UNIT C1.
Basic Principles of Agricultural/Horticultural
Science
Understanding Moisture Holding Capacity
Lesson 6
Next Generation Science/Common Core Standards Addressed!
CCSS.ELA-Literacy.RST.9-10.4 - Determine the meaning of symbols, key terms, and other domain-specific words and phrases as they are used in a specific scientific or technical context relevant to grades 9–10 texts and topics.
CCSS.ELA-Literacy.RST.9-10.5 - Analyze the structure of the relationships among concepts in a text, including relationships among key terms (e.g., force, friction, reaction force, energy).
CCSS.ELA-Literacy.RST.9-10.6 - Analyze the author’s purpose in providing an explanation, describing a procedure, or discussing an experiment in a text, defining the question the author seeks to address.
CCSS.Math.Content.HSN-Q.A.1- Use units as a way to understand problems and to guide the solution of multi-step problems; choose and interpret units consistently in formulas; choose and interpret the scale and the origin in graphs and data displays.
Bell Work
What is soil moisture holding capacity?
Why is it important for a soil to hold water?
Can a soil hold water to tightly?
Interest Approach
Place a jar of pure sand and a jar of good black soil before the students. Ask them which of the samples would produce the best crops. There may be a variety of answers. Lead discussion in the direction of how much water would be available to plants growing in each of the samples. Which sample will provide the most moisture to the plants? Why?
Learning Objectives
Explain moisture holding capacity.
Explain what determines a soils moisture holding capacity.
Determine the moisture holding capacity of a given soil profile.
Important Terms
Available soil moistureAvailable water holding capacityCapillary moistureGravitational moisture
Hygroscopic moisture
Moisture holding capacity
Soil moisture tension
What is moisture holding capacity?
Moisture holding capacity is the ability of the soil within the soil profile to retain water.
Moisture is retained in three ways:
Gravitational moisture is the water that moves downward through the soil.
It may help replenish groundwater supplies.
It is also available to plants.
Moisture is retained in three ways:
Capillary moisture is the water that is held within the pore spaces between soil particles.
It is also available to plants
Moisture is retained in three ways:
Hygroscopic moisture is the soil water that tightly clings to the soil particles.
This moisture is usually not available to plants.
What is available to the plants?
Available soil moisture is the water in the soil that can be used by plants.
When moisture levels are high, plants can easily extract moisture from the soil. As the water is used, soil moisture tension increases.
Soil moisture tension is the force by which soil particles hold on to moisture.
What is used to determine how much moisture a soil can hold?
Moisture holding capacity is determined primarily by the soils texture.
As a rule, the finer the texture of the soil, the more moisture it will hold.
A soil high in sand will hold less water.
Soils high in clay, hold water and keep it from percolating out of the root zone.
What is used to determine how much moisture a soil can hold?
If the soil is entirely clay, it will hold the water too tightly.
This means less water is available to plants than if silt were present.
A good silt loam holds the most moisture available for plants
How do you determine the amount of moisture a soil profile can hold?
The amount of moisture the soil can hold for plants is referred to as available water holding capacity.
Available water holding capacity depends on:
1. How deep the soil profile is.
2. The type of soil texture found throughout the soil profile.
Available water holding capacity depends on:
On average, the following textures will hold the designated amount of moisture per inch of soil:
fine textured .20 inchesmoderately fine textured .25 inches medium textured .30 inches moderately coarse textured .20 inches coarse textured .10 inches
How do you know figure the water holding capacity
To determine the available water holding capacity for a given area, multiply the depth of each horizon, to a maximum depth of 60 inches, by the amount of water the texture within that horizon can hold.
Add the totals for each horizon to calculate total water holding capacity.
Example
A horizon: 9 inches deep, medium texture = 9 × .30 =2.70 inchesB horizon: 23 inches deep, moderately fine texture = 23 × .25 = 5.75 inchesC horizon: 28 inches deep, medium texture = 28 x .30 = 8.40 inchesTotal = 16.85 inches of water
Summary
Explain moisture holding capacity.
Explain what determines a soils moisture holding capacity.
Determine the moisture holding capacity of a given soil profile.
Additional Resources
VAS U4052a, Understanding Soils. Urbana, Illinois: Vocational Agriculture Service.
