1. Soil Water
Chapter #5

2. How much Nitrogen is in the Air above one acre?
There are 69,155,856 pounds of nitrogen in the air above an acre.

3. Why is water important to plants?
it is a nutrient
serves as a solvent for other nutrients

4. What are the water requirements for plants?
plants use several hundred pounds of water for each pound of dry matter produced

5. How does water enter the soil?
through pores in the soil
sandy soils have the largest pores, but are often filled with other material
medium textured soils (loamy) have good water entry properties
clays, pores swell shut when they get wet

6. What is Adhesion?
it is the attraction of two different molecules (water to soil)

7. What is Cohesion?
it is the attraction of two similar molecules (water to water)

8. What is the importance of adhesion and cohesion in soils?
water molecules are strongly attracted to soil minerals and organic materials due to adhesion
water molecules are also strongly attracted to each other due to cohesion
soil holds a thin layer of water, that layer holds a thin layer of water etc., the farther away from the soil particle you get, the looser the water is held

9. What is the importance of adhesion and cohesion in soils?
-water then tends to move from wetter places to drier places. Roots remove water, then water is drawn from surrounding areas
in dry conditions, water moves slowly and only fractions of an inch, thus roots must be well distributed

10. How is soil water classified?
1) Hygroscopic Water is held so strongly by the soil particles (adhesion), that it is not available to the plants
2) Capillary Water is held by cohesive forces greater than gravity and is available to plants
3) Gravitational Water is that water which cannot be held against gravity
as water is pulled down through the soil, nutrients are"leached" out of the soil (nitrogen)

11. What is Field Capacity?
when the soil contains the maximum amount of available water, the greatest amount of water it can hold against gravity

12. What is Wilting Point?
the soil has so little water, that plants can no longer recover from wilting
roots can no longer take in water

13. What is Available Water?
the amount of water between field capacity and wilting point

14. What is Water Table?
the level beneath the soil which the soil is saturated with water
marshes develop where the water table is just below the ground surface
if the water table is not too low, dryness tends to correct itself through capillary movement

15. What affect does soil particle size have on its water holding ability?
the smaller the individual particles are, the more surface area there is, thus it will hold more water
consider a book compared to all of its pages

16. How is soil moisture measured?
soil is weighed dry, then wet (expressed as a percent of dry wt)
electrical conductivity

17. What factors determine the amount of soil moisture?
amount of snow, rain, irrigation
rate which soil permits water entry
water storage capacity of soil
evaporation rate
height of water table
rate which water can move within the soil
amount and type of vegetation

18. How can we increase the amount of avialable water to the plant?
Precipitation: better utilize snow and rain by keeping the water on the soil surface.
windbreaks
keep snow in place (cover)
strip cropping
contour strips
grassland farming
Irrigation: may not be practical
Water entry: reduce cultivation, green manures, correct alkaline soils

19. How can we increase the amount of avialable water to the plant?
Storage capacity: increase O.M. (manures)
-Evaporation: conservation tillage, shelter belts, strip cropping
-Water movement in soil: O.M.
-Vegetative covering: holds water

20. How much water do plants need?
Transpiration Ratio: pounds of water to produce a pound of dry matter.
Corn = 350# Wheat = 500#
Potato = 630# Grass & Alfalfa = 860#
It takes 4,533 gallons of water to produce enough food for one person for 3 meals. (Thats 1,654,545 gallons per year)
It takes 10,000 gallons of water to make one bushel of wheat
It takes 7.35" of rain to make a 20 bushel per acre crop.