Soil water Flashcards
How can you increase the water storage capacity of a soil with low SA?
Adding OM to stabilize the aggregates and create more available pore space
Is water storage capacity high, medium, or low for clays, silts, and sands?
Clays = high silts = medium - high sands = low
What are the 2 forces involved in the movement of soils?
- gravity
2. capillary action (cohesion and adhesion)
How does water move through soil?/simple rule of physics
Water moves from areas of high concentration to areas of low concentration
Rate sandy loams, loams, and clay loams from best to worst for irrigation agriculture?
why?
Sandy loam
loam
Clay loam
sandy loams will drain the quickest and will not become over-saturated or build up a water puddle on the surface
Rate sandy loams, loams, and clay loams from best to worst for dry land agriculture (no irrigation)?
why?
clay loams
loams
sandy loams
clay loams have the best capacity to store water due to their small particle sizes and their multiple negatively charged surfaces
For what reason are clay loams difficult to irrigate?
they have low infiltration rates so water will move very slowly through their small pores
Briefly describe the issues of irrigating clay soils?
- not enough water movement through the soil could put water stress on plants
- water can puddle at the surface and run-off due to lack of infiltration
- the adhesion of water to clay particles is very strong and may be difficult for plants to obtain
Define adhesion vs. cohesion?
Adhesion is the bond between water and soil particles
Cohesion is the bond between water molecules and other water molecules
Describe the flow of water in a sandy soil with a clay layer in the middle
Water will infiltrate and percolate downward with gravity very quickly as sand particles are very large and have a small SA so water cannot easily adhere to the particles.
When water reaches the clay layer, it will immediately slow down, but because the pores of clay are very tiny, they can use capillary action to bring in water from the saturated soil above
What is capillary action and how does it work?
It is adhesion and cohesion working together to move water against gravity in the soil
water flows from high to low, so when there are small soil particles (ie. clay), water will bind to itself (cohesion) and then to the particles (adhesion) to move to an area of low concentration
T or F: sandy/coarse soils will have less capillary action?
TRUE because they have larger pore sizes, so the main force of water movement would be gravity
What is soil water potential?
the measurement of the forces of water
it’s on a scale that moves negatively from 0
What is pure water’s water potential?
zero
T or F: the higher the water potential, the more free it is?
TRUE
What is an example of a low water potential?
-1500 kpa
What are the 3 forces of water potential?
- matric - (MAIN FORCE)
- osmotic -
- gravitational +
What is matric potential?
the attraction of soil water for soil particles
T or F: Sand and silts can have the same water potential, but different levels of water content
TRUE
they can both be at -10kpa water potential, but finer textured soils have more pore space filled with water than a coarse textured soil, a coarse soil would have more air pore space.
fine textured soils have much greater water content than coarse
Which soil is drier? one at -30kpa or -10kpa?
-30kpa
Define unsaturated flow of water?
water movement in soils that are not saturated - capillary action required
define saturated flow of water?
water that will move through saturated soils by gravity
What is field capacity?
Matric potential and gravitational potential are balanced = the soils capacity to hold water against gravity
ideal for plant growth
What is the kpa of field capacity?
around -33kpa
What is wilting point?
When there is no more available water for plants and they wilt
What is permanent wilting point?
When most cohesion water is gone and plants cannot get anymore water, they will die and this cannot be undone
What is the kpa of PWP for most plants?
-1500kpa
What is hygroscopic water?
Air dried soil, when most cohesion water has evaporated
What is available water?
The water between the field capacity and wilting point
it can be absorbed by plant roots and it is gravitationally drained
T or F: in saturated soils, capillary action dominates?
FALSE gravitational action will dominate in saturated soils
