Product Manual

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It is possible to grow some crops in some fine soils at even higher readings and get good yields.
However, we know of no cases where yields or quality have been increased by starting irrigations at
higher readings during the vegetable period of growth. On the other hand, there is considerable
evidence that this practice results in a substantial loss of yield and delayed maturity with most crops.
In any case, the above adjustments should not be confused with the “Soil Calibrations” required with
other methods of measuring available soil moisture. The purpose of these adjustments is to maintain
available moisture in accordance with the requirements of the particular crop and allow a safety factor
in the event of delayed irrigations. This is necessary with every method of irrigation control. The use
of “Soil Calibration” charts involves an additional operation and complication.
NOTE: The trend or “rate of change” can be as important as the IRROMETER reading in
making irrigation decisions, as discussed under “IRROMETER Charts.”
Accuracy of IRROMETERS
Exhaustive tests by leading soil scientists have demonstrated that IRROMETER type instruments
provide the most accurate and most sensitive method of measuring soil moisture in the range in which
most crops are grown. In fact, they are widely used as reference instruments to check the accuracy
of soil moisture determinations made by other methods. The slightest variations in available soil
moisture resulting from soil type or compaction, root density or other factors – variations too small to
be measured easily by other methods – are automatically evaluated and registered on the
IRROMETER gauge. This feature is very valuable in many research applications where precise
measurement of soil moisture is required.
However, the same accuracy of control may not be practical nor necessary under field conditions. For
example, if the objective is to start irrigations at a reading of 50, variations of 10 to 15 points in either
direction are to be expected on instruments in various areas due to the extreme sensitivity of the
IRROMETER. Soil moisture will still be maintained well within the range for the optimum crop growth.
Even greater variations may occur for short periods without loss of yield or quality.
Interpreting IRROMETER Readings
The IRROMETER measures energy directly – the energy, that is,
the roots must exert to extract moisture from the soil – whereas
other methods of making soil moisture determinations measure the
total amount of soil moisture and then in effect, convert it into root
energy for each type of soil by means of soil calibration charts.
Obviously then, the IRROMETER requires an entirely different unit
of soil moisture measurement.
The IRROMETER gauge is graduated 0-100, the graduations
representing hundredths of an atmosphere. The unit of
measurement is centibars or kilopascals, with a gauge reading of
50 representing 1/2 atmosphere or about 7 pounds
of negative pressure (vacuum). This reading is a
direct measurement of how hard the root system
has to work to extract water. If this seems
complicated, think of your IRROMETER readings
as you would a thermostat and schedule irrigations
to maintain soil moisture within the desired
“comfort” range.
NOTE: Low Tension (LT & MLT) IRROMETERS have
a gauge which is graduated from 0-40 centibars
(kilopascals). This provides for better resolution in the
very wet end of the soil water spectrum.
Average Percentage of Moisture
Extraction from Normal Root Zones
1st Quarter – 40%
2nd Quar
ter – 30%
3rd Quarter – 20%
4th Quarter
10%