The temperature of soil is an important factor for yielding a productive and early crop. Warm soil will speed up the growing process and give you a head start on the allotment this Spring. But how do you increase the temperature of soil?

Moisture content

Studies have found that a soil with a slightly increased moisture content will absorb more heat from the sun and maintain a steady temperature throughout the night and day. This is process is similar to the effects of water heating in a domestic storage heater.

Organic content

During very cold weather, the heat generated by the decomposition of organic matter is very low, but still present in soil. The more fresh organic matter combined into soil, the warming it will be. As Winter starts to loosen its grip, this process will start to generate more heat.

Solution: A Victorian style ‘Hotbed’

Warming soil with a hotbed
Warming soil with a hotbed

The Victorian gardeners were great innovators. The hotbed is one of the most useful of their practices and it’s well worth spending a little time constructing your own. It is not only useful for warming soil, but also as a great soil conditioner – adding structure and increased organic matter.

In short, a hotbed is nothing more than a trench of organic materials, covered with soil. It makes a better use of land than a compost heap alone (although compost heaps do have an important role in the garden and this is not by any measure a replacement). Here’s how to construct a hotbed:

Note: It’s worth saving the construction of your hotbed to just before you are ready to plant your crops. Don’t plant out any earlier than you would normally. The hotbed itself will accelerate the growing process.

  1. Dig trench: Firstly, dig a trench at about a spades depth. Make it as long as you like, but I wouldn’t have it any wider than a metre or so for ease of access.
  2. Break up soil: Loosen up the soil at the bottom of the trench with a garden fork.
  3. Add hard clippings: Wood chippings, twigs, etc to form a layer of between 10-15 cm.
  4. Add soft clippings: General garden clippings, grass, leaves and soft stems, etc to form a layer of between 10-15 cm.
  5. Animal Manure: Add a 10 cm layer of animal manure. Traditionally, horse manure was always used, although donkey manure is supposed to be very good.
  6. Rotted Compost: Add a further 10 cm of rotted compost. Avoid using spent compost as it may contain diseases. Material from your compost heap should do well. You can also try mushroom compost.
  7. Add Soil: Replace the top layer of soil over the trench to for a small mound.

Now your hotbed is ready to receive its crop. After harvest, you can break the hotbed up and dig in the well rotted organic materials into the surrounding soil as a soil conditioner.

James Middleton

An obsessive gardener since 1982. Day-time job - web designer and developer and University lecturer.

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