Management of Salt-Affected Soils - Practices of Irrigation and On-farm Water Management - page 294

Salinity has five main impacts on plant nutrition:

(i) The first is the osmotic effect. As salts accumulate in soil, the soil-solution
osmotic pressure increases. When this happens, the amount of water available
for plant uptake decreases and plants exhibit poor growth and wilting even
though the soil is not dry. Thus, salts in the soil water solution can reduce
evapotranspiration by making soil water less available for plant root extraction.
When the salt concentration in the soil solution exceeds the salt concentration
inside plant roots, this causes water to move out of the roots. In extreme cases
plants can be left severely dehydrated.
(ii) The second impact of salinity on plant nutrition is a specific ion effect which
has two outcomes. Saline soil can cause toxic accumulation of ions - sodium
and chloride, inside plant cells, which hinder vital physiological processes.
Salinity causes reduced root growth due to osmotic pressure and toxicity.
Thus, high salt concentrations limit the plant root's ability to take up water
and nutrients, which restricts crop growth and reduces yields.
(iii) The third is the interfering uptake of competitive nutrients. Some ions have an
antagonistic effect on the uptake of other ions or elements. Table 8.8 shows
some of the specific ion effects that can occur in saline soil.

Table 8.8 Specific ion effect

Ions accumulating in plant
cells

Ions inhibited from entering
plant

Ca 2+ K +

Na + Zn 2+

Cl - NO 3-

SO 4 2- PO 4 3-

(iv) The fourth impact of salinity on poor nutrition of crop is the slow mineraliza-
tion of organic nitrogen in saline soil.
(v) The fifth impact of salinity is that it inhibits plant growth as a result of stom-
atal closer, which reduces the CO 2 to O 2 ratio in the leaves and inhibits CO 2
fixation. As a result, the rate of leaf elongation, enlargement and cell division
is reduced.

The availability of phosphorous in saline soil depends on the pH and the degree
of salinity of the soil. In general, toxicity of iron may develop in saline soil; and
deficiency of phosphorus and zinc may occur. The soil may become acidic. From
various research findings, it is revealed that for a particular salinity level, the effect
of a single salt is different from that of the multiple salts.

8.4.2 Impact of Sodicity on Soil and Plant Growth

Sodicity in soils has a strong influence on the soil structure of the layer in which
it is present. Sodic soils are low in total salts but high in exchangeable sodium.