Simple
irrigation scheduling
Using the "look and feel" method
EM 8716
August 1998
F. Niederholzer and L. Long
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Contents
Why schedule irrigation?
How does it work?
Why schedule irrigation?
Accurate irrigation scheduling maximizes the
benefits of irrigation while minimizing potential negative impacts
of over-irrigation or under-irrigation.
Over-irrigation (too much water):
- Drowns roots, thus stressing plants
- Encourages root diseases
- Reduces nutrient uptake
- Cools soil, thus reducing root growth
- Leaches nutrients and pesticides from the
root zone to groundwater
- Reduces crop quality
- Wastes money
Under-irrigation (too little water):
- Reduces crop yield
- Reduces crop quality (fruit and vegetable
size)
- Reduces plant growth
- Weakens plants
Many people schedule irrigation by the calendar
rather than by plant need. Calendar-based scheduling can be very
inaccurate since plant water needs and the amount of soil water
available to plants are affected by factors such as climate,
plant size, soil type, and rooting depth.
The goal of accurate irrigation scheduling
is to replace soil water lost by evaporation and plant use as
precisely as possible. To accomplish this goal, you need to accurately
assess soil moisture content. Then you can determine the need
for irrigation and how much water to deliver.
Irrigation scheduling can seem complicated.
It doesn't have to be. Anyone can use a simple, effective method
known as the "look and feel" (or "soil appearance
and feel") method to determine when to irrigate.
top
How does it work?
This method is based on three simple ideas:
- Soil is at "field capacity" when
it is holding as much water as possible after the excess has
drained away. (A wet sponge is at "field capacity"
when it holds all the water it can without any dripping away.)
- It's best to irrigate when half of this water
is depleted.
- Your goal when irrigating is to return the
soil to field capacity.
So, all you need to know to schedule irrigation
is:
- How much water will the soil hold within
the plants' rooting zone when it's at field capacity?
- What does the soil look like when half of
that water is gone?
- How much water should be applied to return
the soil to field capacity?
top
Prepared
by Franz Niederholzer, Extension horticulture agent, Hood River
County; and Lynn Long, Extension horticulture agent, Wasco County;
Oregon State University. Photos are from Estimating Soil Moisture
by Feel and Appearance, and are reproduced courtesy of USDA/
NRCS. Special thanks to Mr. Diego León for his excellent
work with English/Spanish translation.
August 1998.
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