••••
T RADE N E W S
Management of
sub-soil acidity
SUB-SOIL ACIDITY CAN
OCCUR NATURALLY IN
HIGH RAINFALL AREAS,
OR CAN BE THE RESULT
OF ILL-CONSIDERED OR
EXCESSIVE APPLICATION
OF NITROGEN, AND LACK
OF REGULAR LIMING OF
THE TOP SOIL.
A
dvisors find that sub-soil
acidity increasingly occurs
in the lower rainfall areas, which
usually goes hand-in-hand with
the practice of applying nitrogen
before planting.
The placing of nitrogen is usu
ally deeper than in the case of lim
ing. Lime has to be worked deeper
into soil where underground acid
ity is observed in order to neu
tralise the soil acidity, the reason
being that neutralisation is carried
from the top soil to the sub-soil
at a very slow pace. The deeper
placement of lime is an expensive
process which has to be done with
specialised or adjusted equipment.
The high cost of this specific
action and the irregular liming of
the top soil result in a worsening
of the problem of underground
acidity, which has a negative
impact on crop production. The
depth of the soil profile becomes
more shallow due to the chemical
limitation in respect of the devel
opment of roots, which means
that the moisture in the sub-soil
cannot be utilised. If this problem
is not addressed, it can result in
serious consequences, such as
lower yields. Table 1 reflects the
quantity of pure CaCO3 which
is required for the neutralisation
of the resultant acid per kilogram
pure nitrogen (N) applied as fer
tiliser (Moore, 1998).
Nitrogen is applied annually,
but not everything is taken up
by the crops, which means that
the remainder seeps through the
soil profile. Table 1 reflects the
Table 1: Neutralisation requirements relating to the acetification of nitrogen-
containing fertiliser with CaCO 3 .
Nitrogen fertiliser
CaCO 3 needed to neutralise
the addition of nitrogen
(kg CaCo 3 /kg N) No leaching CaCO 3 needed to neutralise
the addition of nitrogen
(kg CaCO 3 /kg N) 100% leaching
Ammonium sulphate
3.6 7.1
Ammonium nitrate 0 3.6
Urea 0 3.6
DAP
1.8 5.4
Potassium nitrate -3.6** 0
Sodium nitrate -3.6** 0
** Neutralisation by the fertiliser itself
36
JUN/JUL 2017 • SENWES Scenario
neutralisation requirements and
the quantity of lime required. The
lime which is applied, moves very
slowly through the soil profile if
it is not worked in. In the case of
normal application, lime is applied
to the top part of the soil profile.
The slow movement of the lime in
the profile is approximately 8 to
12 mm per year, depending on the
texture of the soil and on whether
the limed part of the profile main
tains a favourable pH. The down
ward movement or neutralisation
takes place by means of root action
and insect action, e.g. earthworms.
Should liming programmes be
maintained over years, it is proba
ble that sub-soil acetification will
not take place as fast as is the case
at present. An acceptable practice
is to apply half of the sub-soil lime
requirements with the top soil lime
requirements. This management
practice should be maintained due
to the slow movement of the lime
in the soil profile.
A gypsum and lime combina
tion can be applied to accelerate
the neutralisation of sub-soil acidi
ty, should sesquioxides be present.
The application thereof should
be done with care since seeping
of magnesium and potassium can
take place from the top soil to the
sub-soil in low percentage clay soil.
A safe ratio of gypsum and lime is
20% gypsum and 80% dolomitic
lime. It is advisable to mix dolo
mitic lime, which contains potas
sium, with the gypsum to counter
the seeping of magnesium.