Irrigation
Water Management
Soil-Water-Plant
Relationships
3-Soil
Texture
Soil Moisture
Water adheres to soil particles
and forms a film of water surrounding the soil particles. The
strength of the attraction between the soil particle and the water
film is based on surface tension, and is related to the thickness
of the water film.
- THIN film --> water is STRONGLY
held
- THICK film --> water is WEAKLY
held
Soil
Texture
| Soil
Texture |
Minimum
Particle Size
(mm) |
Maximum
Particle Size
(mm) |
| Sand |
0.5 mm |
2.0 mm |
| Silt |
0.002
mm |
0.05 |
| Clay |
|
0.002
mm |
|
Soil
Particle Size & Surface Area
Consider a large particle subdivided
into two smaller particles:
Note the surfaces marked in
green.
This is additional surface area resulting from the
split. Thus, smaller particles have more surface area per unit
mass of soil than large particles.
|
|
Texture, Moisture and Tension
Consider now two soils, one made
up of larger soil particles (a sand), and the other made up of
smaller soil particles (a clay or silt).
- Suppose they both contain the
same amount of water per unit of soil.
- The water in the fine textured
soil, with the smaller soil particles, and more surface area,
must be spread out over that larger surface area, and
therefore must exist in thinner films.
- The same amount of water in the
coarse textured soil, with larger soil particles, and
less surface area, can be spread out over that smaller
surface area, and can therefore exist in thicker films.
- Therefore, the same amount of
water is held more tightly in fine textured soils
and less tightly in sandy soils.
- Suppose the water they contain
is held at he same tension in each soil. The water film thickness
in both soils must be the same.
- The fine textured soil
will hold the greater water volume, because the given
film thickness covers a much larger surface area.
- The coarse textured soils
will hold a lesser water volume, because the same film
thickness covers a much smaller surface area.
- Therefore, when water is held
at the same tension (same film thickness), there will be more
water per unit of soil in the fine textured soil,
and less in the coarse textured soil.
Practical Consequences
| Example:
The amount of water held by soils depends on the texture
of those soils. |
Clay Soil
Fine Texture
Small Particles |
Sandy Loam Soil
Coarser Texture
Larger Particles |
| % water by volume |
inches of water per foot of soil |
% water by volume |
inches of water per foot of soil |
| Condition
at Field Capacity (FC) 10 cb tension |
47% |
5.64 in/ft |
13% |
1.56 in/ft |
Condition
at Permanent Wilting Point (PWP)
1,500 cb tension |
27% |
3.24 in/ft |
6% |
0.72 in/ft |
| Available
Water Holding Capacity (AWHC) |
20% |
2.40 in/ft |
7% |
0.84 in/ft |
|