Best compost mixes for growing in containers

Growing plants in containers is more popular than ever, but to make sure that you get the best results it is important to choose the right compost for the purpose. And while the easiest option is to buy bags or bulk quantities of ‘multi-purpose’ compost for the job, this isn’t always best in the longer term for your plants, or for the environment.

Tailoring a compost (or growing medium) to suit the plants that you want to grow is nothing new. It is something that the Victorian gardeners did widely before the advent of bagged composts. In fact they based their growing mixtures on good quality soil with well-rotted leaf mould, pine needles or composted garden waste – something that we can learn a huge lesson from today.

Why does the compost level drop in pots and containers

You’ve probably seen the way that the surface of the compost drops in your pots and containers during the course of the growing season. It would be easily to simply put this down to it settling as a result of watering, and in part this can be the case. But with peat-based and peat-alternative composts this isn’t the whole story. The reason the compost level drops in these types of composts is due to the fact that they are almost wholly composed of organic material which continues to decompose and break down, even when it is used in a pot or container.

This decomposition comes about as a result of exposure to the atmosphere which allow oxygen to ‘oxidise’ the carbon (the once living material) in the compost. This releases nutrients to plants for re-use, but in addition gives off carbon dioxide – one of the ‘greenhouse’ gasses that we are most concerned about in climate change.

It is this oxidation of organic matter which causes a big drop in the level of the compost in pots and containers over the course of the season. In addition, as the compost breaks down into finer and finer particles, these can wash into the spaces between other particles to reduce aeration and drainage of the growing medium. Over the course of a single season this is not too much of a problem for the health of plants. But as the structure of the compost deteriorates progressively, over 2, 3, 4 or more years, so the conditions for root growth of plants worsen. Drainage can be affected so much over the years as a compost ages, that permanently planted containers can often become waterlogged in winter, killing the roots of plants.

Releasing carbon dioxide trapped thousands of years ago

As already mentioned, both peat-based AND peat-alternative composts deteriorate in this way. The peat-alternatives are composed of composted materials that have been alive in recent years – green waste, wood and bark waste, sheep’s wool, bracken etc. And in this way, the carbon which they have accumulated from the atmosphere is released back over a comparatively short cycle, without making a difference to carbon dioxide levels overall.

When it comes to peat-based composts, however, these use the material that formed from mosses or sedges alive many ten of thousands of years ago when they trapped their carbon. The organic remains of these plants have stored this carbon in wet conditions until it has been dug up and processed into composts and for other uses. Thus, when exposed to the air, this organic matter comprising the peat then starts to break down and release its stored carbon in the form of carbon dioxide. This gas has not been in the cycle for thousands of years, but the use of peat composts is now releasing it. This is just one of the reasons why I don’t use peat-based composts.

How to maintain a good compost structure

When it comes to using any organic compost in pots and containers we run the risk of seeing the structure deteriorate over the months and years after planting. And it seems that most of peat alternative composts decompose quickly in use. So this bring me back to the Victorians and their use of good quality soil or ‘loam’ in their growing mixtures, something which is easy to do when filling containers and raised beds.

Soil contains precious mineral matter as well as organic matter in its structure. And the mineral matter does not break down in the same way, so that its particles remain to help hold open the soil for aeration and drainage. Mixed in roughly equal parts by volume with pure organic material, this make a great growing medium and retains its structure for many years before needing to be refreshed.

So, when I fill my containers to grow summer plants in spring, I use a 50:50 mix of my own garden soil with either homemade garden compost or a peat-free bagged blend. And the results speak for themselves. The plants grow more healthily and are less reliant on me for watering during the course of the growing season. The structure remains open for a lot longer and can be reused for a number of years. This works out much cheaper even if you have to buy both the peat-free compost and bagged loam.

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