Lab diagnosis of salinity and alkalinity
Soil testing labs can measure the soluble salt content of soils
through electrical conductivity tests. Since salts are electrolytes, the higher
the salt content, the higher the electrical conductivity (EC or Ece). The
readings are expressed in millimhos or micromhos. Readings can range fro. 0 to
over 16 millimhos (16,000 micromhos).
Soluble Salt Content of Soils and its Effect on Crop Growth
Electrical Conductivity (EC)
Less than 2
Less than 2000
No adverse effect.
Yields of some salt sensitive crops are affected.
Crop must be moderately salt tolerant.
Crop must have good salt tolerance.
No profitable cropping possible.
Labs can also measure the amount of adsorbed soil sodium to
determine alkali danger.
Irrigation Water Analysis
The soluble salt, sodium, and boron content of irrigation water
can also analyzed.
SOLUBLE SALTS: The U.S. Salinity Lab classifies irrigation water
in 4 categories according to soluble salt level:
Class 1: Low Salinity
100-250 micromhos/cm of depth (0.1-0.25 millimhos/cm). Safe to
use on nearly all crops and soils. Some leaching is needed to keep salts moving
downward. Salinity problems may develop on poorly drained soils.
Class 2: Medium Salinity
251-750 micromhos/cm (0.25-0.75 millimhos/cm). Can be used to
irrigate relatively permeable soils. Crops need to have medium salt tolerance,
and leaching will be needed.
Class 3: High Salinity
751-2250 micromhos/cm (0.75-2.25 millimhos/cm). Only for
salt-tolerant crops. Adequate drainage is a must as well as ample leaching.
Class 4: Very High Salinity
Above 2250 micromohos/cm (2.25 miIlimhos/cm). Can't be used for
irrigation except under certain these conditions permeable soil, good drainage,
and high water application rates to obtain good leaching.
SODIUM IN WATER: The sodium content of irrigation water can be
accurately measured, but the potential toxicity depends on the proportion of
sodium in the water relative the combined calcium and magnesium content. This is
known as the Sodium Adsorbtion Ratio (S.A.R.) and is determined by the lab. As
with soil, irrigation water can be grouped into 4 categories, ranging from Class
1 (low-sodium water) to Class 4 (high-sodium water), but the actual
interpretation isn't as simple as it might appear.
MANAGING; SALINITY AND ALKALINITY PROBLEMS
Reclaiming Saline Soils
Since saline soils contain only soluble salts, leaching is the
cure. In many cases, however, salinity is caused by a high water table, and
leaching won't be effective until artificial drainage has been installed such as
underground tile drains. Deep ripping (subsoiling) may be needed to break up any
hardpan that is restricting drainage.
Either periodic leaching or continuous flooding may be used.
Salt-tolerant crops like beets, cotton, and barley can be grown during
reclamation if flooding isn't used. The amount of water needed for leaching
depends on the soluble salt content of the soil and water and the final salt
level desired. As a rough guide, about 50% of the soluble salts in the root zone
can be removed with 15 cm of water applied per 30 cm depth of soil (15 cm = a
layer of water 15 cm deep or the equivalent of 150 liters per sq. meter). About
80% of the salts can be removed by applying 30 cm of water per 30 cm depth depth
of soil (300 liters per sq. meter).
The leaching requirement is the ratio of the salt content of the
irrigation water to that of the soil. For example where an EC of 8 can be
tolerated in the root zone and the irrigation water has an EC of 8 the leaching
requirement is 2/8 or 25%. This means that 25% more water should be applied to a
crop than is used up by evaporation and plant transpiration. (To help determine
crop water needs, see Chapter 5 on water management).
Reclaiming Non-Saline Alkali Soils (Sodic-Soils)
Leaching alone won't remove the adsorbed sodium and insoluble
sodium carbonate and bicarbonate. You need to add a soil amendment such as
gypsum (calcium sulfate) first which reacts with the soil in 2 ways:
· It converts insoluble sodium
salts like sodium bicarbonate into soluble sodium sulfate which is mobile and
· It also detaches the adsorbed
sodium ions adhering to the clay and humus particles and replaces them with
calcium. This also results in a lowering of soil pH after leaching.
The gypsom needs to be finely ground and should be hoed oe
harrowed into the surface rather than turned under with a moldboard plow. (The
turning action of a moldboard ends up leaving the gypsum poorly distributed in
The soil should be kept moist to promote the reaction of the
gypsum and soil. At least 5000 kg/ha (500 g/sq. meter) of gypsum is needed, but
the soils lab will determine the exact amount.
On those sodic soils containing calcium carbonate (lime), sulfur
can be used instead of gypsum. The soil bacteria convert it to sulfate which
then reacts with the calcium carbonate in the soil to form gypsum. Much less
sulfur than gypsum is needed (1000 kg of sulfur have the effect of 5380 kg of
gypsum). Allow 2-3 months between sulfur application and leaching to allow the
conversion to gypsum. The presence of calcium carbonate can be detected by
adding several drops of sulfuric or hydrochloric acid to a small amount of soil.
If fizzing occurs, this indicates calcium carbonate.
An alternative to soil amendments: Recent research by the USDA
has shown that sorghum-sudangrass hybrids can be effective in freeing adsorbed
sodium on sodic soils containing calcium carbonate. Sorghum-sudangrass (often
called sordan) roots release unusually high amounts of carbon dioxide; the CO2
dissolves soil calcium which can then displace the adsorbed sodium ions. Heavy
irrigation is needed to flush away the freed sodium. Where suited, the use of
sordan is much cheaper than gypsum and reclaims a greater depth of soil; it is
also a nutritious cattle feed. However, plants less than 45 cm tall or those
that have been drought-stricken or frosted contain toxic levels of hydrocyanic
acid (prussic acid).
Reclaiming Saline-Alkali Soils (Saline-Sodic Soils)
As with sodic soils, leaching alone isn't effective on
saline-sodic soils, because it removes only the soluble salts, leaving behind
the adsorbed sodium. Free fro. the buffering effect of the soluble salts, the
sodium can now exert its full effect in raising the pH and deteriorating soil
physical condition. In other words, leaching, by itself, converts a salinesodic
soil into a sodic soil! In a few cases, leaching without using gypsum or sulfur
may be effective when the soil contains a large amount of soluble calcium or
magnesium which can displace the adsorbed sodium.
Controlling the Buildup of Salinity and Alkalinity
It's seldom possible to permanently rid an affected soil of
salinity and alkalinity, especially if the irrigation water is one of the
causes. The beat strategy is to use management practices that favor crop growth
on these soils and help keep salt content at tolerable levels. Here are some
· Treatment of irrigation water:
There's no economically feasible way to reduce the soluble salt content of
irrigation water, but the sodium hazard can be virtually eliminated by adding
gypsum to the water. Automatic gypsum metering devices can be bought or built,
or a sack of gypsum placed in the irrigation ditch.
· Use of soil amendments: Gypsum
or sulfur may need to be periodically applied where conditions are favorable for
the development of sodic or saline-sodic conditions.
· Crop selection: Choose crops
that are tolerant to saline and alkali conditions. Boron tolerance may be an
important consideration too. Tables at the end of this chapter list co on crops
and their tolerances.
· Improving soil drainage: Deep
ripping of the soil or double-digging by hand (see Chapter 4) may be needed to
break up hardpans or compacted layers to facilitate downward water movement.
· Improving soil physical
condition: Organic fertilizers and soil conditioners (see Chapter 8) have an
especially beneficial effect.
· Land leveling: This will
smooth out depressions and rises to give more even distribution of irrigation
water which will help prevent the development of pockets with high salt
· Watering management: There are
several watering practices that are vital in controlling salinity buildup in
·· Water frequently to maintain a good soil moisture
content to facilitate plant water uptake, which is hampered by the osmotic
"pull" of the salts.
·· When watering, apply about 10-20% more water than
needed in order to produce enough leaching for salt removal. Leaching assures
that the soil salt content will never be higher than that of the irrigation
water. Without leaching, soil salt content will gradually increase well above
that of the water as salts accumulate instead of being flushed out.
·· Where furrow irrigation is used, increasing the water
level in the furrows will aid in seed germination. Placing the seed or
transplants just above the water line will help lessen salt buildup around them.
(See Figure 12-1.)
·· Some sources recommend drip irrigation as a way to
efficiently maintain good soil moisture and leaching of salts in the immediate
plant area. Others caution that drip irrigation can produce salt buildup just as
other methods do, especially at the borders between the wetted area and dry
· Seedbed design: Furrow
irrigation will cause salts to accumulate near the germinating seeds or the
plants unless special attention is given to seedbed design. Figure 12-1 shows
how bed design and irrigation techniques determine where salts accumulate and
where seeds and plants should be situated.
FIGURE 12-1: Seedbed design and
irrigation technique influence where salts accumulate. Single-row beds (A) with
every furrow irrigated cause salt accumulation around plants. Conventional,
multiple-row beds (B) or slanted, multiple-row beds (C) provide crop space in
areas of lower salt concentration. Alternate furrow irrigation (D) concentrates
salts in the unirrigated furrow away from crop roots.
TABLE 12-2 Relative Tolerance of
Crops to Salinity' (Listed in order of decreasing salt tolerance within each
"high", "medium", and "low" grouping)
TABLE 12-3 Relative Tolerance of Some
Crops to Boron
(Listed in order of decreasing tolerance to boron within each