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Author Topic: The Ideal Potting Medium - Soil vs Compost  (Read 2250 times)
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The Ideal Potting Medium - Soil vs Compost
« on: January 25, 2007, 00:47:10 AM »

There are quite a few varied opinions and ideas about the ideal potting medium, as advocated by enthusiasts. Some champion plain old garden soil, some sing the virtues of compost, yet others a self-produced blend of in-between mixed in any and every conceivable ratio.

Unfortunately some of us base our convictions simply on the criteria of cost, considering compost to be an unnecessary expense that can be done without, or maybe only made sparing use of.

Factually, for potting purposes compost is by far superior to ordinary garden soil as found in Malta, irrespective of it's colour (some "grade" soil by the particular reddish hue it may have) or how good it may look in the fields just down the road.

The main problem with using ordinary soil in pots is that Maltese soil is very clayey, composed of very fine particles, and lacking in organic matter. Thus, it is not at all free draining, tending to remain muddy and soggy for long periods, moreso during the cooler wetter months creating the perfect conditions for root rot. Secondly, being fine-particled, it compacts easily and goes hard when it dries out, and even if not allowed to dry out, it's intrinsic heaviness restricts root growth somewhat.

All the above is compounded by the now-predominant usage of plastic pots. Clay pots are porous so a lot of excess wetness will wick out of the sides of the pot itself and evaporate, helping to maintain the correct water content. Plastic pots are completely impervious, so there is no wicking effect and thus far less surface area for evaporation.

The foregoing problems are mainly in reference to growing established plants in soil - if we are talking about propagation, the odds against using garden soil are even higher. Both tender seedlings and unrooted or partially rooted  cuttings are even more sensitive to the water level/humidity content of the growing medium than established and vigorously growing plants.

To compound this, garden soil is naturally full of micro-organisms, fungal spores, and the like, all invisible to the naked eye but always present in unimaginable numbers - ready to overcome any vulnerable plant growth as soon as the conditions become favourable. Seedlings die from "damping off" (a fungal disease) while cuttings may also suffer a similar fate or be consumed bottom-up by the larvae of some insect.

But if ordinary soil is so bad, how is it that this is all nature offers, and that literally thousands of native and introduced plant species grow so well in our gardens, fields and countryside?

Well, for starters, while nature "only" offers us soil, it most certainly does not offer it up in pots. It can take several wet months for the soil in our gardens and fields to become wet beyond the first few inches of depth. The excess wetness is always seeping and wicking downwards, maintaining humidity levels at the ideal, except for very short periods such as immediately after rainfall. So that's one issue sorted.

The other issue, that of the undesirable low-lifes sharing the same space as our plant's roots - well, nature gives everybody a fighting chance, bugs included. In nature, no-one ever wins all the time. A tree may produce thousands of seeds in a single season, yet it may take a long number of years for the result of all that promiscuity to translate into a single, healthy sapling. That's why so many seeds are produced - it's all a lottery, and the more tickets you have, the better your chances. Unfortunately, if we decide to sow some seeds, we are unlikely to wish to sow a hundred thousand in the hope that one seedling will survive. On the contrary, if we have but a handful of seeds, we would rather wish that each and every seed translates into a strongly growing plant. So if we require results that are at considerable variance with what nature can provide, then we must provide conditions that are improved upon nature's offerings. And sterile compost is just one such "condition", and is all that should ever be used for propagative purposes.

So is ordinary soil completely out for anything but the garden? No! One thing that ordinary soil generally does better than compost is provide support. Compost, most commonly peat-based, is rather light and friable, perfect for the development of healthy roots, but somewhat lacking in heavy-duty support, as may be required for larger plants and potted trees. Of course, we may opt to purchase ready mixed soil-based composts, which, guess what, contain soil. But as long as we are potting-up a vigorously growing plant at any time other than it's dormant period, when we should not be potting it up anyway, we should have no difficulties with making up our own potting mix suitable for these larger specimens. A word of caution - The end result should not be soil with just a token amount of compost in it. Rather, the target should be a friable mix of compost with just enough soil added to augment it's weight, yet retain all it's beneficial free-draining properties. In practice, this means that no more than around 40% of the mix by volume should be soil, as the compost will be compacted somewhat resulting in a true soil-compost ratio that is higher.

Another means of making soil even more usable, especially for certain species such as large cacti requiring support but also a very free-draining medium, is by the addition of sand. Most certainly not salty beach sand, nor the variable Malta building sand which often packs rock-hard, but horticultural grade sand available from garden centres. Not very cheap as it must be rather heavy to transport from some far off country such as the UK, but you'll be using this for specific  plants which generally don't need repotting all that often.


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