Page 109 - Calculating Agriculture Cover 20191124 STUDENT - A

CH 9] Calculating Agriculture 9-25

Solution:
% available water = % moisture equivalent − % at wilting point
= 7.6% − 3.7%
= 3.9%

To calculate the pounds of water available as in the previous example, the dry-weight of the soil
must be known.

Example B: Presume a cubic foot of oven-dried soil weighs 80 pounds, with a percentage of
available water at 3.9% (calculated in Example A above), what is the weight of the
water?
Solution: % available water x weight of cu. ft. of oven-dry soil = lb. water/cu. ft. of soil

3.9% x 80 =
0.039 x 80 = 3.12 lbs. water/cu. ft. available

Calculating inches of water. The relationship of the pounds of water per cubic foot (62.3# per
cubic foot) to a 12 inch depth of soil, the weight of water per 1 inch over a square foot can be
determined.

Example C: Using the data from the above Example B, 3.12 pounds per cuft, and that water per
cubic foot weight 62.3 pounds, how many inches of available water in the soil?

Solution:
a) #’s / cuft of water ÷ 12 in. = #’s of water 1 in. deep over a sq. ft. of soil.

62.3 lbs./cu. ft. of water ÷ 12 in. = 5.19 lbs. water 1 in. deep over a sq. ft.

b) lb. water per cu. ft. ÷ 5.19 lbs = inches of equivalent surface water

3.12#
—— = 0.6 inches
5.19#

Not all soils are uniform in their structure, and in a profile the differing soil textures are
discernible. The differing soil textures have different moisture holding capacities as indicated in the
following:

Figure 9.14
Approximate number of inches of available water per cubic foot

Available moisture
Soil Texture per cubic foot
Fine sands, loamy fine sands 0.50-1.5 in.
Loams, silt loams 1.25-2.0 in.
Sandy clay loams, clay loams 1.75-2.5 in.
Silty clay loams, sandy clays,
silty clays, clays 2.25-3.0 in.

Knowing the available moisture holding capacity of a soil texture allows for the calculation of
total inches of available water in a given soil profile based on the depth of the profile. From this
information it becomes possible to extrapolate what amount of water remaining when the evaporation
and plant usage per day is known; and plan the portion of water needing to be stored for a crop or
when to irrigate. 9

Example A: Referring to Figure 9.14, how many total inches of available water are there in a 5-
foot profile of soil, if the top foot is silt loam and the remainder is silty clay loam?
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