Soil temperature
Soil & evaporation ground temperature
Soil temperature is the temperature at a specified depth in the ground. Depth belongs with the value because the surface and deeper soil warm at different rates.
Fast at the surface, slower below
The surface responds directly to sunshine, night-time cooling and rain. Changes reach deeper soil more slowly and with smaller swings. A warm afternoon therefore does not mean that soil fifty centimetres down has warmed just as much.
Reading the values
Temperature is given in degrees Celsius, together with a depth or soil layer. Zero centimetres describes the surface; six centimetres describes a different depth from fifty-four. Different-depth readings are not contradictory answers to the same question.
Why two garden beds can differ
- Sun and shade alter the heat input.
- Water content and soil type affect heat storage and conduction.
- Vegetation, mulch and snow can insulate soil against rapid changes.
Why plants care
Germination, root growth and soil organisms respond to temperature, and suitable conditions differ between plant species. A model also represents a wider area, not the soil beside one seed. For sowing decisions, a measurement in the actual bed is therefore often more useful than a value for the whole neighbourhood.