Transcript Soil Water Chapter #5 How much Nitrogen is in the Air above one acre? There are 69,155,856 pounds of nitrogen in the air above an.

Soil Water
Chapter #5
How much Nitrogen is in the Air
above one acre?
There are 69,155,856 pounds
of nitrogen in the air above
an acre.
Why is water important to plants?
• it is a nutrient
• serves as a solvent for other nutrients
What are the water requirements
for plants?
• plants use several hundred pounds of water
for each pound of dry matter produced
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
What is Adhesion?
• it is the attraction of two different molecules
(water to soil)
What is Cohesion?
• it is the attraction of two similar molecules
(water to water)
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
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
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)
What is Field Capacity?
• when the soil contains the maximum
amount of available water, the greatest
amount of water it can hold against gravity
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
What is Available Water?
• the amount of water between field capacity
and wilting point
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
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
How is soil moisture measured?
• soil is weighed dry, then wet (expressed as a
percent of dry wt)
• electrical conductivity
What factors determine the
amount of soil moisture?
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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
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.
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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
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
How much water do plants need?
• Transpiration Ratio: pounds of water to
produce a pound of dry matter.
• Corn = 350#
• Potato = 630#
Wheat = 500#
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.