Ch 2. of Water Movement in Soils, Capillary Barriers and Water Movement

Summary

This video script delves into the dynamics of water movement in soil, emphasizing that it's not just gravity but also the soil's solid surfaces that influence water flow. It illustrates how coarse sand layers act as a check valve, retaining water until the overlying soil is saturated, then allowing excess to pass. This principle is pivotal in understanding water retention in soils with mixed textures, such as those in Washington State's Columbia Basin, where the presence of coarse sand and gravel enables the soil to hold more water, making it highly conducive to plant growth.

Takeaways

• 💧 Water movement from an irrigation source is not solely due to gravity but also the attraction to solid surfaces like soil.
• 🌊 As soil becomes wetter, gravity's role in water movement increases, and when saturated, gravitational forces predominate.
• 🏖️ A horizontal layer of coarse sand acts as a barrier to water movement due to the size difference in soil and sand pores.
• 💧 The small pores in soil act like blotting paper, holding water through adhesive and cohesive forces.
• 🚫 Coarse sand's large pores cannot hold water at the same tensions as the smaller pores in the wetter soil above it.
• 💧 Water eventually moves into the sand layer when the soil above becomes very wet, similar to how a blotter would release excess ink.
• 🚰 The sand layer acts like a check valve, holding water back until the soil is very wet before allowing excess to pass.
• 🌱 The presence of coarse sand and gravel in soil can significantly affect the soil's ability to support plant growth.
• 📈 Soils with a layer of coarse sand and gravel can retain more water than those without, doubling the water retention capacity.
• 📍 In Washington State's Columbia Basin, the presence of coarse sand and gravel in the soil composition is beneficial for agriculture.

Q & A

What is the primary force driving water movement from an irrigation pearl?

The primary force driving water movement from an irrigation pearl is the attraction of solid surfaces, rather than gravitation.

How does the role of gravitation change as the soil becomes wetter?

As the soil becomes wetter, gravitation plays a stronger role in water movement, eventually predominating when the soil is completely saturated.

Why is the horizontal layer of coarse sand significant in the water movement process?

The horizontal layer of coarse sand is significant because it acts like a check valve, holding water back until the overlying soil becomes very wet, then allowing excess water to pass through.

What is the comparison between the size of soil pores and sand grains in terms of water retention?

The pores in the soil are many times smaller than the spaces between sand grains, which is why water is held in the soil by adhesive and cohesive forces rather than readily moving into the sand.

How does the presence of coarse sand and gravel affect the water retention capacity of the soil?

The presence of coarse sand and gravel allows the overlying soil to retain more than double the amount of water usually held in a fine sandy loam.

What is the principle behind the water movement in soils with a sand layer?

The principle behind water movement in soils with a sand layer is that the sand acts as a barrier, preventing water from moving into it until the soil above reaches a certain level of wetness.

What is the agricultural significance of soil composition in Washington State's Columbia Basin?

The soil composition in Washington State's Columbia Basin, which includes a layer of coarse sand and gravel beneath a fine sandy loam, is one of the best for supporting plant growth due to its enhanced water retention capacity.

How does the soil's ability to support plant growth relate to the presence of coarse materials like sand and gravel?

The presence of coarse materials like sand and gravel in the soil enhances its ability to support plant growth by increasing the soil's water retention capacity, which is crucial for plant health.

Why is the soil in Washington State's Columbia Basin considered one of the best for agriculture?

The soil in Washington State's Columbia Basin is considered one of the best for agriculture because of the presence of coarse sand and gravel layers that allow the overlying soil to retain more water, thus supporting better plant growth.

What is the role of adhesive and cohesive forces in water movement within the soil?

Adhesive and cohesive forces hold water within the small pores of the soil, preventing it from moving into larger pores in the sand until the soil is sufficiently wet.

How does the script describe the behavior of water in soil compared to a blotting paper?

The script compares the pores in the soil to the pores in a blotting paper, explaining that water is held in the small soil pores by adhesive and cohesive forces, similar to how a blotting paper soaks up ink.