Abstract
Pivot line.
NCAT Photo |
Irrigators who monitor soil moisture levels in the field greatly increase their ability to conserve water
and energy, optimize crop yields, and avoid soil erosion and water pollution. This publication explains
how soils hold water and surveys some low-cost soil moisture monitoring tools and methods, including
a new generation of sophisticated and user-friendly electronic devices.
Table of Contents
Introduction
While poor irrigation
practices cause a
host of environmental
problems, irrigation
can also be a sustainable
practice, at times and
places where it does not
deplete or degrade surface
water, groundwater, or soils.
In times of high energy and
water costs, efficient irrigation
is essential to the viability
of many farms and
ranches. In the next few
decades, more efficient irrigation
may offer the best
hope of feeding the world’s
growing population. (Postel, 1999)
Given the importance of irrigation efficiency,
it’s unfortunate that irrigation water
management is often presented as a series
of complicated mathematical calculations
that only an engineer could love. Irrigation
management is nothing more mysterious
than maintaining a suitable environment
for growing crops, mainly by keeping soils
from becoming too wet or too dry. There
are many ways to achieve this goal, including
some that require no calculations at all.
This publication describes several ways that
you can check the soil moisture levels in
your fields, using your hands, inexpensive
probes, or new electronic devices.
Of course, there’s more to irrigation management
than just checking soil moisture
levels. You should follow general irrigation
guidelines for the crops you are growing,
and you should track crop water use (evapotranspiration)
as the season goes by. These
topics are not covered in this publication;
your local Natural Resources Conservation
Service (NRCS), Extension, or soil and
water conservation district office should be
able to assist you. You should also know the
amount of irrigation water you are applying.
(Please refer to the ATTRA publication
Measuring and Conserving Irrigation Water.)
No one knows as much as you do about
your fields, crops, and irrigation system.
So adjust, adapt, or reject any suggestion in
this publication that doesn’t fit your situation
or doesn’t seem to be working. Use every
kind of information you can find about how
your soils and crops are responding, proceed
cautiously, and test every recommendation
with direct observations in the field.
Overwatering can
- drown crop root
systems, depleting
air and encouraging
disease
- leach nutrients,
especially nitrogen,
below the
root zone
- send nutrients into
groundwater
- reduce root
growth by cooling
the soil
- cause waterlogging
and salt buildup
in the root zone
- reduce crop
quality and yield
- waste energy and
money
|
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How Soils Hold Water
The water-holding capacity of a soil depends
on its type, organic matter content, and
past management practices, among other
things.
Soils are classified into one of about a
dozen standard texture classes, based on the
proportions of sand, silt, and clay particles.
Sand particles are larger than clay particles,
with silt particles falling in between.
For example, a soil that is 20 percent clay,
60 percent silt, and 20 percent sand (by
weight) would be classified as silt loam.
Other texture classes are sand, loamy sand,
sandy loam, loam, silt, sandy clay loam,
clay loam, silty clay loam, sandy clay, silty
clay, and clay.
Coarse-textured soils have a high percentage
of sand, and fine-textured soils have a high
percentage of clay. Fine-textured soils generally
hold more water than coarse-textured
soils, although some medium-textured soils
hold as much or more plant-available water
than some clay soils.
Figure 1. Determining Soil Texture by the "Feel Method."
Besides their texture classification, soils are
also classified into soil types or soil series,
based on soil-building factors such as geology,
chemistry, age, and location. There are
more than 20,000 named soils in the U.S.
alone, with names often referring to a town
or landmark near where the soil was first
recognized. For example, the Houston Black
series is a clay soil formed under prairie
vegetation in Texas. The Myakka series is
a wet sandy soil found in Florida. The full
description of a soil series includes a number
of layers or horizons, starting at the surface
and moving downward.
To identify the soil types or series in your
fields, refer to a soil survey. Soil surveys are
generally available from your local NRCS
or Extension office.
Figure 2. Saturation, Field Capacity, and Permanent Wilting Point. |
As water infiltrates soil, it fills the pore
spaces between the soil particles. When
the pores are completely saturated, some of
the water – known as gravitational water
– percolates down through the soil profile
and below the root zone. Gravitational water may take a few hours to drain away in sandy
soils, or days or even weeks in clay soils.
Evaporation at the soil surface pulls water
upward through capillary forces, while capillary
forces also hold water around the
soil particles. When a balance is reached
between gravitational and capillary force,
water stops moving downward and is held
by surface tension in the soil – a condition
known as field capacity.
Figure 3. Effective Root Zone: the top half
of the actual rooting depth, which supplies about 70% of the crop’s water needs. |
Capillary water stored in the root zone is
the most important water for crop production,
but not all capillary water is available
for plants to use. The water-holding force of
soil, or soil water tension, is affected by soil
texture. For example, clay soils have small
pores and hold water more tightly than silt
soils, with their larger pores. As soil water
is depleted, the films of water remaining around the soil particles become thinner, until they are eventually held in the soil
with more tension than plants can overcome, and the plants begin to wilt.
Available water capacity is the amount of
water a soil can make available to plants,
generally defined as the difference between
the amount of water stored in a soil at field
capacity and the amount of water stored in
the soil at the permanent wilting point
Plants get most of their water from the upper
(shallow) portion of the root zone. The term
effective root zone refers to about the upper
half of the root zone depth, where roughly
70 percent of the plant’s water is taken up.
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What Soil Moisture Monitoring Method is Right for You?
In deciding when and how much to irrigate
(a process sometimes called irrigation scheduling),
all irrigators should do some kind
of soil moisture monitoring. You may be surprised to learn how easy and inexpensive
it has become to purchase, install, and
use a state-of-the-art monitoring system.
More devices are coming on the market all
the time, and prices continue to fall.
- Unless you are a scientific researcher,
don’t get too hung up on accuracy or
precision. Methods and devices will give
slightly different readings, but almost
all will track moisture trends similarly.
So choose a method that works for you,
take the exact readings with a grain
of salt, and pay more attention to the
trends and changes you are seeing over
time.
- Consider the limitations imposed by
your irrigation system, and choose a
method that gives you information you
can use. As a general rule, the greater
your control over the rate and frequency
of water applications, the more sophisticated
and detailed the information you
can use.
- Consider your soils and crops. Some
devices work better in coarse soils than
in fine soils, some devices work better
with annual crops than with perennial
crops, and so on. High-value crops will
often justify a more expensive monitoring
system than low-value crops.
- Consider what’s convenient for you.
Some devices are portable while others
are hard-wired in place. Some devices
give you “raw” data, and others do the
calculations for you or display readings
in a graph. Some devices require cables
that may interfere with tillage.
- Be realistic in your expectations. Soil
moisture measurement, even with the
advent of ever-more-accurate devices,
is still an art as much as a science. Soil
sensing and measuring devices don’t
substitute for the judgment, observation,
and local knowledge that good irrigators
acquire over time.
The methods below are arranged roughly in
order of cost, from least expensive to more
expensive. All work just fine if they are used properly and diligently.
Table 1. Determining Soil Water Content by Feel and Appearance |
Coarse |
Moderately Coarse |
Medium |
Moderately Fine and
Fine |
% of Available
Water Capacity
(AWC) |
Free water appears when
soil is bounced in hand. |
Free water is released with
kneading. |
Free water can be
squeezed out. |
Puddles and free water
forms on surface. |
Exceeds field
capacity – runoff
& deep percolation. |
Upon squeezing, no free water appears on soil, but wet outline of ball is left on hand. |
100% – At field capacity |
Tends to stick together,
forms a weak crumbly ball
under pressure. |
Forms weak ball that
breaks easily; does not
stick. |
Forms a ball and is very
pliable; sticks readily if
relatively high in clay. |
Ribbons out between
thumb and finger; has a
slick feeling. |
70 – 80% of AWC |
For most crops, irrigation should begin at 40 to 60% of AWC. Crop-specific guidelines are available from NRCS or Extension. |
Appears to be dry; does
not form a ball under
pressure. |
Appears to be dry; does
not form a ball under
pressure. |
Somewhat crumbly but
holds together under
pressure. |
Somewhat pliable; balls up
under pressure. |
25 – 50% of AWC |
Dry, loose, single-grained
flow through
fingers.
|
Dry, loose, flows
through fingers. |
Powdery dry, sometimes
slightly crusted but easily
breaks down into powder |
Hard, baked, cracked;
sometimes has loose
crumbs on surface. |
0 – 25% of AWC |
Adapted from NRCS Irrigation Guide, USDA Natural Resources Conservation Service, 1997. |
Related ATTRA Publications
|
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Direct Inspection
The least expensive methods rely on digging
up soil samples in the field and then inspecting,
feeling, or weighing and drying them.
Feel and Appearance Method
Take walnut-sized soil samples from various
locations and depths in the field, appropriate
to your crop’s root zone. Then use
the table above to estimate the soil water
content of your samples. With practice and
diligence, the feel and appearance method
can be accurate enough for most irrigation
management decisions. A soil probe, auger,
or core sampler is far superior to a shovel,
especially for retrieving deep soil samples.
Hand-Push Probe
Figure 4. Soil Sampling Tools. |
You can use a hand-push probe (sometimes
called a Paul Brown Probe or Brown Moisture
Probe) to determine the depth of wetted
soil and also to retrieve soil samples. These
extremely useful probes cost less than $50
and are among the fastest and easiest ways
to check moisture anywhere in your fields.
To determine the depth of wetted soil, push
the probe vigorously into the soil by putting
your weight on the handle without turning.
The probe will stop abruptly when it reaches
dry soil. (Rocks and gravel will also stop the
probe, but these are easily detected by a
metallic click.) Check the mark on the probe
shaft to determine the depth of the wetted
soil.
To obtain a soil sample, twist the probe after
pushing it into the ground. The probe will
be full of soil when you pull it up. Then use
either the “feel and appearance” method or
gravimetric weight method to estimate soil
moisture.
Gravimetric Weight Method
The gravimetric method involves weighing
soil samples, drying them in an oven,
weighing them again, and using the difference
in weight to calculate the amount of
water in the soil. While too time consuming
to be used for day-to-day management
decisions, this highly accurate and low-cost
method is often used to calibrate other
tools. Your local Extension or NRCS office
may be able to provide instructions for this
technique, or you can find the instructions
on the Internet.
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Meters and Sensors
Gypsum blocks.
NCAT Photo |
More sophisticated devices measure some physical property that is correlated with
soil moisture. Some portable sensing tools
are pushed directly into the soil or into an
access tube implanted in the soil. Other systems
rely on buried sensors that are either
hard-wired to a fixed meter or else have
long attached wires (electrodes) that are
left above-ground and hooked to a portable
hand-held meter.
Soil Moisture Blocks
The most common sensors, electrical resistance
blocks, work on the principle that
water conducts electricity. The wetter the
soil, the lower the electrical resistance and
the better the block conducts electricity.
The two most common types of electrical
resistance blocks are gypsum blocks (with a
life of as little as one year but a cost of only
$5 to $15 apiece) and granular matrix sensors (lasting three to seven years or more
and costing $25 to $35 each). Freezing
can cause cracking and premature aging in
gypsum blocks, but will generally not hurt
granular matrix sensors.
Granular matrix sensor.
NCAT Photo |
Electrical resistance blocks work by absorbing
water from the surrounding soil. They
need to be buried carefully, with snug soil
contact and no air pockets – something that
may be difficult to achieve in coarse or gravelly
soils. Over the past several years, the
National Center for Appropriate Technology
(NCAT) has installed hundreds of granular
matrix sensors. We have found the number one
problem to be poor soil-to-sensor contact,
usually in coarse or gravelly soils.
When burying any soil moisture sensing
device, minimize soil compaction and disturbance
to the surrounding soil and canopy
cover. Your goal is to install each sensor
in surroundings that are representative
of the field.
Electrical resistance blocks may be read
either with a data logger (see below) or with
a portable hand-held meter. Hand-held
meters, costing $150 to $600, generally
either give electrical resistance readings in
ohms or else convert resistance to centibars.
(See text box below.)
Hand-held meters have their advantages.
You don’t need to bury cables in the field.
And because the meter is portable, you can
check moisture at an unlimited number of
sites, wherever your soil moisture blocks are
buried.
A disadvantage of hand-held meters, though,
is that each monitoring site must be marked
in some way, so you can find the electrodes
in the field and hook them to the meter.
A common indicator of soil moisture is soil water tension.
Some soil moisture monitoring instruments give volumetric readings –
moisture per foot or per inch of soil – while other instruments indicate
the level of soil water tension. Soil water tension is usually measured in
centibars, where a centibar is 1/100th of a bar, and a bar is roughly
equivalent to one atmosphere of pressure. Centibars measure the force
that a plant must exert to extract water from the soil. As the plant works
harder to remove water, the centibar number increases. So larger centibar
numbers mean drier soil.
Soil water tension levels mean different things in different soils and so – unfortunately – there is no simple way to translate centibar readings
into water volumes or vice versa. Depending on soil texture, for example,
field capacity may be between about 10 and 33 centibars. Coarse soils (such as
sands and sandy clay loams) have released 50 percent of their available
water by the time soils have dried out to 40 to 50 centibars. On the other hand,
many clay and silty soils still retain more than 50 percent of available water at 80 centibars. |
Another disadvantage is the challenge of
wading through crops (sometimes tall and
wet) to your monitoring sites. Also, the
meters seem expensive for what they do. For
roughly the cost of a hand-held meter you
could purchase a sophisticated data logger
offering graphical display, automated moisture
readings, and many other features.
Considering the high cost of the hand-held
meters, you may be tempted (as we were) to
measure resistance with an ordinary ohm
meter. Unfortunately, this doesn’t work.
Ohm meters use DC power, which polarizes
the soil moisture blocks and causes readings
to fluctuate wildly. The meters made
specifically for use with soil moisture blocks
convert DC power to AC, avoiding polarization
and giving stable readings.
Thermal dissipation blocks, a less-common
alternative to electrical resistance blocks,
work on the principle that dry objects heat
up faster than wet objects. These porous
ceramic blocks contain small heaters and
temperature sensors. They cost $35 to $50
apiece, with meters costing $150 to $600.
Tensiometers
Tensiometer
Photo courtesy of The Irrometer Company |
A tensiometer is an airtight, water-filled tube with a porous ceramic tip on the end that
is placed in the soil, with a vacuum gauge
on the other end that protrudes above the
ground. Tensiometers measure soil water tension
and display the reading on the vacuum gauge in centibars. These devices work
best in the range of 0 to 80 centibars, making
them better suited to coarse soils than fine
soils. A coarse soil at 80 centibars might cause
severe crop stress, whereas a fine soil such
as clay might still contain more than half of
its available water capacity at 80 centibars.
Tensiometers are fairly easy to use but
must be serviced regularly by filling with
water and using a pump to pull a vacuum.
If the soil becomes too dry, tensiometers
can lose soil contact, requiring re-installation.
Depending on length – from 6 to 48
inches – they cost $45 to $80. Because they
are easy to install and remove, tensiometers
are well-suited to cultivated fields and
annual crops where buried blocks or cable
would be awkward. They are also often
used in orchards.
Tensiometers measure moisture tension at
the depth where the tip is located. To use
two tensiometers as simple irrigation “on-off”
indicators, install one at the center of
the effective root zone and another one just
below the effective root zone (i.e., at approximately
one third and two thirds of the total
root depth). Use the shallow tensiometer as
an indicator to start irrigating and use the
deeper one as an indicator to stop irrigating.
Table 2. Irrigation Guidelines Based on
Centibar Readings |
Reading |
Interpretation |
0-10 centibars |
Saturated soil |
10-20 centibars |
Most soils are at field capacity |
30-40 centibars |
Typical range of irrigation in
many coarse soils |
40-60 centibars |
Typical range of irrigation in
many medium soils |
70-90 centibars |
Typical range of irrigation in
heavy clay soils |
> 100 centibars |
Crop water stress in most soils |
Adapted from Watermark Soil Moisture Sensors,
The Irrometer Company, Riverside, CA. |
Data Loggers
Soil moisture data loggers are typically battery-operated devices, permanently mounted
on a post and hard-wired to buried electrical resistance block sensors. At regular intervals
(generally every several hours), the data
logger sends a current through each sensor,
measuring electrical resistance. The measurements
are converted into soil moisture
readings and stored in memory. Data loggers
with a graphical display show several
days or weeks of readings in a bar graph,
allowing you to see recent soil moisture
trends at a glance on the screen. Depending
on their features, soil moisture data loggers
may cost $60 to $500, not including
sensors or cable.
The arrival of low-cost soil moisture data
loggers on the market in the late 1990s was
great news for irrigators. A major advantage
of data loggers is that no matter how busy
you get, the monitor automatically checks
and records your soil moisture. The monitor
is normally mounted on a conveniently located
post at the edge of the field or near
the control panel of a center pivot, eliminating
the need to walk into the field or find
electrodes amidst foliage. A disadvantage is
that a limited number of sensors (typically
6 to 15) can be connected to the monitor.
Installation generally also requires running
cable from the data logger to each sensor.
When feasible, such as in perennial crops,
burying the cable is recommended.
Figure 5. Data Logger Graphical Display. |
Between 2000 and
2004, NCAT helped
install around 100
soil moisture data
loggers at farms and
ranches in Montana.
Data logger installation
is not particularly
difficult, and the headaches mostly relate to
the cable. We saw dozens of faulty splices,
cables chewed by livestock and wildlife,
cables damaged by machinery during tillage
and hay cutting, cables melted when the
owner was burning weeds, cables melted by
lightning, and (on one memorable occasion)
a cable snagged by a passing car.
Besides displaying recent moisture readings,
soil moisture data loggers store several
months or years of data, which may be
downloaded and viewed in graph form.
Figure 6. Soil Moisture Graph Generated by a Data Logger, Showing an Entire Irrigation Season.
Comments from Data Logger Users
Below are a few representative comments
from interviews with NCAT’s 2000-2004
soil moisture data logger project participants.
(The names are fictitious.)
Jim Clinton intensively grazes grass pasture,
which he waters frequently and for short
periods. After he installed a soil moisture
data logger, he checked it daily and called it “one of the best things to come along for
a long, long time.” Jim quickly became convinced
that he was overwatering. He had
been running five- to six-hour sets through
the growing season, switching to eight-hour
sets during hot weather. In his second year
with the monitor, he ran two-hour sets in
the spring, three-hour sets through May,
and four- to five-hour sets when it got hot.
He told us, “Soil moisture has always been
the missing link… The meter said that six
hours was all we needed. Even if it started
out at 90 centibars, we got down to 10 centibars
within six hours.”
Soil Moisture Data Logger.
NCAT Photo |
After Chester Hendricks installed a soil
moisture monitor, he looked at it “every
day, at least, and sometimes two or three
times per day.” He told us that he bases
most of his decisions on careful observation
of the crop, and he called the data logger
“a tool to manage the crop along with visual
observation of the crop… It’s a tool, but so
is looking at the crop.” Chester believes that
the monitor definitely made a difference to
his total production. During an exceptionally
hot and dry summer, he “didn’t let the
crop get hurt” by the hot dry weather, and
enjoyed excellent yields, “one of our best
crops ever.” Chester was surprised that
evapotranspiration rates skyrocketed once
the plants started getting taller. “An 18-inch crop pulls a lot more moisture than
when the plants are smaller and younger.”
He was also impressed by “how much
effect wind and humidity make on depletion
of soil moisture…. Unbelievable.” He
saw some tremendous moisture drops take
place in just a four to six hour period.
George Adams told us that his irrigation
practices didn’t change much during his
first year using a data logger. The device
did give him a much better idea, though,
how the water was moving down through
the soil profile. He said, “The year before I
wasn’t getting water deep enough. This year
I wanted to saturate it then let it go longer
between passes to let the water go deeper,
by slowing down the pivot. Yield was fantastic.”
In subsequent years, George has monitored
soil moisture in the spring and started
irrigating earlier. He told us that he sees the
device as useful for limited-water situations:
“Instead of trying to water everything, I can
set priorities for short water supplies.”
NCAT Photo |
Three years of using a soil moisture data
logger have not caused John Jefferson to
make major changes to his water management
methods, but have confirmed his belief
that he is not overwatering and is making
good use of water. He told us that the monitor
has helped him save water during spring
rains and late June snowstorms. “We saved
two to three days of watering because the
ground was wet after a late snow,” Jefferson
says. At prevailing electricity rates, he saved
about $100 in these three days alone.
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Tips on Placing Moisture Sensors
- It’s generally not practical to monitor every part of the field, so install sensors
in average soil and slope areas. Avoid
field edges and unusually wet or dry
areas.
- For mature trees, place sensors well
away from the trunk but inside the drip
line (canopy diameter).
- The question of how deeply to maintain
soil moisture is a management decision,
depending on crop and growth stage,
soil conditions, and other factors. In
general, though, management should
focus on the effective root zone; i.e., the
upper half, where plants take up most
of their water.
- For three-foot or deeper effective root
depths, you may want to place sensors
at three depths; e.g., in the top, middle, and deepest third of the total
root depth.
- For effective root zones of two feet or
less, place sensors at two depths.
- Place a sensor below the root zone
for shallow-rooted crops (including
grasses), or in the
Figure 7. Soil Monitoring Sites Under Pivot. |
lower quarter of the
root zone for deeper-rooted crops, as a
way of detecting deep percolation and
overwatering.
- For center pivots, monitor a few sprinkler
diameters from where you normally
start the pivot, in the direction of pivot
movement. Also monitor a few sprinkler
diameters before the spot where you
normally stop the pivot.
- Avoid the inner part of a pivot circle
(inside the first tower), which tends to be
wetter than the rest of the circle.
Changes in soil texture act as a temporary
barrier to water movement.
Fine soil overlying a coarse soil, or vice versa,
must become very wet before water will move
down through the subsoil. Under these conditions,
the overlying soil holds up to three times
as much water as it would in more uniform soils.
If you have distinct layers of soil, you may want to
monitor soil moisture in each layer separately.
|
Figure 8. Water Movement in Stratified Soils
(Adapted from NRCS Irrigation Guide, USDA Natural
Resources Conservation Service, 1997.) |
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Other Tools and Techniques
In 2006, the tools below are generally more
expensive and best suited to high-value
crops, large farms, and scientific research.
In some cases, though, high-tech features
are becoming available at affordable prices.
It’s hard to predict the future of this highly
competitive and rapidly changing market.
Remote sensing systems ($1,000 and up) use
buried sensors wired to a nearby transmitter
that sends readings to a receiver, usually
a data logger connected to a computer.
These systems are often called “wireless.”
Although this term is slightly misleading,
it’s true that the cable connections between
the sensors and transmitter are typically
quite short. The big advantage of these systems
is that they allow large farms to monitor
soil moisture in several fields from a single
computer, without going into the field.
Time domain reflectometers (TDR) send an
electromagnetic wave along two parallel
rods or stiff wires inserted in the soil, measuring
the “dielectric constant” of the soil.
TDR instruments range in price from about
$500 to $4,400.
Frequency domain reflectometers ($475 to
$900) use high-frequency radio waves pulsed
through the soil from a pair of electrodes.
Infrared thermometry is based on the principle
that the temperature of a plant’s leaves
is related to its transpiration rate. Infrared
satellite imagery to detect crop stress is
under research.
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Conclusion
Soil moisture monitors, especially the new
generation of electronic devices, show you
how water is moving through your soils,
with a precision and vividness that most
irrigators have never seen before. The
effect can be startling – almost like having
an x-ray machine that allows you to
look beneath the surface of the soil. With
the cost of sophisticated monitoring systems
dropping into the range of a few hundred dollars, many of these devices are rapidly
paying for themselves in the form of crop yield improvements,
energy savings, water conservation, and peace
of mind.
On the other hand, soil moisture monitors don’t “tell you
when to irrigate.” You’ll still need to develop guidelines
for your own crops and soils, and there is no substitute
for the experience, subtle observations, and judgment
that make someone a good farmer.
Back to top
References
Postel, Sandra. 1999. Pillar of Sand. Worldwatch
Books, New York. 313 pages.
USDA-Natural Resources Conservation Service. 1997.
NRCS Irrigation Guide. Natural Resources
Conservation Service, Washington, DC. 702
pages.
www.wcc.nrcs.usda.gov/nrcsirrig/irrig-hand
books-part652.html
Further References
NCAT Publications
Installing and Using the AM400 Soil Moisture
Monitor. 2004. By Mike Morris and Vicki Lynne.
National Center for Appropriate Technology, Butte,
MT. 17 pages.
Detailed instructions for installing and using the
AM400 soil moisture data logger, including maintenance,
troubleshooting, downloading data, and
advanced settings. To request a free print or electronic
copy, call 800-411-3222 (toll-free).
The Montana Irrigator’s Pocket Guide. 2003. By
Mike Morris, Vicki Lynne, Nancy Matheson, and Al
Kurki. National Center for Appropriate Technology,
Butte, MT. 161 pages.
A take-to-the-field reference to help irrigators save
energy, water, and money, including guidelines for
water management, equipment maintenance, and
handy conversions and formulas. Get a free printed
copy by calling 800-346-9140 (toll-free).
Water and Energy Conservation with the AM400
Soil Moisture Monitor. 2004. By Mike Morris.
National Center for Appropriate Technology, Butte,
MT. 15 pages.
Summarizes four years of NCAT research on the
AM400 soil moisture monitor. To request a free print
or electronic copy, call 800-411-3222 (toll-free).
Other Publications
Soil Water Monitoring with Inexpensive Equipment.
2000. By Richard Allen, University of Idaho,
Kimberly, ID. [four papers]
www.kimberly.uidaho.edu/water/swm
Reviews and research on low-cost soil moisture monitoring
equipment.
Hard copy available from
Kimberly Research and Extension Center
University of Idaho
3793 North 3600 East
Kimberly, ID 83341
208-423-4691
Tensiometer Use in Irrigation Scheduling. 1997.
By Mahbub Alam and Danny H. Rogers. Kansas
State University Agricultural Experiment Station and
Cooperative Extension Service, Manhattan, KS. 6 p.
www.oznet.ksu.edu/library/ageng2/l796.pdf. (PDF / 144K). Download Acrobat Reader.
Tensiometer installation, use, and troubleshooting.
Hard copy available from
Production Services/Distribution
Kansas State University
26 Umberger Hall
Manhattan, KS 66506-3404
785-532-5830
Measuring Soil Moisture. 1998. By Blaine Hanson
and Steve Orloff. University of California, Davis, CA.
34 p.
http://gwpa.uckac.edu/pdf/direct_soil_mositure_measurement.pdf. (PDF / 191K). Download Acrobat Reader.
Good discussion and comparison of soil moisture
measuring devices, although slightly dated and does
not include data loggers.
Hard copy available from
Cooperative Extension Office
Department of Land, Air and Water Resources
113 Veihmeyer Hall
University of California
Davis, CA 95616
530-752-1130
Web Sites
NRCS Irrigation Page
USDA-Natural Resources Conservation Service
A comprehensive source for irrigation reports, guides,
statistics, photos, and links.
The Soil Water Content Sensor discussion group
Moderated by Bruce Metelerkamp
An e-mail discussion list and archives, with discussions
and reviews of all kinds of soil moisture monitoring
devices, concentrating on automated electronic
sensors that can be continuously logged with
data loggers.
An Internet search under “soil moisture monitoring” (or
similar key words) will yield hundreds of additional Web
sites offering products, reviews, and guidelines.
Suppliers
Hundreds of companies make and sell soil moisture monitoring
equipment. Listed below are a few representative
sources of equipment mentioned in this article.
Art’s Manufacturing & Supply, Inc.
105 Harrison Street
American Falls, ID 83211
800-635-7330 (toll-free)
www.ams-samplers.com
Source of soil probes, bucket augers, and other irrigation
equipment.
Ben Meadows Company
2589 Broad Street
Atlanta, GA 30341
800-241-6401 (toll-free)
www.benmeadows.com
Campbell Scientific, Inc.
815 West 1800
North
Logan, UT 84321-1784
435-753-2342
www.campbellsci.com
Source of data loggers and weather
stations.
Davis Instruments Corp.
3465 Diablo Ave.
Hayward, CA 94545
510-732-9229
www.davisnet.com
Source of weather stations and wireless soil moisture
monitoring systems.
Delmhorst Instrument Company
51 Indian Lane East
Towaco, NJ 07082-1025
877-DELMHORST (toll-free)
www.delmhorst.com
Source of gypsum blocks and hand-held soil moisture
meters.
Gempler’s
P.O. Box 44993
Madison, WI 53744-4993
800-382-8473 (toll-free)
www.gemplers.com
Source of soil moisture sensors, probes, tensiometers,
and other irrigation equipment.
Irrometer Company, Inc.
P.O. Box 2424
Riverside, CA 92516
951-689-3706
www.irrometer.com
Source of Watermark granular matrix soil moisture sensors, hand-held soil moisture meters, tensiometers,
soil moisture data loggers, and other soil moisture
monitoring equipment.
Isaacs and Associates, Inc.
3380 Isaacs Ave.
Walla Walla, WA 99362
800-237-2286 (toll-free)
www.isaacstech.com
Source of remote soil moisture monitoring systems.
M.K. Hansen Company
2216 Fancher Boulevard
East Wenatchee, WA 98802
509-884-1396
www.mkhansen.com
Manufacturer of the AM400 soil moisture data
logger.
Soil Moisture Equipment Corporation
801 S. Kellog Ave.
Goleta, CA 93117
805-964-3525
www.soilmoisture.com
Source for soil augers, soil moisture sensors and
meters, tensiometers, time domain reflectometers,
and other soil moisture monitoring equipment.
Soil Moisture Monitoring: Low-Cost Tools
and Methods
By Mike Morris
NCAT Energy Specialist
Paul Williams, Editor
Sherry Vogel, HTML Production
IP277
Slot 277
Version 040306
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