Home' Grower : May 2012 Contents The South Australian Grower -- May 2012
Acidification in Horticultural Systems
Step up Soil Condition Program
Hortex Alliance Incorporated
Soil acidification is a natural
process that has become a
serious land degradation
issue through agricultural prac-
tice. Acid soils limit the choice of
crops and pastures that can pro-
vide an economic return. Growers
have fewer options to manage
other land degradation issues such
as compaction, shallow water
tables and erosion.
What is Acidity?
Acidity is due to the presence of
hydrogen ions in soil. Ions are
charged particles. The hydrogen
ion has a positive charge and is
represented by the symbol H+.
The pH is a measure of the con-
centration of H+ in a solution.
The pH scale ranges from 1 to 14.
Each unit change in pH is a 10-
fold change in acidity. Neutral
soil has pH 7 and acid soil has a
pH below 7. Most plants grow
between pHCa 5 and 8.
Acidic soil pHCa<5.5: Over
time, acidity can move down from
acid topsoil to the subsoil where it
becomes more difficult to man-
age. Sensitive plants may become
impacted by higher availability of
manganese (Mn) in certain soil
types. Affected plants tend to
crinkle and cup.
Strongly acidic soil pHCa<4.8:
Effects of aluminium (Al) tend to
dominate over those of Mn.
Soluble Al can enter roots passive-
ly and damage their growth.
Affected plants are unable to take
up sufficient water and nutrients
from soil for shoot growth, reduc-
ing yield and quality.
Very low pH can also lead to
deficiencies in calcium, magne-
sium and potassium due to leach-
ing, and molybdenum and phos-
phorus due to reduced availability.
Microbial populations and worms
are also impacted, affecting nutri-
ent cycling, nodulation of
legumes and nitrification of
ammonium to nitrate. There is
less dry matter production and
surface cover, increasing the risk
of wind and water erosion.
Very strongly acidic soil
pHCa<4: The acidity begins to
break down the soil minerals and
organic matter causing permanent
structural decline. Serious nutri-
tion problems are likely. Plants are
sur viving rather than producing.
Measuring Soil pH
Laboratories measure pH in
either water pHw or calcium chlo-
ride solution pHCa using a ratio of
1 part soil to 5 parts solution. Soil
pH measured in water is a better
reflection of pH that roots are
exposed to, but is subject to sea-
sonal variation. Use of calcium
chloride solution largely over-
comes the seasonal variation in pH
allowing samples taken at different
times of the year to be compared.
Results of soil tests are as good
as the sample taken hence instruc-
tions need to be followed when
taking samples, returning to the
same general area each time. Field
test kits and pH meters can be
used to check soil pH.
The major sources of acidifica-
tion attributed to agricultural
practice are linked to management
of the nitrogen and carbon cycles
(Figure 1). Gaining a better appre-
ciation of the linkages allow risk
factors to be more readily identified
and targeted by management.
1. Increasing the source of H+
through use of ammonium fer-
tilisers and growing legumes.
Acidity is released into the soil
solution when bacteria convert
ammonium NH4+ to nitrate NO3-
The process is very rapid, which
sets up the potential for acidifica-
tion. In natural systems, the acidi-
ty is neutralised when plants take
up nitrate and when the organic
--N is eventually mineralised back
2. Nitrate Leaching
Nitrate produced from ammoni-
um releases H+ the soil solution.
Plants must uptake the NO3- to
release the agent responsible for
removing H+ from the soil solu-
tion. The neutralising potential is
lost once NO3- is leached past the
active root zone. For a given con-
centration of NO3- leached, an
equivalent concentration of H+ is
left behind in the soil solution.
Due to its negative charge,
NO3- is not held by soil particles.
Nitrate is highly soluble in water
and will readily move in soil with
the passage of the water front fol-
lowing rainfall. Due to these rea-
sons some ammonium derived
nitrate will naturally leach, partic-
ularly in regions with high winter
dominant rainfall. However, agri-
cultural practice promoting NO3-
leaching can rapidly acidify soil.
The release of H+ into the soil
solution is avoided when NO3- is
applied as fertiliser. Consequently
leaching of fertiliser nitrate will
not in itself contribute to acidifica-
tion. However uptake of fertiliser
nitrate can "mop up" some of the
acidity produced from other
sources, thus leaching should be
3. Har vest or export of crops
or plant products
N-Cycle: Har vested legume
products -- hay or seed, which are
high in organic-N, will remove
some of the neutralising potential,
leading to acidification.
C-Cycle: Plants take up more
positively charged nutrient cations
(e.g. potassium, calcium, and
magnesium) than negatively
charged nutrients (e.g. NO3- and
phosphate). To maintain neutral
charge plants release H+ into the
soil solution. The amount of H+
released equals the amount of
excess positive charge resulting
from the uptake of cations.
Many plants source the H+ from
organic acids converted from sug-
ars produced by photosynthesis.
As with the nitrogen cycle, the
acidity is neutralised when plant
remains are mineralised and the
excess nutrient cations are
returned to the soil.
When plant products and crops
are har vested, or exported from
one paddock or another through
animal grazing or movement of cut
hay, the stored alkalinity or plant
ash cannot be returned, leaving the
excess H+ to acidify the soil.
Figure 1: Acidification in agricultural systems.
Source: J Chapman
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