Thermometers
Any thermometer that will measure temperature at a specific depth can be used to measure soil temperature. Ordinary glass bulb thermometers are adequate, provided they are long enough to allow the temperature to be read whilst the bulb is in the ground at required depth.

It is easy to break the thermometer when pushing it into the ground, so it is best to make a hole with a metal rod or screwdriver, before pushing the thermometer down. Metal sleeves can be purchased to sit the thermometer in, to avoid breakage.

Test site
Make sure the position in which the soil temperature is measured is representative of the bulk of the vegetable garden:

• soil cover - whether full of weeds, straw-covered, bare, etc. Soil warms from the top down, and bare soil warms most quickly.

• aspect (amount and direction of slope) - South-facing slopes warm more quickly than north-facing slopes or level land. Trees or buildings shade from the sun, and soil in the shade will be cooler.

• soil moisture - Dry soils warm faster than wet ones; moisture content is strongly related to soil cover, aspect, texture and drainage.

To read soil temperature

• place the bulb or sensing part of the thermometer at the intended seeding depth. In the case of glass thermometers, use a pilot hole to reduce the risk of breakage. The bulb should be in firm contact with the surrounding soil.

• allow a minute or two for the thermometer and the soil to reach temperature equilibrium before taking a reading. If there is bright sunshine, shade the thermometer during this time interval.

• record the temperature.

Measuring soil temperature is an interesting exercise, but it must be done safely to avoid breaking glass thermometers.