How do I measure soil temperature and why does it matter more than air temperature?

Almost every failed early sowing is a soil temperature failure. The seed sits in cold wet ground, does nothing, and rots or is eaten. Sown a fortnight later into warmer soil, the same seed germinates in days and overtakes the earlier sowing within a month — which is the single most useful and least believed fact in vegetable growing.

What you'll need

• A soil thermometer with a probe
• A notebook or a note on your phone

Step by step

1. Buy a probe thermometer rather than guessing.A basic soil thermometer with a spike and a dial costs very little. A digital kitchen probe works too. What you cannot do is judge soil temperature by hand — everything below body temperature feels cold.
2. Measure at seed depth, not at the surface.Push the probe in to the depth the seed will sit. For small seed that is a couple of centimetres; for beans, potatoes and larger seed it is deeper, and the deeper reading is meaningfully lower in spring.
3. Take the reading first thing in the morning.Soil warms through the day and cools overnight. The morning reading is the daily minimum and it is what governs whether the seed does anything. An afternoon reading in spring sunshine flatters the bed by several degrees.
4. Measure the bed you will sow, not the garden.A raised bed, a south-facing slope and a shaded border can be several degrees apart on the same morning. Soil under a cloche or black polythene is warmer again.
5. Take it over several days and look for a trend.One reading tells you about one morning. What you want is a run of mornings holding above the threshold your crop needs, because a single warm day followed by a cold night achieves nothing.
6. Match the reading to the crop.Different seeds have very different minimum germination temperatures, and seed packets or catalogues generally state them. Cool-season crops germinate at temperatures where warm-season ones will simply rot.
7. Warm the ground if you want to be earlier.Cover the bed with clear polythene, black plastic, a cloche or a cold frame for two or three weeks before sowing. Clear plastic warms fastest; black warms more slowly but also suppresses weeds. Keep the cover on until you sow.

Tips

• The traditional test of sitting on the soil to see if it is warm enough is not a joke — it is a crude but real proxy for the same thing. A thermometer just removes the argument.
• Wet soil warms far more slowly than drained soil, because water takes a great deal of energy to warm. Improving drainage is also a way of getting an earlier spring.
• Soil under a mulch stays cold longer in spring. If you want early sowings, pull the mulch back a few weeks beforehand and put it back once the soil has warmed.
• Record the date the bed reaches your target temperature each year. After a few years you can predict it, and you will find it moves far less than the air temperature does.

Common mistakes

• Sowing on a warm week in early spring because the air feels like spring — Air responds to a warm spell in hours; soil takes weeks. A week of sunshine in March barely moves the ground temperature at seed depth, and the seed goes in to sit in the cold.
• Reading in the afternoon in direct sun — The surface layer can be many degrees above what it will be at dawn. The reading that matters is the low, not the high.
• Assuming the whole plot is at one temperature — It is not, by a wide margin. Sow the warm beds first and the cold ones a fortnight later rather than treating the sowing date as a garden-wide event.

Questions people ask