Understanding soil, and comparing it with compost
Soil, compost, fertility
No Dig Gardening

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.

1. The different colour and texture of soil left and mushroom compost right
The different colour and texture of soil compost (left) and mushroom compost (right)
16. No dig lettuce in a bed with no compost mulch. Soil cracking open in dry weather
No dig lettuce in a bed with no compost mulch – the soil is cracking open in the dry weather
16a. No dig lettuce in a bed with compost mulch. Dry weather and 0.75kg just harvested
No dig lettuce in a bed with compost mulch – dry weather, and 0.75 kg (1.7 lb) has just been harvested

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!

23. I’m checking the soil before buying Homeacres. Lovely topsoil and subsoil in my hand
Checking the soil before buying Homeacres – lovely topsoil, and subsoil in my hand
28. Soil profile April 2017 & mulch layer on 4 year old bed: on a course. Photo John Reith
A soil profile from April 2017 – a mulch layer on a four-year-old bed

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.

21. Texture difference is highly visible: soil from underneath a raised bed and the top mulch of compost
Soil from underneath a raised bed, and the top mulch of compost – the texture difference is clearly visible
22. Clay soil was dug and now the surface is cracking in a dry spring: parsnip seedlings on the dig bed at Lower Farm
Parsnip seedlings on the dig bed at Lower Farm – the clay soil had been dug, and here the surface is cracking in a dry spring

Biology matters
Weeds
Nutrition
Soil as a base ingredient in new beds
Soil and compost trial
Comparing weed growth
Comparing plantings
Step 15
Step 15
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