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SOIL MOISTURE
FORMS OF SOIL MOISTURE (1 bar = 0.987 atm)
1) chemically combined - occurs as a water shell around compounds and particles in soil;
                                         plants cannot utilize.

2) hygroscopic water - water adsorbed onto soil particles, held at less than -31 bars of tension;
                                     plants cannot utilize.

3) capillary water - water held by capillary attraction in the capillary pores in soils; held at  -1/3 
                               to -31 bars; plants can extract water in the larger capillary pores down to
                               approximately -15 bars.

4) gravitational water - water in large pores immediately after watering or a rain, which drains
                                     from the soil (within 24 hr.) by the force of gravity; held at greater 
                                     than -1/3 bars (0 to -1/3 bars); plants can utilize when present. 
 
 

SOIL WATER TERMINOLOGY
field capacity - the amount of water a soil can hold against the force of gravity; 
                      - water is held at -1/3 bars at field capacity.

wilting- the loss of plant turgidity due to excessive water loss.

incipient wilting - when a plant wilts, but recovers when placed in a saturated atmosphere
                             (100% R. H.), ex. overnight.

incipient wilting point - the soil moisture content when a plant wilts, but recovers when placed
                                      in a saturated atmosphere (100% R. H.), ex. overnight.

permanent wilting - when a plant wilts, but cannot recover when placed in a saturated 
                                atmosphere (100% R.H.).

permanent wilting point - the soil moisture content when a plant wilts, but cannot recover
                                         when placed in a saturated atmosphere (100% R.H.). 
 
 

HOW TO DETERMINE WHEN TO WATER PLANTS
1) Based on soil
    a) experience - feel, looks, etc.
    b) measure soil water potential, usually use a tensiometer - an instrument that is inserted
        in the soil and measures the soil moisture tension.

2) Based on plant
    a) experience - wilting, looks, etc.
    b) measure plant water potential