Home' Grower : October 2011 Contents The South Australian Grower -- October 2011
cause more damage.
Crops with poorer root systems have to
spend more energy to access the soil water,
energy that would other wise go into
growth. Limited movement of soil water to
the roots may result in plants suffering
water stress earlier even though the soil
may appear moist.
Plants are more likely to wilt during heat
waves because they cannot take up the soil
water rapidly enough to replace amounts
lost through the leaves and stems.
Stronger, compacted soil can cause
deformation of carrots, parsnips and other
root crops making the product unsuitable
for human consumption, with considerable
loss of profit.
Plants need to continually develop feeder
roots throughout the growing season to
take up nutrients with limited mobility in
soil such as phosphorus, potassium,
calcium, and magnesium.
Root development can be severely
restricted in stronger (compacted), poorly
aerated soil, leading to nutrient
imbalances. Adequate phosphorus is
needed to aid cell division of the
developing root tip while calcium
strengthens the cell walls to assist
penetration of the root tip in soil.
Calcium is associated with the surfaces of
clay in soil, along with magnesium, sodium
The relative balance of these
exchangeable cations is important for crop
nutrition. In a well balanced soil cations
are present within a defined ratio.
When the exchangeable sodium
percentage is greater than 6 per cent then
the availability of exchangeable potassium,
calcium and magnesium is restricted, the
plants will take up sodium instead of
potassium. This weakens cell walls and the
ability of the plants to control moisture
loss from their leaves. As a result plants will
be susceptible to wilting in warm weather.
The pi chart of optimal percentages to
the left shows that in healthy soils, calcium
(Ca) is the dominant exchangeable cation
whereas sodium (Na) is relatively minor.
Hydrogen is also important in helping to
buffer the soil against pH changes.
Continual irrigation with water high in
sodium promotes displacement of
exchangeable calcium by sodium.
The example to the right is from a soil
that is affected by sodicity. Calcium levels
have dropped to the low 60pc, and
hydrogen has been lost. Soils that are low
in hydrogen ions are alkaline.
Sodic soil has a highly alkaline pH,
which adds to nutritional problems.
As pH becomes more alkaline
phosphorus and trace elements including
iron, manganese, copper and zinc become
less available to plants even when there is
an adequate reser ve.
Do you have a salinity or sodicity
SODIUM chloride is a common
component of salty water sources. It is
therefore important to test both the soil
and plant tissue to determine whether you
have either a salinity or sodium problem,
• Details: 08 8523 7718 or
Figure 3: Cavity spot in carrots aggravated
by low calcium and high sodic soils
Desired Balance of Exchangeable Cations (Per Cent)
Exchangeable Cations Percentage of a Sodic Soil
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