One of the challenges with container gardening is what you do with used compost. Replacing it every year seems expensive, labour intensive and not a good use of resources.
The good news is that you can reuse it.
To re-use it successfully, you need to replace the nutrients used by the previous crop. You can either add a general purpose organic fertiliser like chicken manure or blood fish and bone. And / or you can mix in some fresh organic matter – like home made compost, a good quality peat free commercial compost, municipal compost (made from recycled waste), manure, or, my personal favourite, worm compost.
It’s not possible to offer precise recipes for how much to add as there are too many variables – the type of organic matter or fertiliser you’re adding (worm compost is richer than normal compost for example), the previous crop grown in the compost (hungry crops like tomatoes or courgettes will take out more nutrients than say salads), and the crop you plan to grow next.
The key is to experiment and observe the results. As a very rough guide, I would usually add 10 – 20% worm compost if I’m growing leafy crops, 20 – 30% if I’m growing a hungry crop like courgettes or tomatoes. For salads I often also add a small handful of nitrogen rich fertiliser (nitrogen promotes leaf growth) like chicken manure pellets. For tomatoes I might also add a handful of general purpose organic fertiliser like seaweed meal. I’d then feed the tomatoes with a potassium rich fertiliser like comfrey juice or organic tomato feed once they are flowering (potassium is needed for fruit growth).
If you’re using a commercial fertiliser, start with the dose recommended on the pack and adjust with experience. I’d recommend only using organic fertilisers – and not just out of principle. Inorganic fertilisers often result in rapid but ‘soft’ growth that is vulnerable to pests and disease. They can also kill the microbial life in the soil, which is essential for strong, healthy plants.
If all this sounds very complicated, I promise you it’s not. True, achieving very vigorous growth with old compost requires some tweaking. But getting good growth is not difficult. I’ve found it easier to re-use compost successfully for salad and leafy crops than for fruiting crops like tomatoes or chillies.
How do you know if you’ve added enough nutrients? Tell tale signs of a lack of nutrients include leaves with yellow or purple tinges, small leaves or slow growth. On the other hand, soft lush growth that is attacked by pests is often a sign of adding too much. Remember that it’s always easier to add more nutrients later – like a top dressing of compost or chicken manure pellets or to feed with liquid fertiliser like nettle tea or seaweed extract – than it is to take nutrients out!
One last thing – avoid growing the same crop in the same compost in succession. In this way you’ll reduce the stress on nutrients in the soil and avoid the build up of pests and disease.