Soil Water: Characteristics and Behavior

Chapter 5 – NR 200

Water Uses  How the plant uses water.  60-90% of plant mass is water  When a plant has a full complement of water it is said to be turgid, loss of turgidity results in wilting.  Is essential for cell functions  Photosynthesis  Transpiration helps cool the leaf  Plant nutrients are in solution  Carries carbohydrates in phloem

 Improper amount of water  Stress – too little  Wilting  Permanent wilting plant  Water stress weakness plant  Too much water  No air space  Anaerobic conditions exist when no oxygen available Water Uses

Water Molecule structure  Structure  Two hydrogen atoms one oxygen atom  this attachment is held at 105   this hydrogen side is positively charged

 Creating a polar molecule  Causing it to be attracted strongly to itself, cohesion  Also attraction towards other material, adhesion.  The force that attracts water to other objects  In small spaces this force can move water  In large spaces the affect is minimal  Responsible for the surface tension of water Water Molecule Structure

 Capillary water  The ability of water to move upward against gravity or outward.  This ability is directly related to adhesion and cohesion  The meniscus is pulled up a small tube by adhesion properties of the tube and the cohesion pulls the other water molecules with it. The height of travel is related to the size of the tube. h =.15/r with the radius in cm Water Molecule Structure

Soil Water Energy forms: The energy that acts on soil water  Matric force is the soil solid’s attraction (adhesion) to water which causes adsorption and movement of water through the soil (capillarity) not counting the force of gravity.  Osmotic force is the movement of a high concentration of ions to a lower concentration.  Gravity

Water movement and retention  Movement  Texture and the wetting pattern  Clay soils produce a more round ball pattern  Sandy soils produce a more oblong pattern; water percolates more quickly into the soil

 Types of water movement  Gravitational Water or Saturated flow  Water that moves through the root zone and below  Water movement varies as to the soil texture  Water movement in the soil changes when water moves from one different/unmixed texture soil to another Water movement and retention

 Types of water movement (cont’d)  Capillary movement  Small pores water moving laterally  Conditional on texture and structure  Large pores  Little water retention  Sand  Small pores  Great water retention  Clay Water movement and retention

 Types of water movement (cont’d)  Capillary movement (Cont’d)  Medium pores  Fine sand and silt hold the most available water  Infiltration - the process in which water moves into the Soil  Percolation is the downward movement of water through  the soil  soil texture  Compaction  Soil stratifications affect the soil how?

Water soil terms of wetness  Maximum retentive capacity  Water has filled all pore spaces  Can only be maintained by more water or no percolation  Field Capacity  After gravitational water has moved out and the soil is holding the water  Sometimes called capillary water

Water soil terms of wetness  Permanent Wilting Point (PWP) or wilting coefficient.  That point of no return for a plant wilting  Water in the soil is no longer available to the plant  Available water is the difference between Field capacity and PWP  A fine sand or silt loam will have the highest available water  Hygroscopic water - water held so tightly it is unavailable to the plant

Water removal by the plant  Removal of water from the surface area first then the lower areas  Water around the soil particle is removed by the root hair and used by the plant.  The root must be in contact with the soil to remove its water.

Measuring soil water  Gravimetric Measurements  Weight difference between soil and oven dried soil = percentage of weight of the water.  Tensiometers  Measures the moisture pull of the soil in a tube or the measures the water potential of the soil

Measuring soil water  Electrical Resistance block  Measures the conductivity through a block of gypsum or other buffering material. More water the less resistance. This device can easily be hooked to a watering device to automatically water a given field.

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)

Saturation Percentage  Immediately following an irrigation, the film of water is thick, and smaller pores are full of water. The soil particles can not hold the water. It is easily lost to the plant.  1/10 Atmosphere (Near Saturation)

What is Field Capacity?  when the soil contains the maximum amount of available water, the greatest amount of water it can hold against gravity.  1/3 Atmosphere  ½ saturation %

What is Permanent Wilting Point?  the soil has so little water, that plants can no longer recover from wilting.  roots can no longer take in water.  15 Atmospheres.  ¼ Saturation %

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

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

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

When to irrigate  1. Tensiometer – measures in atmosphere  2. Electrical Resistance Blocks (gypsum blocks) electrical resistance  3. Neutron probe – neutrons collide with hydrogen atoms soil (water in soil)  4. Thermal dissipation sensor – measures dissipation of heat  5. Time domain reflector – high speed microwave pulse

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

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