From the Ground Up

by Derek Burch

The Roots and the Soil - Part C: The Soil Mix

We have seen how the soil particles and the space between them interact in determining the drainage of water from a soil and its aeration. What materials can be used to make a good growing medium?

Making a mix is more or less a matter of putting large and small particles together, although the wrong combination can turn out like concrete if the small particles just fill the gaps between the larger ones. This is where organic materials such as peat and coir (coconut husk) and some types of bark are valuable. They are large particles, but have an internal structure that allows water to enter them as though each large particle was made up of smaller ones. They give both aeration and water-holding capacity to a mix. Would they work by themselves for growing? Yes and no. They are very light and don't give the roots much of a grip to hold the plants upright, and if they dry out too much, they are hard to rewet.They are also subject to breakdown over time, which can be a nuisance when a container winds up only part full of medium.

A material called calcined clay, made by heating clay until the tiny clay particles join up in clumps, is more bulky to give support, and has a similar combination of large particle size and internal water holding ability. In theory, it could be used as a growing medium by itself, but it is more often used as only part of a mix.

What do we have so far in looking for the growing medium that combines aeration and water-holding? The materials that come originally from plants (peat, coir and bark), and the material that began as rock (calcined clay), all have good qualities, and are a great starting point.

Other materials of a rock origin are often added to keep a mix open or give it a longer life. Probably the most common are coarse sand, and two manufactured materials, perlite and vermiculite.

  • Coarse sand gives a good "feel" to a mix, and may be important for root growth because of its abrasive nature. It holds only the film of water around each particle as the soil drains: very few of the spaces in a pot of coarse sand are small enough to retain water. Very good aeration, poor water holding.
  • Perlite is made by heating volcanic rock to a high temperature. It has a rough surface which increases the amount of water held, but none penetrates into the particles, which can be graded to whatever size is needed.Very good aeration little to moderate water holding,
  • Vermiculite is also made by heating a rock, mica, until it expands. It has a layered structure, and water can be stored between the layers as well as in a film around the outside. The big disadvantage of this material is that it loses its structure and becomes slimy with time. Good aeration for a time, good water holding.

These are the main components of "soilless" mixes, although there are a number of others worth considering. A combination of two or three of these will usually give the required physical characteristics to the mix that ensure the combination of good aeration and some water holding. One part of peat to one of sand or perlite, by volume, is a good start for propagation or for growing. Aged pine bark, peat, and perlite or coarse sand in a 2:1:1 ratio by volume is an easy-to-manage growing medium for most sizes of pots.If you want to experiment with mixing your own, you need to get into the habit of keeping close notes so that you know what works, and what is difficult. The principles here are the same for a single plant or for the largest nursery.

What about using topsoil, either by itself or as part of the mix? It is always an option,of course, but one about which it is hard to write without getting into the whole subject of soil science. Put into a pot, topsoil has very different characteristics from those which you may understand well in your garden. It may need to be mixed with something to increase aeration or to hold more water, or simply to make it lighter in weight. Nothing wrong with the soil that has served you in your garden, but treat it as a friendly stranger when you use it for your potted plants until you get to know its strengths and weaknesses for your purpose.

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