Do you garden in containers? If so, revitalising old potting mix and checking 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!
This is a trouble shooting article to help you improve your potting mix and solve some common problems.
Can you Reuse Old Potting Mix? If so, HOW?
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. Not only that, but it dries out fast and your plants suffer.
Making your own homemade potting mix make sense (and cents!) because it lasts longer, provides all your plant needs and contains no nasty chemical additives like water crystals and soil wetters. Re-using your old potting mix 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. Check 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 10 Tip Checklist First!
1. 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.
2. What is the soil level in your pot?
- If the growing medium has dropped well below the lip of the pot, then it needs topping up. 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. Bagged potting mixes are primarily made up of cheap pine bark. This ingredient breaks down and you may notice the soil level dropping in your pot over time. You can top up the growing medium with additional ‘ingredients’ that won’t break down so quickly.
3. What is the soil structure like?
- If you used a bagged potting mix to start with, it may have a good structure but be a bit tired. If you used soil from your garden, it may have compacted. You may be able to amend your potting mix with natural ingredients that help hold moisture and improve structure. However, like any recipe, for healthy soil, it’s important to get the quantities right. Too much of one ingredient can cause problems! If you’re not confident with how much to add, use a recipe to get the balance correct.
4. Is your plant a long term resident?
- If you have a dwarf fruit tree for example, it’s likely to be in a large pot for the long term. Rather than re-potting every 1-2 years, you can keep the potting mix healthy by adding nutrients every season. If the plant has outgrown the current pot, then consider moving it into a larger container, so it doesn’t become pot bound.
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5. Have you had any problems with disease in the pot?
- Fungal and bacterial diseases in particular can be a problem. e.g. different types of root rot. Plants can be 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. Avoid adding to your compost. Start again with fresh potting mix.
- Even if you use advanced hot composting techniques, it’s important to respect the importance of good soil hygiene practices. Occasionally, this may be the best option, rather than causing more ongoing serious problems. Cleaning the pot afterwards is also important. Avoid chemicals. Just warm soapy water is all that’s necessary.
6. Have your plants been attacked by pests?
- If pest insects have been a problem rather than disease, it’s very likely the soil needs attention. Adding nutrients to build up soil health is one vital step. You may also need to check soil moisture and the suitability of the plants you are growing. You can also use a variety of integrated pest management (IPM) strategies to resolve the pest issues.
7. When watering your pot, does water run quickly out the bottom?
- If you see water spilling out the drainage holes almost as fast as you are pouring it in, this 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. 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. The potting mix likely needs to be improved or replaced with long-lasting moisture and nutrient-retentive ingredients. Alternatively, as a trial you could add a product like Eco-Hydrate, an excellent soil wetting agent without the nasty chemicals in other polymer based wetting agents and water crystals.
8. 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 peas, swap to another plant family. 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.’
9. Are you feeding your beneficial soil microorganisms?
- 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! These beneficial microbes help feed your plants by turning soil nutrients into a soluble form the roots can take up. In a pot or container, this doesn’t happen to the same extent. You need to give this natural process a helping hand! You can add beneficial ‘effective microbes’ (or E.M. as they are also known) to boost the life in your soil. Maintain a healthy growing environment for your plants by ‘feeding’ the microorganisms with diluted molasses and kelp. This quickly activates soil life and improves plant health. Watch the difference in your plant health – you’ll be amazed!
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10. Have you checked 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, check the pH balance. You can also adjust your soil with simple amendments. I share how to do this in my How to Make Potting Mix Guide at Home.
- 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. Compost also provides a buffer zone. 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. You can continue to plant in this pot for another season.
- The nutrients available to the plant or next crop may have diminished and likely need replacing. It’s a bit like us losing energy if we don’t eat regularly! 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:
- 1. Refresh Old Potting Mix with New Ingredients. You can add pre-soaked coir (coconut husk fibre which helps retain moisture). I soak a compressed block in HOT water to speed up hydration and add seaweed and molasses to double as a slow release fertiliser. Vermiculite can be added for drainage, nutrient retention, insulation and aeration. Compost and worm castings (vermicast) also help add vital healthy microbes to your tired soil mix. If you need confidence on quantities with a recipe you can use year after year, check out my How to Make Potting Mix at Home Guide.
Add slow release pelletized or powdered organic fertiliser (preferably one that has rock minerals in it). To add a balance of minerals into your soil, so your plants are not lacking any nutrients for healthy growth, make sure you include a mineral mix (also known as soft rock phosphate, rock minerals or mineral fertiliser).
- 2. 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. 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.
- 3. 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 fill in these areas.
- 4. 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.
- 5. 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.
- 6. 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.
If you want to save money, why not make your own potting mix at home? I have created a practical guide to help you with 4 simple steps.
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