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Copyright 2005, Board of Trustees, University
AGRICULTURAL RESEARCH AND EDUCATION CENTER
/ A, IFAS, University of Florida
C Bradenton AREC Reseatch Report GCi976-12 October 1976
PHYSIOLOGICAL EFFECTS OF HIGH SOLUBLE SALTS ON lPLATrr GROWTH
SS. S. woltz
The two main effects of high soluble salt contents in the soil solution are:
First, to reduce or stop water uptake by roots, or even to cause a loss of water
from the roots to the soil; and second, to cause nutritional imbalances or toxi-
Osmotic pressure (OP), or water-pulling-power, is the key to the balance of
water movement between roots and soil solution. OP expressed in units of atmos-
pheres of pressure determines how readily water moves into roots or whether it
moves into roots or whether it moves out into the soil. The normal OP of plant
roots is in the range of 3-5 atmospheres. If the OP of the soil solution i less,
water is taken up; if more, it is lost. This is where the cotiept of "salt"' con-
centration becomes important. Thus, a plant with an average root OP of 4 atmos-
pheres would be in equilibrium with a salt solution of 5500 ppm and would take
up no water to replace any water loss from the top of the plant. If the OP in
the root zone were higher, the roots would lose water under any environmental
Plants use more water, by far, than any of the other chemical compounds they
take up. They probably do not need to transpire large amounts of water for pur-
pose of growth and nutrition but plant leaves are "designed" for gas exchange,
especially of carbon dioxide and oxygen; water vapor is lost in the process but
does have a cooling effect on the leaf in the process of evaporating. They are
therefore quite vulnerable to excess water loss, with the resultant depletion of
available soil moisture.
The OP of most agricultural soils is less than 1 atmosphere except when they
are dry. The soil itself competes with the roots for water. As OP is increased
in nutrient solutions, water uptake by plants is decreased. There is an adapta-
tion, however, whereby plants from a lower OP solution are able to adjust when
moved to a higher OP solution by increasing their internal salt concentration.
Also, plants can adapt to a limited degree to increasing OP or salt content in
soil solution if the change is not too rapid.
Most species of plants do not develop normally when the soil solution OP
exceeds a few atmospheres (exceptions are xerophytes, desert plants and halophytes,
salt-tolerant plants). Mangrove trees, for example, can extract relatively pure
water from sea water by the expenditure of energy.
In addition to the effects on water uptake, salts from irrigation water may
produce nutritional imbalances and toxicities and may adversely affect the physi-
cal condition of soils. Sodium tends to make soils "run together,' to be wet,
non-aggregating and subject to poor drainage and aeration; light, sandy soils are
not affected greatly, however. Sodium also interferes with the uptake of other
positively charged ions that are nutritionally important including potassium,
magnesium and especially calcium. Excess sodium may induce calcium deficiency
or poor quality of plant products due to low calcium availability. Potential
toxic elements in irrigation water include boron, fluoride, lithium and bicarbonate.
In the case of boron, there is a need to learn how to fertilize properly according
to the amounts of boron present in the irrigation water. Analyses of water and
leaf samples will aid in clarifying this situation.
Large amounts of the negatively charged ion, bicarbonate, are desirable.
If bicarbonate is present much in excess df the chlride plus sulfate content, the
bicarbonate can cause the precipitation of calcium and magnesium and the production
of sodium-saturated soils that have poor physical condition. The bicarbonate ion
also increases the incidence of Ve deficiency.
In regard to the reduction or elimination of the effects of high soluble salts,
we may suggest the following procedures:
1. Avoid excess use of chloride, sodium and sulfate in fertilizer to reduce
the salt input of unnecessary elements.
2. Test your irrigation water and soils regularly to see if salt levels are
3. Select sources of water of the best quality available.
4. Provide good drainage to remove salts; leach as much as possible during the
5. Double row beds that are not thrown up too high are less subject to salt
damage for seeded crops. Salts move to the highest place in the bed or to the top
center of a double-row bed. Sloped beds may sometimes be used with seeding on the
6. Be careful to avoid letting very light soils dry out since a 50% loss of
available soil moisture approximately doubles the salt concentration.
7. If a salt problem has been encountered especially in a heavier soil -
apply 1000 to 2000 lbs/A of gypsum to improve soil condition and help remove sodium
by subsequent leaching.
8. When possible, decrease the salt level in the soil by leaching even with
slightly salty water if necessary. With seep irrigation one may raise the water
level and then drain to help remove dissolved salts. Overhead irrigation is the
ideal way to remove salts by leaching. The saltier the water, the more leaching
one will have to do in order to use the salty water in production.
9. Salts accumulate in hard-pan subsoil pockets or depressions in the hard-pan
profile. These small areas that will damage a planting may be corrected by break-
ing the hard-pan at the center of the affected area and leaching out the salts or
by the use of tile drainage. The areas described will surprisingly persist even
with heavy rains or heavy irrigation in the absence of corrective measures.
A physiological character of root growth that has not been discussed is the
failure of roots to grow into a zone of high soluble salts because of water
limitations to the root. If fertilizer is placed too close to roots, they will
probably be burned, but roots will not grow into excessively salty soil, because
of the water deficit that occurs for these roots.
If soluble salts build up too high in soil, the plants will make little growth
and leaves may be burned. Plants grown under high salt conditions will have an
appearance as if they were grown in a drought with inadequate irrigation. They
will have dull-green leaves, slow rate of growth and poor yield of produce.