Page 107 - Calculating Agriculture Cover 20191124 STUDENT - A

CH 9] Calculating Agriculture 9-23

Field Capacity. In soils it is recognized that soil already contains water. Soils have a Field
Capacity for water, a saturation level, which affects the amount of water to apply. To calculate the
amount of moisture to apply it is necessary to know the Field Capacity of the Soil, its current
moisture content, the volume weight of the soil in the field and the depth to irrigate.

Example: In a soil having a volume weight of 1.3 and a field capacity of 25%, how many
inches of irrigation water must be applied to soak the soil to a depth of 2 ft. when it
now has a moisture content of 15%?

Solution:
Field Capacity (FC inches) = Volume Weight (V) x Δ Moisture % (Δ P) x Depth to soak (D inches)
Δ : delta Change in moisture. Connotes subtraction
FC(inches) = V x Δ P x D(inches)
FC = (1.3) (25% - 15%) (2ft.)
FC = (1.3) (25% - 15%) (24") Perform the subtraction
FC = (1.3 ) (10%) (24") Convert % to decimal: divide by 100%
FC = (1.3) (0.10) (24")
FC = 3.12 inches of water

USABLE SOIL MOISTURE and UNAVAILABLE at Soil Moisture Capacity
Soils have differing maximum moisture holding capacities. It is also true that plants are unable to
use all of the moisture in the soil. The percent of water in a soil that is not available for plant growth
is called the Permanent Wilting Percentage (PWP). The PWP percent is based on the dry weight of
the soil. If the volume weight of soil is 1.2 and the PWP is 10%, this means that the plant is unable to
use moisture equal to 10% of the weight of the soil. Presume a cubic foot of soil weighs (1.2 x 62.5 lb.
=) 75 lb. If the soil contains 22% moisture, then it contains ( 0.22 x 75 lb =) 16.5 lb. of moisture. If
this soil’s PWP is 10%, then (10% of 74.9 lb =) 7.5 lb. of this moisture is not usable by the plants,
based on the soils weight, and [(22% - 10%) x 75 lb. =] 9.0 lb. is usable moisture.
This can also be stated arithmetically as:
% based
Pounds on Soil Wt. Calculations
Total Weight (Soil + Moisture) 75.0 # 100 % (100% — 22% Moisture = 78%)
— Soil Weight — 58.4 # — 78 % (78% x 75 # = 58.4#)
Total Weight Moisture 16.6 # 22 % (16.5 # ÷ 75 # x 100% = 22%)

— Moisture unavailable — 7.5 # — 10 % (7.5 # ÷ 75 # x 100% = 10%)
Moisture available 9.1 # 12 % (9.0 # ÷ 75 # x 100% = 12%)

Figure 9.13 PWP SOIL MOISTURE

22 % Water weight changes PWP = 75 # x 10% = 7.5 #
through evaporation and
16.5 # plant uptake. Not all
Moisture moisture is available for 10% PWP = 7.5#
plants (PWP).

75 lbs 78 % Soil weight does not change. Soil 90 %
58.5 # weight remains 58.5 pounds 58.50 # 66 lbs
Soil Weight while drying. Soil Weight

58.5 # = 58.5 # 9

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