Soil Moisture Behavior

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Soil Moisture Behavior
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Why is water important to plants?

• it is a nutrient
• serves as a solvent for other nutrients

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What are the water requirements for plants?

• plants use several hundred pounds of water for
  each pound of dry matter produced

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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

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What is Adhesion?

• it is the attraction of two different molecules
  (water to soil)
• Water held by adhesion not available to the
  plant

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What is Cohesion?

• it is the attraction of two similar molecules
  (water to water)
• Water held by cohesion IS available to plant

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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

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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

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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)

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What is Field Capacity?

• when the soil contains the maximum amount of
  available water, the greatest amount of water it
  can hold against gravity

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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

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What is Available Water?

• the amount of water between field capacity and
  wilting point

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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

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What is saturation?

• When all pore spaces are filled with water
• Unhealthy state for plants when prolonged because
  the oxygen dissolved in the water that is needed
  for respiration is quickly utilized

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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

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How is soil moisture measured?

• soil is weighed dry, then wet (expressed as a
  percent of dry wt)
• electrical conductivity

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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

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How can we increase the amount of available 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

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How can we increase the amount of available 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

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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. (That's
  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.