organic products like Organic Xtra, Nutri-Store Gold and Organic Link. A handful of rock minerals is also an important addition (I use NatraMin but Alroc or even crusher dust will help add a balance of minerals to your mix).
Add enough potting mix to top up the pot and water in.
- Use Refreshed Potting Mix to Top up Other Pots – If you want to revitalise the entire pot contents, rather than just topping up the level in the pot, water the old potting mix while it is still in the pot to make it easier to remove. Sieving and breaking it up will be much easier too. Moistened (not soggy!) mix will also be less dusty and easier to mix with the new ingredients. Spread out a small tarp or plastic sheet and tip the potting mix out (or depending on your space, use a wheelbarrow or large plastic bucket). Use a mask and gloves for this exercise.
If you have drainage gravel in the mix, use a large sieve to strain off the stones and any root masses and allow the aerated mix to fall to the bottom of a bucket, barrow or onto the tarp. This can then be mixed with the above ingredients (use 3:1 new to old as a general guide but if you’re a really frugal gardener, you can use a 50:50 mix of old and new or any other ratio that you like!). Then you will have extra refreshed potting mix to top up other pots as well.
- Top Dress your Lawn – If you have depressions in your lawn, ruts or areas that are subject to run-off and spots where you lose valuable topsoil, you can use old potting mix to top up these areas.
- Make a Raised Garden Bed– Build up the level of your garden with the additional mix, add some new potting mix as well and plant straight into it.
If you have a no-dig garden bed, these tend to shrink as the plants use up the organic matter so another use is to top up the level of your garden.
- Add to Your Compost– A hot compost heap or system that reaches between 65-70°C will destroy any pathogens if you are worried about soil health. Even if you don’t have a problem with disease in your pot, your old potting mix will turn into rich living humus full of nutrients that you can add to your new potting mix.
- Add to Clay Soils – Clay soils tend to hold too much moisture, are heavy and often difficult to grow a wide variety of plants in. Turning in some lighter, used potting mix which is still rich in organic matter can improve soil structure and drainage.
These are just some of the ways you can re-use old potting mix and revitalise your plants.
Related Articles: If you want to save money, why not make your own potting mix? You can get a copy of my easy DIY Potting Mix Recipe here and I explain the role of the ingredients I use and their benefits.
© Copyright Anne Gibson, The Micro Gardener 2010-2013 – http://www.themicrogardener.com. All rights reserved.