Agricultural Science (AS)·Soil · NSSCAS 2.3
Soil water: movement, pores & availability
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You water a sandy garden near Rundu and a clay garden in a river valley on the very same evening. By the next afternoon the sandy one is bone dry, while the clay one stays cool and damp for days. Same water, same sun — so why the difference? In this lesson we uncover how soil grips and even lifts water, why pore size decides everything, which water a plant can actually drink, and the precious band of available water between field capacity and the wilting point.
What you'll learn in this lesson
By the end you should be able to (NSSCAS Agricultural Science (AS) 2.3):
- Explain cohesion, adhesion and surface tension, and how they cause the capillary movement of water in soil
- Explain how soil pore size (macropores, mesopores and micropores) relates to soil water and drainage
- Explain why plant growth needs a balance of both water and air in the soil pores
- Compare gravitational water, capillary water and hygroscopic water, stating which is available to plants, which drains away and which is held too tightly
- Explain how sandy and clay soils differ in the way they hold and release water
- Define field capacity and the permanent wilting point, and explain the available water held between them
- Calculate the available water of a soil from its field capacity and permanent wilting point
- Suggest how a Namibian farmer can improve the water-holding of a sandy soil
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