Suitable soil and compost for container plants

Plants that grow in a limited space and whose soil does not produce its own nutrients, need very high quality compost in order to be healthy and vigorous. Its composition should be such that the main nutrients and trace elements are present in the correct quantity and in a balanced ratio. Good compost should be loose and structurally stable, well ventilated and not compressed. On the one hand, the water must drain away easily and on the other hand, enough of it should be retained to meet the plant’s needs. The compost used for perennial container plants should be able to store nutrients for a long time because the plants will often remain in the same container for several years.

Organic components

White peat: This is a still fairly little-decomposed peat from the upper layer of a peat bog with a large number of pores that ensures the stable structure of the compost mixture. It improves the compost’s aeration and it has the ability to store water and nutrients. A disadvantage is that if it dries out it is very hard to get it moist again.

Black peat: This is older, very decomposed peat with fewer pores than white peat. It improves the supply of nutrients but has a tendency to silt up. Excessive amounts of black peat in the compost may lead to water-logging.

Compost: In this sense, compost is not the growing medium as a whole but a mixture of decomposed organic waste. It improves the ability of the soil mix to absorb water and nutrients, makes for better aeration and provides humus. The degree of decomposition and source of the organic detritus used play a decisive part in the quality of the compost, (no sludge sewage because of the heavy metals it contains)

Bark humus: This composted waste product from the timber industry is increasingly replacing peat. It usually consists of crushed bark from conifers. The degree of aeration and water absorption depends on the level of decomposition.

Mineral components:

Clay: A mixture of various clay minerals that have a great water and nutrient absorption power. Aeration decreases as the amount of clay in the soil mixture increases.

Loam: A mixture of clay, poor clay and sand that improves water absorption and reduces air permeability.

Sand: The presence of sand in the soil mixture ensures good drainage and breaks up heavy soil. Because it is so heavy, it increases the stability of the plants. Sand is chemically neutral.


Expanded clay, expanded slate, pumice gravel: These are used to improve the aeration and water drainage of the soil mixture.

Volcanic rock such as perlite and vermiculite and synthetic plastic foam such as expanded polystyrene have similar properties.


You use either standard ready-made soil mixtures or mix them yourself. There are several kinds of soil mixes such as:

Standard soil mixes: These are produced industrially and consist chiefly of white peat, clay and humus, containing varying amounts of nutrients and aggregates depending on their use. The main advantage is the standard composition and quality while a pH of 5.6 to 6.5 is ideal for the majority of balcony and container plants. There are several kinds of standard soil mixes, ranging from seed compost, low in nutrients, to soil mixtures with slow-release fertilizer for long-term cultivation of perennials and shrubs.

Peat culture medium: The main component is de-limed white or sphagnum moss peat. It is true that white peat has a very loose structure but it has the disadvantage that it dries out very quickly and does not absorb water easily. In addition, it provides little stability, which is a serious disadvantage for large container plants. It is best used to as an addition to heavy, loamy soil mixtures so as to loosen them up.

Potting compost: There is a very wide range of potting composts and they also vary considerably in quality. Inexpensive potting composts contain a high proportion of black peat which – if too high – is not ideal for plants. On the plus side, black peat is able to store nutrients but against that it is heavy, relatively badly aerated and has a tendency to becoming waterlogged. For these reasons is better to use high-quality potting composts, even if they costs a little more.

Special soil mixes: These are specially formulated to meet the requirements of special particular plants such as palms, rhododendrons and so on.

Expanded clay, expanded slate: Soilless mixes consisting of expanded clay have the advantage that they do not decompose unlike organic composts, and aeration always remains constant. Even with complete water-saturation, the roots still have sufficient oxygen. The need to water is greatly reduced because of the medium’s great ability to store it. The disadvantage is that it does not store nutrients and plants must therefore be given regular applications of fertilizer.

Home-mixed composts: If you have a garden whose soil is loamy with a good humus content, as well as a compost heap, you can use it to make your own potting compost. A good recipe is: Take garden soil, well-rotted compost, sand and white peat in the ration 3:3:2:2. Mix well together and add some fertilizer according to requirements. The disadvantage is that the soil and compost may contain weed seeds, and also the quality will not be as consistent as commercial products.

Half-and-half: You can also mix unfertilized or lightly fertilized garden soil with bought compost. A good mixture consists of soil, loamy compost and sand in the ratio of 1:1:0.2, to which you add organic fertilizer or slow-release fertilizer.

In order to halt the destruction of irreplaceable peat bogs, while or sphagnum moss peat is increasingly being replaced by the use of other components such as bark humus.

You can contribute to the protection of the environment by buying bark culture mediums instead of soil mixtures containing peat.

Making compost on the balcony?

It is undoubtedly a desirable idea to make compost out of the humus-rich plant detritus and kitchen waste. This can be done easily enough in a small garden but it is not really practical on a balcony. Apart from the space required, many soil creatures are necessary, from micro-organisms to earthworms, to decompose organic waste. To ensure that these creatures are able to penetrate the compost, it must rest on natural soil and not on a solid base such as concrete. There is also the matter of the smell given off by the decomposing organic material. So in these circumstances it is best to buy organic compost from garden centres, or possibly from official compost heaps in your area.

Closed thermo-compost makers are ideal for small gardens. Once the enclosed container is filled the compost is not touched until it is ready, which may take as little as three weeks.

18. April 2014 by admin
Categories: Tips and Advice | Comments Off on Suitable soil and compost for container plants


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