Agriculture Reference
In-Depth Information
Tests frequently employed in diagnosing salinity/sodicity problems are summa-
rized below:
•
pH
•
Electrical Conductivity (EC)
•
Sodium Adsorption Ratio (SAR)
•
Exchangeable Sodium Percentage (ESP)
•
Cation Exchange Capacity (CEC)
•
Lime Estimate
•
Total Dissolved Solids (TDS - water only)
Anions and cations: e.g., Ca
2+
,Mg
2+
,Na
+
,Cl
-
,SO
4
2-
,CO
3
2-
,HCO
3
-
•
•
Available gypsum (CaSO
4
·
2H
2
O) and gypsum requirement
•
Soil texture estimate
•
EC of Irrigation water
Soil Sampling
The soil samples should represent the salt-affected areas only, instead of the entire
field. The top 10 cm or plow depth should be sampled if the salt accumulation is
induced by irrigation or other farming practices. On the other hand, samples from 0
to 15 cm and 30 cm increment below the surface need to be collected to evaluate the
extent of brine contamination. A similar sample from nearby normal soil is often
useful as a benchmark for comparison. Multiple subsamples (15-20 cores with a
soil probe) are needed to make a representative composite.
Composite soil sampling may not provide an accurate measurement of the overall
salinity level of a field. To assess a suspicious area of a field for salinity, take soil
samples to 0.6 m from the affected area and an adjacent non-affected area. In many
cases, comparison of soil samples from the affected area and surrounding normal
appearing areas will be beneficial in diagnosing the problem. If you wish to map an
entire field for its salinity status, there are indirect measurements using specialized
equipment that can be used.
To determine the source of salinity, dig a pit in the soil of both the affected
and unaffected areas, check for salt particles and check for carbonates using dilute
hydrochloric acid (HCl). Since soluble salts are more mobile than carbonates,
this can be used to determine the net direction of water movement. Observation
wells and piezometers may be installed to identify recharge and discharge
areas.
In general, when sampling for salinity or sodicity problems, collect a compos-
ite sample of several soil cores from the active root zone or the zone you plan
to manage, concentrating on the areas that appear most impacted. High pH, salt
or sodium levels are rarely uniformly distributed across fields. Areas of the field
suspected of these problems should be mapped and sampled separately to fully
understand the severity of the problem. Be sure to tell the laboratory that you sus-
pect a problem and ask that they analyze for SAR or ESP and perform a gypsum
test, if necessary. Tentative test required for different problems are summarized in
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