Chemistry Reference
In-Depth Information
which contain relatively high concentrations of salts, including sodium, there is
an increased risk of salinisation of soils.
The response of plant growth to salts differs markedly between species,
ranging from highly sensitive species such as rice, through moderately tolerant
species such as barley and alfalfa, to highly tolerant (halophytic) species such
as saltbush (Atriplex amnicola).
39
Often saline and sodic soils affect plant
growth through one or more sets of processes: (i) physiological drought,
resulting from a reduction in water-availability caused by reduced osmotic
potential; (ii) ionic imbalance in plant cells; and (iii) toxicity due to specific
ions, such as sodium, chloride or borates. In addition, high pH may also result
in plant stress and have effects via nutrient availability and plant metabolism.
In controlled experiments, plant-growth responses to salt stress are frequently
observed as a two-phase process.
40
Initially, the response can be attributed to
osmotic stress but as the exposure to salt increases, salt-specific effects in the
older leaves of plants result in premature leaf senescence and associated
reductions in leaf photosynthetic area, leading to reduced plant growth. The
initial response to salt is due to osmotic stress which causes cell dehydration
and shrinkage in the root, reducing the ability of the plant to take up water; if
the salt is removed, this is a transient effect that can be reversed, but under
typical field conditions the plant must expend increasing amounts of energy to
maintain turgor. For example, in a sandy loam, plants under non-saline
conditions were able to maintain turgor with a water content of only 5%,
whereas at an electrical conductivity in a soil-water (1 : 5) extract of 1 dS m
21
,
a water content of at least 18% was required to prevent plants wilting.
25
Crop plants exhibit a variety of mechanisms that confer some degree of
tolerance to saline or sodic conditions, including: (i) selective exclusion/
inclusion of ions; (ii) induction and up-regulating of antioxidative enzymes;
(iii) accumulation of certain organic solutes to increase hyperosmotic
tolerance; and (iv) efficient salt excretion via salt glands or tissue shedding.
Typically, salt-tolerant plants have slower rates of sodium and chloride ion
transport to leaves than sensitive plants and a greater ability to compartmen-
talise these ions into vacuoles, so preventing their accumulation in the
cytoplasm or cells walls.
39
Tolerance of salt is not a simple property and is a
genetically complex trait affected by many genes with additive, dominant and
reciprocal effects.
41,42
d
n
1
r
2
n
g
|
1
d
n
6
h
3
2.3 Soil Strength and Structure
The rate of root growth, and hence the ability of plants to exploit water and
nutrient reserves in soils, is affected considerably by soil strength.
43
This
strength is a consequence of capillary forces and bonds between particles in
soils. Management practices resulting in compaction or loss of organic matter
generally increase soil strength and decrease root growth. The arrangement of
soil materials, gases and water determines soil structure, which has been
defined as the ''spatial heterogeneity of the different components or properties
