Estimating available soil moisture

Once you know the textural class for the soil, you can use the table below to estimate available soil moisture. Note that the amount of available moisture (mm) at field capacity is expressed per 1 meter of soil. If the rooting depth is less than 1 m, the amount of available moisture is reduced proportionally.

The relationship between moisture and volume/depth are:

Moisture (by weight) x bulk density = Moisture (by volume)

If Moisture (by weight) = 20%, and
Bulk density = 1.2 g/ cm3,
Moisture (by volume) = 1.2 x 20 = 24%

Moisture (by volume) x soil depth = Moisture (by depth)

If Moisture (by volume) = 24%, and
Soil depth = 50 cm, then
Moisture (by depth) = 24 x 50 = 12 cm = 120 mm

Table. Guide to soil moisture content.

Coarse (sand, loamy sand) Moderately coarse (sandy or silt loam) Medium (loam, clay loam, silty clay loam, silt, sandy clay) Fine (clay, silty clay or light clay)
At field capacity, contains: 60-100 100-150 150-200 200-250
(mm available moisture per meter of soil)
Soil moisture content
Above field capacity Water appears when soil is bounced in hand Water released when soil is kneaded Can sqeeze out water Puddles and water form on surface
Field capacity Upon squeezing, no free water appears on soil but wet outline of ball is left on hand
75-100% available moisture Tends to stick together slightly. Sometimes forms a weak ball under pressure. Forms weak ball, breaks easily, will not slick. Forms a ball and is very pliable, slicks readily if relatively high in clay. Easily forms a ribbon between fingers, has a slicky feeling.
50-75% available moisture Appears to be dry, will not form a ball under pressure. Tends to ball under pressure, but seldom holds together. Forms a ball, somewhat plastic, sometimes slicks slightly with pressure. Forms a ball, ribbons out between thumb and forefinger.
25-50% available moisture Appears to be dry, will not form a ball under pressure. Appears to be dry, will not form a ball under pressure. Somewhat crumbly, but forms a ball. Somewhat pliable. Will form a ball under pressure.
0-15% available moisture Dry, loose single-grained. Flows through fingers. Dry, loose. Flows through fingers. Powdery, dry, sometimes slightly crusted, but easily broken down into powder. Looks moist but will not quite form a ball.

Source: Irrigation Practice and Water Management 1 Rev. 1, FAO, 1984.