6.6 Soil Acidity Explained
What is soil acidity?
Soil acidity occurs when there is a build up of acid in
the soil. The production of acid in the soils is a natural process and
many soils in the high rainfall areas of SA are inherently acidic.
Acidification is a slow process but it is accelerated by agriculture.
As soils become more acidic, plants intolerant of acidic
conditions do not thrive and productivity declines.
How fast does soil acidify?
The rate at which a soil acidifies depends
on:-
-
Soil type
Light sandy soils with little clay or
organic matter have lower buffering capacity and therefore acidity
develops more quickly than on heavier soils.
-
Rainfall
Higher rainfall increases leaching of
nutrients which in turn increases acidification.
-
Land use
Higher production increases the rate of
acidification. Shallow rooted plant systems also increase
acidification compared with deep-rooted plants.
Figure 1:
Examples of acidification rates for
different farming systems
|
Farming system
|
Acidification rate*
|
|
Extensive grazing
|
10-25
|
|
Improved pasture
|
50
|
|
Cropping
|
75-100
|
|
Cropping with high N input
|
400
|
|
Horticulture with high N input
|
up to 500
|
|
Typical hay paddock
|
300
|
|
Lucerne hay
|
500-600
|
*kg lime/ha/year required to neutralise
acidity
What causes soil acidity?
The natural rate of acidification is
accelerated by agricultural practices. These include: Use of nitrogen
fertilisers The impact of nitrogen fertilisers on acidification depends on
the type of fertiliser and what happens to the nitrogen (see Table 2).
Figure
2:
Lime required to counteract
acidity caused by fertilisers in acid soils.
|
Fertiliser
|
Acidification
(kg lime/kg of N (or S) fertiliser)
|
|
minimum
|
maximum
|
average
|
|
Anhydrous ammonia
|
0
|
3.6
|
1.8
|
|
Urea
|
0
|
3.6
|
1.8
|
|
Ammonium nitrate
|
0
|
3.6
|
1.8
|
|
Ammonium sulphate
|
3.6
|
7.2
|
5.4
|
|
DAP (18:20)
|
1.8
|
5.4
|
3.6
|
|
MAP (10:22)
|
3.6
|
7.2
|
5.4
|
|
Goldphos (0:18:0:10)
|
|
3
|
|
|
Superphosphate
|
|
|
nil
|
|
Muriate of potash
|
|
|
nil
|
Removal of plant products
The removal of plants products leaves the soil more
acidic. Highest acidification rates occur when hay is cut and fed on a
different paddock or sold off the property. Similarly, the use of night
paddocks transfers alkalinity from the other paddocks to the night
paddocks.
Figure 3:
Lime required to counteract acidity caused
by product removal.
|
Plant product
|
Lime requirement*
|
|
Lucerne hay
|
60
|
|
20% subclover/annual grass
|
15
|
|
40% subclover/annual grass
|
30
|
|
60% subclover/annual grass
|
40
|
|
80% submedic/annual grass
|
50
|
|
perennial ryegrass hay
|
40
|
|
cereal hay
|
22
|
|
phalaris/cocksfoot hay
|
30
|
|
wheat grain
|
5 - 10
|
|
barley grain
|
5 - 10
|
|
Wool
|
14
|
|
Milk (1000L)
|
4
|
*kg CaCO3 per tonne
Soil pH
Soil pH is a measure of the acidity or alkalinity of a
soil. Acid soils have a pH lower than 7, alkaline soils have a pH greater
than 7. The lower the pH, the more acid the soil.
There are two common ways of measuring soil pH - in water
and in calcium chloride. The latter is preferred for acidic sols because
results are generally more consistent. pH measured in calcium chloride are
generally 0.5 to 1 pH unit lower than if it is measured in water. Make
sure you know which method has been used!
|
Animal product removal
The removal of animal products also cause soil
acidification but rates are generally low.
For example at normal
management levels for milk, wool or meat production, the acidity produced
is only equivalent to about 1 to 10 kg of lime per ha per year.
Note: Superphosphate does not directly cause acidity,
but may increase the process indirectly by promoting better pasture growth
or higher productivity.
Module 6- Barriers to Root Growth
6.6 Soil Acidity Explained
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