Geoscience Reference
In-Depth Information
13.6
ELEMENTS OF LAND RECLAMATION
13.6.1
Salt Tolerance of Plants
Effect of salts
The osmotic pressure
j
is related to conductivity by:
j
atmospheres
= -0.36 EC
mmhos/cm
Thus a conductivity approximately equal to that of seawater (50
mmhos/cm) is required for the osmotic pressure to prevent most plants
from pumping water (18 atmospheres). Actually, in unsaturated soils,
this osmotic pressure is added to the hydric pressure related to release
of the water in soil.
At high salt contents in the soil, physiological disorders appear. These
are nutritional problems caused by an excess of Na compared to Ca or
the result of chloride toxicity. When the concentration of
Cl
-
exceeds 5-10
meq/l, many plants are affected. Their leaves turn brown and necrose
laterally starting from the tip. The most resistant plants tolerate 30
meq/l. The mechanisms are more or less clear: inhibition of chlorophyll
production, deficiency of K, Ca and Mg and disrupted absorption of
nitrates (Dellal
et al
. 1988). Table 13.2 gives a simple interpretative key
(FAO 2005).
Table 13.2
Tolerance of plants to salts (FAO 2005).
EC, dS/m
Salts, g/l
Effects
<2.0
---
Negligible
2.0-4.0
<1.5
Some harm to sensitive crops
4.0-8.0
1.5-3.5
Serious harm to most crops
8.0-15.0
3.5-6.5
Only tolerant plants resist
>15.0
>6.5
Few plants survive
Lastly, the frequent corollary of excess salts is often ignored, at least
in maritime environments: many plants suffer because their roots are
submerged in water. Scientists have tried their best to differentiate these
two factors (salt and water) in the reduction of growth (Rogers and West
1993). The results obtained greatly depend on species. Some species have
systems whereby they take in atmospheric oxygen, permitting them
to tolerate submergence (Chap. 12, ยง 12.6), but others do not. Along
with salinity, the duration of submergence seems to be a fundamental
ecological criterion for distribution of plant species (Rogel
et al
. 2001b).
