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Water
Sampling Procedure
The quality of
irrigation water plays an important and significant role
in the successful production of agricultural crops. The
quality of irrigation water can vary substantially
between areas and also from one farm well to another. It
is essential that growers have a knowledge of the
chemistry of their water as well as an understanding of
potential problems that may be associated with it use
for irrigation.
To accurately evaluate a specific water source, it is
important to collect samples in a proper manner. The
following guidelines are suggested for proper sample
collection:
- Collect samples at
the appropriate time. Ground water samples should be
collected only after the well has pumped for a
period of one to two hours. A longer time period is
desirable if the well has not been used on a regular
basis. Surface water samples should be taken during
the period of time when the water is used for
irrigation.
- Collect an
adequate amount of sample. Generally an 8 to 16
ounce sample of water is adequate for most quality
assays.
- Use a proper
sample container. Clean plastic containers may be
used in most instances. Samples to be analyzed for
organic residues, such as pesticides, should be
collected in glass containers. Contact the
laboratory for special instructions.
- Use preservatives
when required. Accurate tests for certain
constituents in water require the addition of
specific compounds as preservatives during the time
the samples is collected. Contact the laboratory for
special instructions.
- Deliver water
samples to the laboratory as soon as possible.
Samples should be delivered within 24 hours. Samples
may be refrigerated or kept in a portable cooler for
short periods. Do not store samples at room
temperature or expose them to heat or direct
sunlight.
The following
explanations of the water analysis can be used to
interpret the results of the laboratory analysis.
However, caution should be used in the interpretation of
laboratory results. Accurate interpretation of
laboratory results requires experience and knowledge,
therefore, the use of trained and experienced
agronomists is strongly recommended.
Explanation of Water Analysis
-
pH – The pH expresses the acidity or alkalinity
of water. A pH value below 7.0 is acidic, 7.0 is
neutral, and above 7.0 is alkaline. Most water has a
pH between 6.5 and 8.4.
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ECw – The total salt content of water is
expressed as electrical conductivity (ECw). The ECw
provides an indication of both potential salt
accumulation and water penetration problems as
follows:
|
Problem Type
|
No Problems |
Slight
Problems |
Severe
Problems |
|
Salinity
|
<0.75 dS/m |
0.75-3.0 dS/m |
>3.0 dS/m |
|
Penetration
|
>0.50 dS/m |
0.5-0.2 dS/m |
<0.2 dS/m |
-
Ca+Mg – Calcium (Ca) and Magnesium (Mg) are
essential plant nutrients that are normally present
in water. Together with Sodium (Na), they are used
to calculate SAR and adjSAR.
-
Na – High levels of Sodium are toxic to plants
and lead to water penetration problems in the soil.
The detrimental effect on soil is primarily related
to the SAR and adjSAR.
-
HCO3+CO3 – Bicarbonates (HCO3) and Carbonates
(CO3) are common in natural waters. High levels may
lead to sodium dominant soils and to precipitation
of calcium and magnesium salts.
-
Cl – Chloride is an essential nutrient in small
amounts. However, excessive levels are toxic. The
following guide can be used to predict toxicity
problems due to chloride in the water:
|
Cl levels (meq/L) |
Expected plant
response |
|
<2 |
Generally safe |
|
2-4 |
Sensitive
plants show injury (trees/vines) |
|
4-10 |
Moderately
sensitive plants show injury (annuals and
short-lived perennials |
|
>10 |
May cause
severe problems (most crops) |
-
SAR and adjSAR – The sodium adsorption ratio
indicates the relative activity of sodium, calcium,
and magnesium ions in soil or water reactions. The
adjusted SAR includes the additional effect of
carbonates and bicarbonates. The following guide can
be used to relate adjSAR values to potential soil
and plant problems.
|
Problem Type
|
No Problems |
Slight
Problems |
Severe
Problems |
|
Toxicity
|
<3.0 |
3.0-9.0 |
>9.0 |
|
Penetration
|
<6.0 |
6.0-9.0 |
>9.0 |
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Boron – Small amounts of Boron (B) are essential
for plant growth. However, high levels may be toxic.
Most trees, vine and bean crops are sensitive to
excess boron, while forages, field crops and certain
vegetables are relatively tolerant.
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Nitrogen – In waters and wastewaters, the forms
of nitrogen of greatest interest are nitrate,
nitrite, ammonia, and organic nitrogen. All these
forms of nitrogen, as well as nitrogen gas, are
biochemically interconvertible and thus are
components of the nitrogen cycle.
Nitrate generally occurs in trace quantities in
surface water, but can reach high levels in
groundwater. In excessive amounts, it contributes to
the illness known as methemoglobenemia. A limit of
10 mg/L Nitrate-N has been set in order to prevent
this disorder.
Ammonia is naturally present in surface and
groundwater and in wastewater. It is produced
largely by the de-amination of organic
nitrogen-containing compounds and by the hydrolysis
of urea. It is also produced naturally by the
reduction of nitrate under anaerobic conditions.
Organic nitrogen includes such natural materials as
proteins and peptides, nucleic acids and urea, and
numerous synthetic organic materials.
Traditionally “Kjeldahl nitrogen” is the sum of
organic and ammonia nitrogen. JMLord, Inc. uses a
LECO Nitrogen analyzer which detects all forms of
nitrogen during the analytical process. Thus the
“Total Nitrogen” reported is a sum of all the forms
of nitrogen discussed here.
-
TDS – Total Dissolved Solids is another measure
of total salt content in water. It may be estimated
my multiplying the ECw result by 640 and expressed
as mg/L (ppm).
Useful Conversions
1 dS/m = 1 mmho/cm = 1000 micromhos/cm
1 percent = 10,000 ppm
1 ppm = 2.72 lbs/Ac-Ft of water
1 Gallon water = 8.345 lbs
1 Ac-Ft = 325,851 gallons
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