CIVE1145 Irrigation (Exam)
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Define field capacity (FC), wilting point (WP) and available water for plants.
- FC = when the soil pores are so full of water that the next drop will leach downward out of the rooting zone (0.1 to 0.3 bars of suction)
- WP = the level at which plant roots can no longer take in water (15 bars of suction)
- Available water = FC - WP
What is meant by soil texture? Draw a suitable soil texture classification chart.
- the relative proportion of sand, silt and clay
- indicates the relative proportion of sand, silt and clay
- see USDA soil textural classification chart (12 textural classes)
What is soil structure, what are the properties effected due to soil structure?
- indicates the arrangement of individual particles with respect to each other into a pattern
- Soil structures may be grouped as
- • Cube-like
- • Columnar
- • Platy
- • Angular and/or sub-angular
- • Granular, etc
Why is loam most suitable for plant growth?
because they hold more available water and cations than sand and are better aerated and easier to work than clay
What are the variables we limit to classify our irrigation water quality? How can we improve soil drainability?
- Na with carbonate – alkali soils
- Na with chloride (salt) or sulphate – saline soils
- Boron – necessary in small quantities, toxic in large quantities
What is meant by soil-water potential? What are the components in soil-water potential? Why does finer particle soil have higher soil-water potential?
- it is the energy status of the soil water
- gravitational potential (force of gravity pulling on the water)
- matric potential (force on water by soil matrix. Also called soil-water tension
- osmotic potential (difference in salt concentration across plant root)
- Finer particle soils:
- have a larger number of small pores
What is osmotic pressure? Why we can not apply saline water for irrigation purpose?
- When solutions of different concentrations are separated by a semi-permeable membrane (e.g. plant root)
- salt decreases the total available water in the soil profile. As salt increases in soil-water, the energy expended by a plant to extract water must also increase, even though the soil moisture tension remains the same
What equipment we use for measuring soil suction? How it works? What is the usual unit of soil suction measurement?
Tensiometer for measuring soil-water potential?
Explain different soil-water measurement techniques, their advantages and disadvantages.
- Gravimetric: Measures mass water content (θm). Advantages: accurate; multiple locations, Disadvantages: labor; time delay
- Feel and appearance: Advantages: low cost; multiple locations, Disadvantages: experience required; not highly accurate
- Neutron scattering (attenuation): Measures volumetric water content. Advantages: samples a relatively large soil sphere, repeatedly sample same site and several depths, accurate
- Disadvantages: high cost instrument, radioactive licensing and safety, not reliable for shallow measurements near the soil surface
- Dielectric constant: A soils dielectric constant is dependent on soil moisture.
- Primarily used for research purposes at this time
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