Soils & Soil Physical Properties
Part 2 – 22 | Unit 2.1
Lecture 2: Soil Properties
q EXAMPLE 1 | CALCULATION OF TOTAL AVAILABLE WATER CAPACITY IN THE ROOT ZONE
ESTIMATING AVAILABLE WATER CAPACITY
Determine AWC for each layer soil texture.
Reduce AWC for each layer for gravel.
Reduce AWC for each layer for salts.
Calculate AWC for entire soil.
(In this example we assume no salts or coarse fragments)DEPTH TEXTURE LAYER THICKNESS AWC PER FOOT AVAILABLE MOISTURE
(FOOT) (INCHES/FOOT) (INCHES)
0 to 8 inches sandy loam 8/12 x 1.5 = 1.0
8 to 20 inches sandy clay loam 12/12 x 1.9 = 1.9
20 to 48 inches loamy sand 28/12 x 0.9 = 2.1
48 inches rock (rooting depth)
TOTAL AVAILABLE MOISTURE 5.0 inches
If you wanted to irrigate at 50% depletion, which is often the case, then in this case you would irrigate with 2.5
inches of water when the available water reached 2.5 inches (50% of 5 inches)iv. Estimating total available water
You can estimate approximate total available water by hand using the methods in
q Example 1, Calculation of Total Available Water Capacity in the Root Zone.
You can obtain a more precise estimate by using soil moisture sensors to monitor
the wetting and dry-down of the soil following irrigation events or precipitation.
This requires electronic sensors that are installed at several depths within the rooting
zone for a given crop at several locations in a field, the total number of which
would depend on the budget available. While these methods have in the past been
used primarily by researchers, some commercial growers are moving to this kind
of monitoring to better understand the water holding capacity and total available
water for their fields and crops, leading to more precise watering. This is of particular
importance for areas that experience regular or periodic droughts or water rationing.
As technology improves and prices of electronic monitoring come down, these
methods are becoming available to more growers; see Supplement 3, Soil Moisture
Sensing Instruments Commonly Used for Irrigation Schedules, in Unit 1.5.