Soil moisture
Soil & evaporation soil water content
Soil moisture describes water held in the ground. Soil type, depth and the stated unit matter as much as the number itself.
What 0.20 m³/m³ means
A common measure is water volume divided by soil volume. 0.20 m³/m³ equals 20 per cent by volume: one litre of this soil contains about 0.2 litres of water. The rest consists of solid material and air-filled pores. It does not mean that the soil has filled twenty per cent of its storage capacity.
Plants cannot reach all soil water
- Field capacity: water content remaining after excess water has drained away.
- Permanent wilting point: a state where plants can no longer extract enough water.
- Available water capacity: the water held between those two states.
Sand and clay make the same number mean different things
Soil types differ in the size and distribution of their pores. Clay can contain plenty of water while binding some of it tightly. Sand drains faster. Equal volume percentages therefore do not imply equally good water supply.
Depth makes a difference
A brief shower wets the surface first; deeper roots may still be in dry ground. Conversely, the surface may look dry while water remains below. A model value for a layer is guidance, not an inspection of every garden plant.