Agriculture Reference
In-Depth Information
Chapter 2
Salt stress and leguminous crops:
Present status and prospects
P.S. Sha Valli Khan
1
and P. Osman Basha
2
1
Department of Botany, Yogi Vemana University, Vemanapuram, India
2
Department of Genetics and Genomics, Yogi Vemana University, Vemanapuram, India
2.1 Introduction
salt-rich irrigation water or causing poor drainage
(Ghassemi
et al.,
1995). Salt-affected areas occur widely
in arid and semi-arid areas (Rengasamy
et al.,
2003).
Based on soil and ground water processes found all
over the world, Rengasamy (2006) has further classified
salinity into three different types:
1
groundwater-associated salinity (GAS);
2
non-groundwater-associated salinity (NAS);
3
irrigation-associated salinity (IAS).
GAS occurs in discharge areas of the landscape. Water
permeates from groundwater to the soil surface carrying
the salts dissolved in it. The motivating force for upward
movement of water and salts is evaporation from the
soil plus plant transpiration. Generally, the water table
in the landscape is at or very close to the soil surface.
Salt accumulation is high when the water table is less
than 1.5 m below the soil surface (Talsma, 1963).
NAS is found in landscapes where the water table is
deep and drainage is insufficient. Salts carried by rain,
weathering and aeolian deposits are stored within the
soil solum.
IAS results from irrigation water accumulating
within the root zone because of inadequate leaching.
Poor quality irrigation water, low hydraulic conduc-
tivity of soil layers as found in heavy clay soils and
sodic soils, and high evaporative conditions all promote
IAS. Utilization of highly saline effluent water and
inadequate drainage and soil management enhance the
risk of salinity in irrigated soils. In many irrigation
regions, rising saline groundwater interacting with the
soils in the root zone can compound the problem
(Rengasamy, 2006).
Salinity is the build-up of mineral salts in soils and
waters. The dissolved mineral salts form electrolytes of
cations such as Na
+
, Ca
2+
, Mg
2+
and K
+
, and anions such
as Cl
−
, SO
4
2−
, HCO
3
−
, CO
3
2−
and NO
3
−
. Other elements
contributing to salinity in hypersaline soils and waters
comprise Al
3+
, B, Ba
2+
, Sr
2+
, SiO
2
and Mo (Hu &
Schmidhalter, 2002). Soil salinity is assessed by the soil's
electrical conductivity, and the SI unit of electrical
conductivity (ECe) is dS/m (decisiemens per metre).
Richards (1954) classified salt-affected areas into non-
saline (<2 dS/m), slightly saline (2-4 dS/m), moderately
saline (4-8 dS/m), highly saline (8-16 dS/m) and
extremely saline (>16 dS/m) based on the ECe.
Ghassemi
et al.
(1995) categorized salinity as 'primary'
or 'secondary' salinity. Primary salinity occurs due to
the accumulation of salts over long periods of time by
two natural processes. The first natural process is the
weathering of rocks comprising soluble salts of several
types, primarily Cl
−
of Na
+
, Ca
2+
, Mg
2+
and to a lesser
extent, SO
4
2−
, HCO
3
−
and CO
3
2−
. The second process
follows due to the deposition of oceanic salt carried
inland by wind and rain. The deposited salt mainly con-
tains NaCl, which is a component of oceanic salt
(Munns, 2005). Secondary salinity results from anthro-
pogenic activities that alter the hydrological balance in
the soil between water applied (irrigation or rainfall)
and water consumed by crops (transpiration). The most
common causes for secondary salinity are (i) land
clearance and the substitution of perennial vegetation
with annual crops, and (ii) irrigation schemes using
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