Outline:

1. Soil Water Movement
2. Water Movement to Plants
3. Soil Aeration

 

1. Soil Water Movement
1. response to a gradient

Wet soil           to	Dry Soil
low soil moisture tension	to    high SMT
high soil water potential	to     low soil water potential

2. Saturated conditions
   
   water moving mainly in the macropores,
   all of the pores are filled.
3. Unsaturated conditions:
   
   macropores full of air
   micropores filled with water and air
   moisture tension gradient creates unsaturated flow

 

2. Water Movement to Plants and Plant Roots
   
   Plant roots get the water from the soil in two ways:
1. Capillary movement of soil water to plant roots. Plant root removes water. Tension in the soil right around the root increases gradient flow of water from low tension to high. This keeps a source of capillary water flowing to the plant root.
2. Growth of plant root into the moist soil. Capillary forces only affect the zone right around the root.
1. Rapid root growth to moist areas will help supply most of the moisture a growing plant needs
2. Commonly, roots only come into contact with 1% of the total surface area of the soil.

Both types of water uptake then are important for maintaining good plant growth.

Texture	% Water	Field Capacity
sand	6%	-1/3 bar
loam	15%	-1/3 bar
clay	24%	-1/3 bar

 

Note: bar - pressure exerted by a column of water 1023 cm in height. approx. = to 1 atmosphere >= 14.7 lb in²; or 760 mm of mercury.

3. Soil Properties That Affect Soil Water

 

1. Soil Consistence:
1. The amount and type of clay that is present affects water holding capacity.

 

2. Soil Structure:
1. Good structure makes the soil more permeable allowing gravitational water to drain more quickly. Better developed the structure the greater the permeability.
2. it may allow the plants roots deeper to use moisture from more of the soil profile.
3. this may be one of the only ways to increase effective available moisture.

 

3. Organic Matter:
1. enhances soil structure and aggregation
2. increases the soils moisture holding capacity. Organic matter is very light and swells when wet. It may hold more than 100 percent of its weight in water.

 

4. Bulk Density:
1. Density increases -- % Pore space decreases

 

5. How This Relates to Plants
   
   Plants take water out of the soil by exerting forces that are greater than the forces holding water in the micropores.
   
   As the soil dries, water is left in smaller and smaller pores.
   
   In the smallest micropores, only adhesion water exists and the attraction to the soil is so strong that the plant root can't extract the water.
   
   At this point, plants wilt and die if no water is supplied.
6. Saturated and unsaturated flow
   
   Saturated flow (also called gravitational flow) occurs only under saturated conditions when the force of gravity is greater than forces holding water in the soil.
   
   Capillary flow occurs in unsaturated soil (also called unsaturated flow). Unbroken films of water spread through connected capillary pores.
   
   A sand or gravel layer in a soil, which has a finer texture soil above it will stop unsaturated flow downward in the profile. Sand contains large pores which don't exert much force and can't pull capillary water out of the finer layer.
   
   Only when the finer texture layer becomes saturated will water move into the sand.
   
   For any soil layer, field capacity is increased when the layer is underlain by coarser textured material.
   
   The important point is that capillary forces will result in movement from large pores to small pores. Water will not move from small pores to large pores until the soil with the larger pores is saturated.

 

4. Soil Air-Water Relationships
1. Importance of air in soils
1. plant root respiration
2. microbial oxidation of organic residues

 

2. Soil Aeration
   
   Ability of soil to exchange gases with the atmosphere -- avoid depletion of O₂ and toxicity of CO₂
1. Affected by
1. Air Capacity
   % PS - % water = % air filled pores
   This will be a function of the bulk density and the water content.
2. Oxygen diffusion rate
   Pore arrangement (continuity) -- mainly based on the macropore arrangement
3. Drainage
   frequency and duration of saturation. Well drained soils are well aerated soils.
3. Measuring soil moisture
1. Gravimetric Method
   
   Take a moist sample, weigh, oven dry, weigh and determine % moisture
   
   Requires time, which you may not have

 

4. Tensiometers
1. measures soil moisture tensions
2. measures the suction that the soil is exerting on the water
3. only good for tensions between 0 and .8 bar.

 

5. Electrical Resistance Methods:
1. Two electrodes are imbedded in a block of gypsum. When placed in moist soil, the gypsum block becomes moist. A meter is used to measure electrical resistance between the electrodes. The greater the moisture the greater the flow of current. The meter is calibrated to estimate soil moisture.