Page 108 - Calculating Agriculture Cover 20191124 STUDENT - A

9-24 Land, Soils and Fertilizers CH 9]

Note that 10% PWP does say that 10% of the moisture is not usable by the plants. To determine what
percent of the moisture is not usable, you must compare the unusable moisture with the total moisture
content; and this can be calculated with the pounds of moisture or the percentage of moisture.
Presume that the moisture percent of soil is 22% and using the 10% PWP the percent unusable
would be stated thusly:

Unusable % moisture
——————— x 100% = % unusable
Total % moisture

10 %
——— x 100% = % unusable
22%

0.455 x 100% = % unusable

45.5 % = % unusable or unavailable (PWP%)

The percentage of unavailable moisture (PWP%) can also be calculated using the
pounds of moisture and the solution is:

Lbs unusable moisture
—————–——–— x 100% = % unusable
Total pounds moisture

7.5 #
—— x 100% = % unusable
16.5 #

0.455 x 100% = % unusable

45.5 % = % unusable or unavailable (PWP%)

Again, 45.5% of moisture present is not usable.

Example: The percent of water in a soil that is not available for plant growth is called the
Permanent Wilting Percentage (PWP), which is based on the dry weight of the soil. In
a soil with a 20% moisture content and a PWP of 5%, what percent of the water in the
soil is not available for plant growth?

Solution:
PWP%
—————— x 100% = % unavailable
Water content %

5%
——— x 100% =
20%

0.25 x 100% =

25% = % unavailable (of the water present)

Available Soil Moisture. When soils contain moisture, part of the moisture is available and part
of the moisture is unavailable. The percentage of moisture available is a factor of the total moisture
and the PWP%.

Example A: What is the percent of available water if the moisture equivalent percentage of oven-
dry soil (fine sand) is 7.6% and the wilting point percentage of oven-dry weight of
soil is 3.7 percent?

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