Available Water Capacity

Soil holds water in its pores, and only some of what it holds is any use to a plant. Drain a saturated soil for a day or two and gravity takes the water out of the largest pores; what remains is field capacity. Let a crop draw on that reserve and it eventually reaches a point where the remaining water is held so tightly by the soil that roots cannot pull it out, and the plant wilts and does not recover overnight; that is the permanent wilting point. The water between the two is the only water the crop can ever spend, and its depth is what this formula computes.

Both contents are volumetric fractions — cubic metres of water per cubic metre of soil — which is what makes the multiplication work: a fraction times a depth of soil gives a depth of water. A silt loam at 30% field capacity and 12% wilting point has an available fraction of 0.18, so a crop rooted to 90 cm can reach $0.18 \times 0.90 = 0.162$ m, which is 162 mm of usable water. Expressed the way a soil survey prints it, that is 1.8 mm of water per centimetre of soil. The same soil at 36 inches of rooting gives $0.18 \times 36 = 6.48$ inches.

Texture drives the available fraction and it drives it in a way that surprises people. Sands hold little because their pores are large and drain freely — 5 to 10% by volume. Clays hold an enormous amount of water but grip much of it below the wilting point, so their available fraction is middling. The winners are the medium textures, silt loams above all, at 18 to 22%, because their pore sizes sit in the range that holds water loosely enough to be released. Organic matter raises the figure in every texture, which is one of the quieter arguments for building it.

The classic mistake here is mixing gravimetric and volumetric water contents. A laboratory that reports water content by weight is giving a number that must be multiplied by the bulk density before it means anything in this equation — for a soil at 1.3 t/m³ the volumetric figure is 30% larger than the gravimetric one, and using the wrong one understates the reserve badly. The second mistake is using the depth of the soil profile instead of the depth the roots actually occupy: a crop with 40 cm of roots cannot spend water sitting at 90 cm, however much of it is there, and early in the season the effective rooting depth is a fraction of its final value.