How to Re‐use Old Compost
Traditional gardening books often tell you to replace the compost or soil in your pots each year, but if you have more than just a few pots, emptying and refilling all of them will be a time consuming and messy job as well as being expensive. It also seems a waste to throw this compost and soil away.
However, it is possible to re‐use the soil and compost in pots and it will last for several seasons, perhaps even indefinitely.
These are the basic things you need to know:
- Some crops are easier to grow than others in old compost.
- After a season of growing, the nutrients (i.e. plant food) in your pots will have been largely used up. To re‐use the soil, you’ll have to add more nutrients.
- The ‘structure’ of your growing mix may also start to break down over time. For plants to grow healthily, you might need to add something after a year or two to help rebuild this.
Which crops are easier to grow in old soil:
It’s possible to grow any crop in old compost, but some are easier to grow successfully than others. This is because some have quite precise and demanding nutrient needs, while others are less fussy.
Why you need to add nutrients and how to do it:
By the end of a season most of the nutrients in your pots will have been used up ‐ particularly if you were growing hungry plants like tomatoes. This means that to grow healthy plants in used compost, you’ll need to add nutrients. There are a wide range of organic ingredients and fertilisers you can chose from.
How much to add:
It’s hard to be precise about quantities to add as it depends on several things. These include the size of pot, how old or depleted the existing soil is, what was grown in the pot last season, and what you plan to grow next season (for example, fruiting vegetables like squash will need more food than leafy vegetables like salads). In addition, most organic ingredients like manure or worm compost, will vary considerably in strength and what nutrients they contain.
In general, it is better to add too little than too much as you can always add more nutrients during the season, either to the top of the pots or as liquid feed, but you can’t take them away.
Rejuvenating old compost is not really a precise science; it’s more trial and error. Don’t let this put you off. Experiment, observe the results and you’ll soon learn what works for you. It might be hard to get superb growth using old compost, but getting good, healthy crops is not so hard.
When you do experiment, label or make a note of what you did. It’s easy to have a very successful pot and then forget what you added.
If you want to grow organically, it’s also a good idea to think about how you will add and support soil life in your containers. This is vital because it will break down the organic materials in your soil to release nutrients for your plants.
How to maintain the structure of old compost:
After a season or two, the structure of your growing mix can start to collapse. This is due to the organic particles in your mix decomposing and breaking down. When this happens, the air gaps in the soil which are so important to enable the roots to breathe, begin to disappear. To fix this, you’ll need to add something to improve the structure again and add air gaps back into the soil.
A variety of things will help do this. Medium or coarse grade vermiculite or perlite are two (add between 5% and 20%). Both are natural ingredients made from volcanic rock and both work well. However, they're also high energy to make and costly to buy, particularly in smaller quantities. Two other options are biochar and green waste compost.
Things you can add to rejuvenate old compost:
Worm compost is the urban container gardener’s best friend. It’s one of the most useful and best ingredients you can have to rejuvenate old compost. It’s usually rich in the major nutrients, trace elements and is teeming with soil life.
Add to old compost between 10% for salad items and up to 50% for squash or tomatoes. Take care not to add too much as it can be very rich; you can always add a layer on top later. If you have lots of containers, you may find you can’t make as much as you need but even a little can make a big difference.
You can either buy a wormery or it is easy to make your own.
If you have more space, you can make your own compost. Some composters are particularly designed for small spaces. Use it like worm compost, but it is less rich so add in slightly higher percentages.
Bokashi is a Japanese method of composting. It uses Effective Microorganisms and needs even less space than a wormery. It is faster than composting and can be done inside or outside. It’s a good alternative if you don't like worms!
The end product of bokashi is pickled, partly composted vegetables. These can be used like worm compost but are less versatile because what comes out of the bin is not really a ‘finished product’. They must be left in the soil for a few weeks to settle before growing in. You can mix 10 ‐ 30% pickled bokashi veg into a container, then leave for several weeks before planting, or add to the bottom of pots before filling with old compost. As the pickled veg breaks down it will release nutrients and lots of beneficial soil life to your pots.
You can buy bokashi buckets online or make your own out of buckets with lids. You’ll also need to buy bokashi bran.
Manure is another useful ingredient for reinvigorating used container soil, particularly if you can’t make your own worm compost, or aren’t making enough. It is teaming with soil life and has most of the essential nutrients plants need in moderate quantities. However, it does vary considerably in quality. To use manure safely and successfully, you need to check that it’s well rotted (as a general rule, it needs to smell like garden soil rather than animal poo) and it hasn’t come from animals farmed inorganically or fed antibiotics. If you can, try to find other growers who have tried it and if you’re still not sure, try a small quantity first before adding it to all your containers.
You can mix well rotted manure into old compost, or put a layer at the bottom of your pots or add a layer to the top of your pots half way through the season, known as ‘mulching’. If you are not sure how well rotted it is, either leave it in a bag to rot for longer or put it at the bottom of a big pot where it can slowly break down. Depending on the manure and what you are growing you might add between 15% for less hungry crops like salads and 50% for crops like a squash plant.
Compost can be obtained from City farms that are run on organic principles and produce excellent manure. Also check out riding stables or police stables but be aware of the animal feed and any drugs used.
Green waste compost is often fairly rich in nutrients and is ideal for rejuvenating old compost. It typically contains quite large particles, which can be sieved out if you like, or you can leave them in as these can help add structure and air gaps back into your growing mix.
Mix 15 to 50% into your old compost, depending on how rich a mix you want to make. Or you can add a layer to the top of your containers so the nutrients can slowly work their own way in. It is usually low in soil life so you should also add something like worm compost or manure if you can.
A more reliable but more expensive option to green waste compost, is to buy new multipurpose or potting compost from the shops or online. Use it in a similar way to green waste and mix it in to add nutrients to your old compost or add a layer to the top of your old compost.
Chicken manure pellets contain the main nutrients that plants need and are particularly high in nitrogen. Because plants need nitrogen for leaf growth, these pellets are excellent to rejuvenate old soil in which you want to grow leafy veg like salads or kale.
Use it in moderation, or not at all, for fruiting crops like beans or tomatoes. If you add too much, you’ll get very leafy plants without many fruits.
The guidelines on the packaging are a good place to start but in general add about a handful or two to each window box size container. This is excellent for salads but make sure it comes from organic reared chickens.
Bought fertilizer can be very useful, particularly if you haven’t got a supply of any of the above ingredients. And, even if you have, it can be useful in moderation to help raise nutrient levels.
For reusing old compost, you’ll want a balanced fertiliser. To know if a fertiliser is balanced, look for the ‘NPK’ details on the side of the packaging. You want one with roughly equal numbers for N P K (something like 5: 5: 5). Generally, avoid anything too strong and try and choose one below 8 if you can. The number refers to the percentage, so N 5 means 5% nitrogen.
A common general purpose fertiliser is ‘blood, fish and bone’. There are also vegan equivalents. Mix it into the soil before planting. The fertilizer should come with guidelines on how much to add so use these as a starting point, observe the results and then add a little more or less next time. You can also add a sprinkle to the top of the soil, (known as ‘top dressing’) to top up nutrients as your plants grow.
Biochar is charcoal ground fine. It won’t add nutrients but it will improve structure and the water holding capacity of your growing mix. Biochar needs to be charged with nutrients before you add it to your pots. You can either buy it ready charged or you can add it your wormery or compost heap.
Add 5 to 15% to your pots to improve structure and water retention. You can also use artists charcoal or barbeque charcoal, but check that the wood is from a sustainable source. You’ll also need to crush it so that it is in small pieces.
Rock dust is ground up volcanic rock that is rich in trace elements and minerals. It won’t provide plants with their basic nutrient needs but it can contribute both to plant health and flavour. Soil life also benefits from it. Mix 5 to 10% into your old compost.