Using Wireless Soil Moisture Sensors For Increased Yields
PORTAGEVILLE, MO.
Wireless soil moisture sensor technology
combines two things: (A) traditional soil
moisture monitoring (e.g., with gypsum
blocks, tensiometers, etc.).
(B) wireless communication.
The end result is that the soil moisture status
of one’s crop can be monitored 24-
7 from a computer or smart
phone. A farmer can see how
much rain the field got, how deep
it soaked, and the water being
slowly extracted from the soil at
different depths. The sensor technology
has been available for
about 100 years, but coupling it
with wireless technology has imbued
it with new synergism. Ordinarily,
sensors were read
periodically, in the order of once a
week. This provided a SNAPSHOT
of the root/soil/moisture complex
for the farmer to make management
decisions about irrigation.
After sensors were tied to data-loggers
the farmer was given a HOME
VIDEO of the root/soil/moisture
complex for making these decisions.
Tying the data-logger to
wireless technology quickly followed,
and now an irrigator (once
the sensors are installed) only
needs to turn on his computer to
see the current soil moisture situation
in his fields.
Farmers seem ready to embrace
this new technology. Three Wireless
Irrigation Sensor workshops
were scheduled a year ago in Missouri
with over 100 attendees,
most of them farmers, signed up
for the workshops. Wireless sensor
companies present were
Decagon, Irrometer, Campbell
Scientific and Onset.
Normally, growers put sensors
in at three depths (6, 12, and 18
inches) at each locale
and have three locales in their
field. Growers are told to carefully
monitor the 18-inch depth,
since when it starts to deplete it
means that the 6 and 12-inch
depths are running short of
water. No one was arguing that
the data from the sensors would
not help farmers make informed
choices about irrigation.
However, past research by the
University of Missouri showed
that one of the most critical factors
in making this technology
successful is that the communication
between the sensors in the
field and the farmer’s home computer
be flawless. Cotton Incorporated
(CI) likewise felt that
trouble-free links must exist are
the busy farmer will abandoned
using the technology. Therefore,
they initiated several demonstrations
in various states in the mid-
South to see if the technology
could be taken off the shelf and
made to work in the fields of typical
cotton farmers.
In the CI demonstration the
project manager asked me to be
the cooperating researcher and
asked me to test Decagon sensors/
wireless equipment. I was
to locate a suitable cotton famer
in southeast Missouri to evaluate
everything about using wireless
sensors to grow cotton. The farm
I found was run by three generations
of cotton farmers: Max Ray,
Marty & Ryan. I visited them in
early April to explain the project
and look at the field. They indicated
that they would be involved,
but wanted the right to make the
final call on whether to irrigate or
not; that was perfectly fine with
me and, in fact, that was how I
had planned to work with
them. Ryan is the most computer-
savvy, so most of the
work was with him. The location
had good AT&T service
Since Decagon makes both a
sensor that records soil moisture
tension (like a tensiometer
does with units in centibars),
as well as one that records volumetric
soil moisture that
records data in % moisture or
inches per foot, we hoped to
use both types to see if the
growers had a preference. Unfortunately,
their new tensiometric
sensors were not ready
for delivery, so we evaluated
just volumetric sensors. We
also added a rain gage to the
group of sensors, which turned
out to be very useful.
I had asked Ryan to sit in on
a “webinar” that Decagon was
going to give on their data- logger.
It was totally Greek to him
and I had to assure him that
the information he was going to
receive would be helpful. To
this end, I wrote an Excel program
that took all the data that
was being stored at the company’s
data storage site and allowed
the data to be viewed in
three additional graphs:
• By site (averaging the 6-,
12-, and 18-inch depths into
one graph [Fig. 1]).
• By depth (averaging each
depth from the three sites into
one graph [Fig. 2])
• Averaging all nine sensors (3 sites
X 3 depths) into one graph.
My staff did the actual installation of
the sensors (Figs. 3 and 4). We also
had to initially help Ryan get on the
website, after this we was able to do
so unaided and could put the data
into the Excel graphs we had developed.
He and his father looked at the
graphs & understood when they
should irrigate. Ryan and I texted
each other concerning when to irrigate.
Through this I had the wake-up
call that their world did not center
around this one pivot, that they had
watermelons to harvest and sell,
spraying that need to be done and the
keep-out periods adhered to, etc. One
of the points we were keying in on was
not to let the 18-inch depth start to
dry out (meaning that the 6- and 12-
inch depths were heavily depleted
[Fig. 5]).
Several problems encountered during
the study were:
• One sensor that “bled” (Fig. 6)
making it useless as a sensor as is (in
the Excel program I cleaned up the
data so it could be used)
• One of the data-loggers that
stopped sending data (I could go out
& manually download it [Fig. 7]).
However, over all, the operators of
the farm felt that it caused them to
change the way they were irrigating.
They feel that this field (one of 17 pivots
they operate) will out-yield the
other ones. One of the things that
helped them was seeing the amount of moisture
that actually got into the soil after a rain or irrigation.
Figure 8 shows the number of bolls in
three feet of row.
As already mentioned, one of the original stratagems
was to apply enough irrigation water so
that the water content in the 18-inch depth did
not noticeably decline. It can be seen in Figure
2 that the content at this depth began to decline
about mid-July. We had a very hot, dry summer
and this pivot has watermelons planted on a
quarter of it, so water was shared between two
crops. What ended up as the practice was to
keep smaller, frequent applications continuing
so that the 6-inch depth had adequate water.
Near peak bloom the water was ramped up so
that the 12-inch depth was also being supplied
water. So what was happening was the 18-inch
level declined and the 6- and 12-inch were
“spoon-fed”. It reminded me of sub-surface drip
irrigation. You could say that with these wireless
sensors, the soil moisture was managed as
if by a conductor. Δ
DR. JOE HENGGELER: State Irrigation Extension
Specialist, University of Missouri