Baseline Systems BaseStation 3200 - Manual (Page 10)

I n t r o d u c t i o n
Page 8
Baseline 3200
Soil Moisture Content
Soil scientists and Agronomists have been studying the plant-water-soil system for over 100
years. Early work in irrigation efficiency focused on the estimation of soil moisture based on
weather information, plant water requirements, and soil information such as soil texture and
slope. With the availability of inexpensive and highly accurate soil moisture sensors, we are able
to take soil moisture based irrigation to a whole new level of efficiency and effectiveness.
With soil moisture sensors, your controller can operate like a thermostat for your landscape –
applying water when it is needed, and where it is needed.
To understand soil moisture based smart irrigation, it is important to understand some industry
standard terms for soil moisture content:
Saturation The soil pores are filled with water and nearly all of the
air in the soil has been displaced by water. Gravity will
exert force on the water contained in saturated soils,
moving it deeper into the ground (if possible). This is
known as “gravitational water”.
Field Capacity The level of soil moisture left in the soil after drainage
of the gravitational water. Irrigation to levels above
field capacity will result in runoff or drainage as
gravitational water.
Maximum Allowed
Depletion (MAD) Desired soil moisture deficit at the time of irrigation,
typically set well above the wilt point.
Permanent Wilt
Point The minimal point of soil moisture where the plant(s)
wilt and begin to die off.
Oven Dry When soil is dried in an oven, nearly all water is
removed. This moisture content is used to provide a
reference for measuring saturation, field capacity, and
MAD.
One key point is that water applied above Field Capacity is generally wasted – it gravitationally
moves down through the soil and becomes unavailable to plants. Excess water will also leech
nutrients from the soil into deeper soil layers, reducing the efficiency of fertilizers and soil
treatments.
To understand field capacity, it is often useful to think of a sponge. If you dunk a sponge in a
bucket of water and pull it out, water will gravimetrically drain from the sponge for a period of
time. When this stops, the sponge will still be very wet, but will no longer be dripping. This is
roughly equivalent to Field Capacity in soils – water is no longer draining into lower soil layers
and is held in the root zone of the plants.
Best plant health and plant water efficiency results are gained when soil moisture content is
maintained between Field Capacity and Maximum Allowed Depletion. Studies also show that
appropriately varying the time between irrigation events in order to allow the soil to dry to the
chosen depletion point promotes deeper root growth and subsequently more efficiency and
drought tolerance from the plants.
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