Gardeners ask: ‘How can I mulch on top of my raised bed without it overflowing?’ This question suggests that a lot of the bed’s original content was soil.
- Compared to compost, soil reduces little in volume through time, and therefore the surface level of a soil-filled bed will barely reduce from one year to the next. This leaves no empty space on top to add compost and maintain fertility.
In contrast, compost does reduce in volume, so one can fill beds with compost to a depth of say 15 cm (6 in) initially; then, within a year, the surface level may be 5 cm (2 in) lower. Or lower than that if, when first filling the bed, you did not firm the compost by walking on it.
Soil profile and quality
A great aspect of no dig is being less involved with soil such as sticky clay, which is difficult to work and to pull weeds from. You rarely even see your soil, let alone need to work it.
When starting out I recommend digging a hole to see what is there, for your information. I did this before making an offer to buy Homeacres back in 2012. The selling agent found this very funny – her first house-viewer to have brought a spade!
The most noticeable thing you see when digging a hole of more than about 30 cm (12 in) deep, is the paler colour and denser nature of soil at lower levels. This is called ‘subsoil’, to differentiate it from the darker ‘topsoil’. Depth of the layers varies, and the main differences are:
- Topsoil has more organic matter (carbon) which gives the darker colour.
- Topsoil has more organisms living, moving, eating and excreting.
- Soil organisms maintain more structure, drainage and air in the topsoil.
The topsoil you can buy is probably low in microbes, and often contains some subsoil. If using it for vegetables you will probably be disappointed with their growth. I have seen samples of supposed topsoil which looked very pale and shiny rather than crumbly, and contained gravel, roots of perennial weeds, and thousands of annual weed seeds.
Stones are interesting, because sometimes they add fertility and hold some moisture, especially limestone. However, although on the whole they reduce the chances of roots finding food and water, I don’t recommend sifting soil to remove them. Best to use a thicker compost mulch if your soil is very stony.
Follow with:
Further reading:
- Professor Victor Stewart’s work on managing man-made soils