September 2020 - 11

MSUE VEGETABLE NOTES
Maximizing irrigation impact - the soil link
By Ron Goldy
Michigan State University
Most vegetable growers, even those
in high-water states like Michigan,
have learned the value of irrigation. It
is one of the primary ways to reduce
production risks, maximize yield, and
stabilize year-to-year product output.
Many Michigan fresh vegetable
producers have utilized the
combination of raised, plastic
mulched beds with drip irrigation
(plasticulture). These steps go a long
way in helping maximize water and
nutrient use efficiency. It is good to
remember that water and nutrients
cannot be separated, since wherever
water goes so do the nutrients it
carries.
Another important aspect to
consider in any irrigation system is
soil type. Unless you are growing
hydroponically, water goes through
soil first before the plant takes it
up. Therefore, it is important to
understand that in the process of
irrigating, you are directly irrigating
the soil and indirectly irrigating
the plant, and how you irrigate soil
depends on soil type.
As you move through the soil
triangle from sand to silt to clay, how
water behaves varies greatly. There
are different rates of penetration,
absorption and retention. A dry clay
can actually have more water than
a wet sand; it is just not available to
the plant. Soil and plants compete
for water, and the soil will only allow
the plant to have a certain amount
before the soils physical and chemical
properties kick in and say, " Enough.
You can't have anymore. " A good
indicator of how water will behave
* Apply water for shorter times.
* Apply water more often.
* Use higher flow drip tape.
* Use 12-inch emitter spacing.
Irrigating high CEC soils:
* Apply water slowly.
* Apply water for longer times.
* Apply water less often.
* Use low-flow drip tape.
* Use 16-inch or greater emitter
spacing.
The question then becomes, " At what
Graphic: Michigan State University
in soil is the soil's Cation Exchange
Capacity (CEC).
Soil CECs range from near one for
coarse beach sand (the lowest I have
ever seen is 0.9) and goes up from
there as the soil contains more silt,
clay and organic matter. Pure organic
soils can have CECs over 100. This
CEC number tells you a lot. It lets
you know how anything you put in
that soil will behave. This includes
water, nutrients, herbicides, and
other products. Low numbers mean
there are fewer binding sites and
anything you add to a soil having a
lower number will move quickly into,
through and out of the soil. Higher
numbers mean more binding sites
and therefore slower penetration and
greater retention.
So how does this affect irrigation
strategy and effectiveness? Water
penetrates quickly in low CEC soils
and therefore has little horizontal
movement before moving vertically
(down). In higher CEC soils,
water will have greater horizontal
movement before moving vertically.
Irrigators should consider the
following general irrigation strategy:
Irrigating low CEC soils:
* Apply water quickly.
number do I make the switch between
irrigating for low CEC and high
CEC? " Certainly, from an irrigation
standpoint any soil with a CEC of 10
or below would be a low CEC soil and
anything 20 or above would be a high
CEC soil. The transition zone between
10 and 20 becomes difficult. Generally,
in glaciated states like Michigan, any
field over 5 acres is rarely one soil type
and, therefore, not one CEC number.
So getting an average CEC might be
helpful in making your irrigation
decision. USDA Natural Resources
Conservation service soil surveys can
be of some help since they include
percolation rates for various soil types
(websoilsurvey.sc.egov.usda.gov/App/
HomePage.htm).
My experience is that most producers
tend to over irrigate just to be safe
since they know under irrigating can
lead to economic loss. However, over
irrigating produces losses in other
areas, such as nutrients leaching beyond
the root zone, extra expense and wear
on irrigation systems, and excess use
of one of Michigan's most valuable
assets - water. So, irrigate wisely and
everyone benefits.
Ron Goldy is a senior Extension
educator at Michigan State University
Extension. VGN
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September 2020

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