Are you buying the right kind of soil for your greenhouse or garden?
We think we are buying soil, but most of what is in the bag we buy for our gardens’ plants are not yet what we normally understand by soil. It is sphagnum, which can be mixed with different things like clay, sand, fertilizer or lime. A growbag for the greenhouse is available from 1 pound to about 5 pounds. The difference. In price can be caused by three things.
- The extra material added to the sphagnum.
- The quality of the sphagnum product
- The volume of the bag, how much is actually in the bag.
The quality of sphagnum depends on how light or dark it is. On the bag, the light sphagnum is described as coarse and the dark as fine grade. The lighter the sphagnum, the better quality. Light sphagnum is coarse and contains lots of air and cavity, which benefits the roots of the plants. The light sphagnum is ideal for capillary boxes in the greenhouse or in the ericaceous bed. The light sphagnum does not react as quickly as the dark one. This means that the light sphagnum will not turn dark and floppy as fast. The dark sphagnum quickly turns watery. On the other hand, it is an advantage in pots which quickly dries out on a warm summer day. Always remember drainage so the roots don’t dabble in water.
Sphagnum which has nothing added is naturally cheaper than sphagnum with added material. Some sphagnum has added natural fertilizer which is more expensive than chemical fertilizer. Some has added Leca to drain. In sowing and planting soil, sand is often added and lime is added in the soil for roses. Adding something costs money in both production and materials, which is what we pay for.
It is not without importance how much volume there is in a growbag, if there are 40 litres or 50. The more volume the better for the plants. There are ecological and more expensive growbags, but if you read the declaration on the back some eco-products are not ecological as it appears on the packaging. This is due to especially the content of the sphagnum, which from an ecological view should not be used because the production itself destroys our nature.
Generally, the quality of sphagnum drops quickly. The reason for the drop is that the raised bogs eventually have been drained for good sphagnum. Formerly, we thought of the Swedish sphagnum as the best you could get, but the quality of Swedish sphagnum has also dropped significantly because the Swedish are depleting their natural resources.
Coconut coir and compost made from horse manure are good alternatives to sphagnum. The problem with the alternatives is that consumers don’t want to pay extra for the quality, and it is difficult to change people’s mind about what is good soil to grow in.
Coconut has the same qualities as sphagnum. It is aerated and absorbs water and nutrients for the plant quickly. Also, it is made from recycled coconut shells, that’s been through a grinder. Coconut coir is difficult to obtain. Occasionally it can be bought in ALDI and IKEA.
Om Lars Lund
Danish horticulturist and journalist
Lars Lund has for many years engaged in the garden and greenhouse. Lars has published many books about greenhouses, and he has participated in many Danish horticultural TV shows. He is a walking garden encyclopaedia, and he has answers for most basic cultivation questions – also the more ambitious ones.