Posts Tagged ‘Soil preparation’

Trench gardening, double digging and raised beds

Despite the fact that so few people practice trench gardening in southern Africa (and most of those that have, have given it up) this is probably the most written about gardening system in the region. Writing about trench gardening is obviously a lot easier that putting it into practice! (Ouch! I can almost feel you wince.) What’s more the recommended trench depth appears to be getting deeper and deeper and the last article I read on trench gardening was recommending trenches at least 1m deep.

Just so that you know what I’m talking about, the following directions are usually given for building a trench garden bed.

  • First dig a trench about 1m wide and 1m depth. Keep the topsoil and the subsoil seperate.
  • Next fill the trench with a layer of organic matter or rubbish to a depth of about 200mm and water well.
  • Add 50-100mm of subsoil, and again, water well.
  • Continue filling the trench with alternating layers of rubbish and soil until all the suboil is used, watering each layer well as you go along.
  • Finish with a layer of topsoil, raised above the original ground level.
  • The trench is now ready for planting.

The directions do not give any guidelines on how you should deal with the resulting backache or heat exhaustion.

There are several reasons – apart from the backache – why trench gardening is problematic.

Plant roots need air in order for the plants (especially vegetables) to grow well, and they are therefore unlikely to grow well in a water-logged soils. Trench gardens are problematic because they become prone to water logging, especially where they have been dug deeply enough to penetrate the subsoil. And, given the recommended depth, this is more than likely. Subsoil is not only rather compact and difficult to dig, it has a high clay contact. Water is therefore unlikely to drain readily out of the trench into the surrounding soil. To make matters worse, in very wet conditions, water may actually drain out of the surrounding subsoil and accumulate in the trench.

Burying organic matter in a trench is, moreover, an extremely wasteful practice. When organic matter is buried, the whole composting process and accompanying release of nutrients slows down because the micro-organisms responsible for this process no longer have enough air. Once the trench collects water, as will happen during the rainy season, the organic matter becomes waterlogged and you end up with a smelly mess rather than useful compost.

Another difficulty I have with this method is the fact that in the unlikely event that the deeply buried layers of organic matter actually break down and release nutrients, these nutrients would probably be too deep for the plants to access, given that most food crops do not send their roots down any deeper than 300mm. Rather a waste of organic matter, don’t you think?

Interestingly, if you build a trench garden you end up with a layer of top soil well above the original ground level. In other words you create a raised bed. Raised beds help to ensure that the soil is well drained, and the vegetables in your garden will grow well provided you can maintain that layer of soil above the original ground level high enough to accommodate the plant roots. Raised beds are also a joy to work with since you don’t have to do so much bending to tend to your plants. It is not however necessary to dig a meter deep trench in order to build a raised bed!


There are any number of ways that one can go about building raised beds – it rather depends on what materials you have available. In the garden at Lizard’s Leap I used recycled concrete blocks to outline the beds and then filled them with compost and soil from the pathways. There are however other, far more attractive alternatives – railway sleepers immediately come to mind.  At Cleopatra’s Mountain Farm House the beds are edged with low brick walls, beautifully set out in a formal garden. In larger gardens, where space is not an issue, you need do no more than heap the soil and cover the sides with mulch.

Double digging is another system of soil preparation which is commonly recommended to new gardeners. In this system the gardener starts off by digging a trench, usually about 200mm deep and setting aside the soil. A second trench is then dug alongside, and this time the soil from the second trench is dumped into the first trench. Trench after trench is opened up and the soil from each trench is used to fill the preceding trench. Eventually the whole area has been dug over. The only possible reason I can think of for double digging, is perhaps to remove very large rocks which might impede root growth. The usual argument for double digging is that it aerates the soil. Unfortunately, with all this digging you run the risk of destrying the soil structure – especially if you are digging wet clay soil, in which case you are most probably going to end up with hard clods of very badly aerated soil. A far more effective and much easier way to aerate the soil is to cover it with a layer of mulch. In no time at all earthworms will move in under the mulch and aerate the soil for you.

For more information – and photographs – on raised beds visit the post Raised bed designs