THE ROLE OF THE NITROGEN AND CARBON
CYCLE IN SOIL ACIDIFICATION
Greg Fenton1
Keith Helyar1
SYNOPSIS
I
n nature most eco-systems have evolved to an almost
stable state where the balance between acidity and
alkalinity changes slowly towards becoming more acid. In
this natural stable state the eco-system is producing a small amount
of acid that releases sufficient cations from the parent material to
maintain fertile soil.
Acidification rates increase when the land is turned over to
agriculture and the natural stable state for pH is destroyed. The
normal agricultural practices of fertilising and harvesting, along with
increases in leaching of nitrate nitrogen and occasional soil erosion,
accelerate the acidification of agricultural lands.
The challenge for farmers, and their advisers, is to establish
within their farming eco-system a new stable state of acid/alkali
balance that will to continue for 100’s and maybe 1,000’s of years.
An understanding of the processes that acidify the soil is the basis
for planning how this can be achieved.
INTRODUCTION
In general soils become more acid over time. The acidity
comes from biological cycling of nitrogen and carbon by plants,
usually called the nitrogen (N) and carbon (C) cycles. Effective
management of the problems associated with soil acidity is based
on an understanding of these chemical processes within the cycles.
1
Greg Fenton, Project Coordinator, Acid Soil Action, NSW Agriculture & Dr. Keith
Helyar, Director, Wagga Wagga Agricultural Institute, PMB, Wagga Wagga NSW
2650, Australia. E-mail: greg.fenton@agric.nsw.gov.au
Soil acidity is part of a whole complex of chemical factors that affect
plant nutrition and growth for soils with a pH less than 5.52.
As soils become more acid some plant nutrients become
deficient while others increase in availability to toxic levels. These
changes in availability of nutrients cause most of the effects on plant
growth attributed to acidic soils. In addition, soil acidity may decrease microbial activity affecting nitrogen fixation by some legumes
and mineralisation of organic matter (HELYAR & PORTER, 1989).
Acidification of the soil starts when nitric and organic acids
are produced in the nitrogen and carbon cycles by lichens and algae
that first colonise the rock surface. These acids react with the rock
breaking it down to start soil formation and to release the cations
that the plant needs to grow. This is called weathering. In time plants
replace the lichens and algae. The same C and N cycles occur in
plants and continue the weathering process. Thus there is a close
relationship between acid production and the accumulation of soil
fertility.
In an ecosystem that remained closed there would be no
change to the acid/alkali balance of the system, only redistribution
over time through the system. However loss of organic anions with
removal of plant material and through leaching of nitrate nitrogen
before the plant can utilise it leaves the eco-system a little more
acid. If these losses are small then the small amount of acid added
is sufficient to maintain a fertile soil. Such an ecosystem acidifies
slowly and this is a major part of weathering of parent material.
When the acid produced exceeds the weathering rate then
the soil becomes acid. Within the experience of the authors this
acidification is not reversed naturally. In time the acidification will
mean permanent loss of plant species that are sensitive to acidic
soils and reduced the growth of those that remain. In addition the
activity and biodiversity of the soil fauna and microbial populations is
decreased.
Agricultural practices of removal of crops and hay, increased
leaching of nitrate nitrogen and erosion of fertile soil (Figure 1)
greatly increases net acid production (FENTON & HELYAR, 1999).
2
In this paper soil pH is determined in a solution of 5:1 0.01M CaCl2:soil. This gives
almost identical results to the standard method used in Brazil of 2.5:1 0.01M
CaCl2:soil.
Figure 1. A diagrammatic representation of the processes and causes of
agriculturally induced soil acidity. Those processes marked as
'permanent' means that the acidification by that process is
permanent.
This paper will visit the processes and effects of soil acidity,
how acidity varies through out the soil profile and how to manage
soil to prevent the formation of acidity. It is written for those who
wish to hand their farm on to the next generation in as good, if not
better, condition than they received it or if they are not farmers then
to assist those who are. In writing this paper the authors recognise
that it is difficult for farmers to implement soil conservation practices
if they are not making money and suggestions for changes in
management are made on the understanding that they will only be
adopted if profits are increased.
SOIL ACIDITY AND PLANT GROWTH
As soil becomes more acid the availability of aluminium to the plant
increases to toxic levels, while the availability molybdenum, phosphorus, magnesium and calcium decreases. In weakly weathered
soils manganese toxicity will develop as the soil becomes more acid.
These changes in availability of aluminium and plant nutrients cause
most of the effects on plant growth attributed to acidic soils. Soil
acidity may also reduce nodulation and nitrogen fixation by some
legumes, decrease rates of mineralisation of organic matter and decrease microbial activity in general (ROBSON & ABBOTT, 1989).
ALUMINIUM (Al)
Available aluminium increases as pH falls. The available Al at a
given pH varies between soils with more aluminium available in well
weathered soil than in a weakly weathered soil (Figure 2) (FENTON
& HELYAR, 1999).
Figure 2. As soil pH falls below 5, aluminium (measured as Al in 5:1 0.01 M
CaCl2:soil) becomes available to plants.
The principal effect of aluminium toxicity is to restrict root
development by disrupting the structure and function of roots (Figure
3). Symptoms of aluminium toxicity in the leaves are similar to those
of phosphorus deficiency and moisture stress as it decreasing the
ability of plant roots to grow into acidic soil and extract nutrients and
moisture. Crops wilt more quickly during dry periods and an overall
reduction in water use efficiency of both pastures and crops occurs.
Plants vary in their tolerance of aluminium. Tolerant plants
can survive high levels of aluminium in the soil. These plants are
also affected as high levels of aluminium immobilise phosphorus in
the soil and can affect the uptake and utilisation of calcium and
magnesium.
Figure 3. The effect of increasing concentration of Al in solution on the
roots of wheat.
Plants vary in their tolerance to soluble aluminium. Table 1
gives the tolerance of a number of commonly grown agricultural
crops and pastures grown in Australia. The plants are arranged into
four tolerance classes. (RING et al., 1993).
Table 1. Aluminium sensitivity (tolerance) of some crop and pasture
plants.
Highly
sensitive
Sensitive
Tolerant
Highly
tolerant
Durum wheat, faba beans, Chickpeas, Alfalfa, medics,
strawberry, Balansa, Berseem and Persian clovers, Buffel
grass, Tall Wheatgrass
Canola, some wheats, barley, albus lupins, phalaris grass,
red clover, Caucasian and Kenya white clovers.
Some wheats, annual & perennial rye-grass, Tall fescue,
Haifa white and subterranean clovers
Narrow leaf lupins, oats, triticale, cereal rye, cocksfoot,
kikuyu, paspalum, yellow & slender serradella, Maku lotus,
common couch, Consul love grass
MANGANESE (Mn)
Both toxicities and deficiencies of manganese can occur in
plants. Soils most likely to develop manganese toxicity are weakly
weathered soils that are high in easily reducible manganese oxides,
such as red earths and red brown earths and more highly weathered
soils that are high in iron and aluminium oxides that adsorb and
accumulate Mn2+ (CONYERS et al., 1997). Not all plants are sensitive to Mn toxicity. In those plants that are sensitive, Mn toxicity
causes chlorosis and crinkling of the leaf margins.
Deficiencies are only expected where there is an absolute
deficiency as in the case of some siliceous sands, or in soils that are
very high in calcium carbonate (e.g. > 50 %). High Fe soils do tend
to accumulate Mn rather than being depleted in Mn during soil
development. This could be a problem in Brazil but the limited
literature seen by the authors (CAMARGO & FALICI, 1974) suggest
that an Mn deficiency may be a bigger problem.
Increasing the soil pH to above 5.5 with lime can lower high
levels of available manganese in the soil. On the other hand Mn
deficiency can be accentuated by liming.
MOLYBDENUM (Mo)
Molybdenum is a trace element required for nitrate conversion to the ammonium form used in plant proteins and in the nitrogen fixation process. Where the pH of a soil is below 5.5 in soils
naturally deficient in Mo an application of 50–100g Mo per hectare is
usually sufficient to eliminate Mo deficiency for 5 to 10 years. In soils
high in iron and aluminium oxides more may be required.
CALCIUM (Ca)
In the experience of the authors most agricultural soils
contain an adequate supply of calcium as a nutrient for the crops
and pastures grown. Where there is a moderate calcium deficiency,
it is seen first in parts of the plant that are furthest from the main flow
of water within the plant such as production of peanuts. Low levels
of soil calcium can also adversely affect the nodulation of subterranean clover. Short term severe deficiencies can cause petiole
collapse of young expanding leaves.
Examples of very severe calcium deficiency are most likely in
soils with a pH less than 4 that are sandy and low in organic matter,
or where there has been excessive use of highly acidifying fertilisers. Under these circumstances the exchangeable calcium can
drop to concentrations less than 40 per cent of the exchangeable
cations other than aluminium and this can only occur in soils with a
low cation exchange capacity. As these soils usually have very high
levels of soluble aluminium, all but the most acid tolerant plants,
such as sugar cane, are killed by the aluminium before the
symptoms of calcium deficiency become apparent. The symptoms
are stubby, weakly branched and discoloured roots and dead shoot
growing points. The root symptoms are difficult to distinguish from
symptoms of aluminium toxicity.
MAGNESIUM (Mg)
Losses of production in crops and livestock due to a nutritional deficiency of magnesium are most unusual. In Australia most
of soils have adequate supply of Mg in the subsurface soil layers if
not in the surface layers. Excess Mg can in some circumstances
cause loss of soil structure.
PHOSPHORUS (P)
Phosphorus is absorbed onto iron and aluminium oxides
where it is less available to the plant and this effect increases as
soils become more acid. Thus acidic soils high in iron and aluminium
oxides need to contain higher concentrations of phosphorus to avoid
P deficiencies in plants.
THE PROCESS OF ACIDIFICATION
The source of the acids in the ecosystem is principally the
carbon and nitrogen cycles within the eco-system.
THE CARBON CYCLE
Description of the C cycle starts in the plant leaves with the
creation of sugars from carbon dioxide (CO2) and water using the
energy of the sun through photosynthesis. These sugars are converted in part to organic acids that dissociate into H+ ions and
organic anions. The negative charge on the anions is balanced by
the “base” cations, Ca2+, Mg2+, K+ and Na+ to form salts of organic
acids. These base cations are taken up through the root. The H+
ions are consumed in metabolic reactions or are excreted at the root
surface in exchange for excess cation uptake. Note that the plant
requires that the charge remain constant across the root membranes when taking up water and nutrients but does not require that
a constant acid-base balance be maintained.
The result is that the plant material becomes alkaline and the
soil becomes more acid. The internal pH balance within the plant is
maintained through the Malate – Pyruvate buffer within the plant.
When the plant loses its leaves, or dies, the plant tissue becomes part of soil organic matter that in time breaks down to
become CO2 and water again, and releasing the alkalinity stored as
organic anions. (Figure 4) Fire is the same process but occurs far
more quickly. The alkali released in the break down of the organic
matter will neutralise the acid in the soil.
Sunshine
CO2 + H2O
In the leaf
CO2 + H2O
The organic matter
becomes
CO2 + H2O
& energy
RCOO- + H+
In the root
cations are absorbed
(eg Ca2+, Mg2+)
and H+ is excreted into the soil
This means the
In time the plant dies and
becomes organic matter and the
acid in the soil neutralises the
alkali in the plant.
PLANT becomes AKALINE
(RCOO- + M+)
and the SOIL becomes
ACID (H+)
Figure 4. The Carbon cycle in a closed system where organic acids are
produced from CO2, water and sunshine as components of
plants, then organic matter and return to CO2, water and energy.
As the alkali is accumulated in the above ground biomass an
equal amount of acid is added to the soil. The return of biomass to
the soil neutralises the acid as it breaks down. Some acid will be
utilised in the weathering process otherwise the eco-system will
retain an alkali/acid balance that, if the system was totally closed,
would continue indefinitely. However if plant material is removed
from the eco- system as in the harvest of grain or the making of hay
then the eco-system is left more acid (Figure 5).
In a cropping system
The grain is removed taking the
organic anions (RCOO- + M+) and
leaving the soil a little more acid
Figure 5. If product such as grain is removed from an permanently
removed from an ecosystem then that system is left more acid.
THE NITROGEN CYCLE
The description of the N cycle starts with nitrogen being taken
from the air by legume plant/Rhizobia symbiosis and incorporated
into the plant. When the plant dies the nitrogen becomes part of the
organic matter that in turn breaks down releasing ammonium nitrogen. The ammonium nitrogen through mineralisation and nitrification becomes nitrate nitrogen with a net release of acid into the soil.
As the nitrate nitrogen is taken up by the plant root it excretes an
equal amount of anion that will neutralise the acidity as the charge
across the root membrane must remain constant (Figure 6.)
N fixation
Atmospheric N2
Plant Organic N (RNH2)
Produces OH - H+
NO3 - is taken up
by the plant
Soil Nitrate
NO3 - N
Plant dies
Mineralisation
Produces 2H+
Nitrification
Uses H+
Soil Ammonium
NH4+ N
Of the 2 H+ produced by nitrification one is used in the
mineralisation and one is neutralised by the OH -.
Figure 6. The completed nitrogen cycle which always returns to the same
acid/alkali balance. Not on this diagram is the limited return to
the atmosphere of nitrogen gas through denitrification.
If all the nitrate nitrogen produced is taken up by the plant
then the acid alkali balance in the eco-system will continue indefinitely. However if any of the nitrate is leached from the root zone
before the plant can take it up then the soil is left more acid. (Figure
7.) Some nitrogen may return to the atmosphere through a further
step called denitrification. This is an alkaline process that balances
the acid produced during the mineralisation and nitrification process.
Note that contrary to some older texts, the removal of the
“base” cations, Ca2+, Mg2+, K+ and Na+ is not the cause of acidity in
the soil but rather the result of acidification of the soil by acids
added in the C and N cycles within the eco-system.
N fixation
Atmospheric N2
Plant Organic N (RNH2)
H+
Plant dies
Mineralisation
Uses H+
Produces 2H+
Soil Nitrate
NO3 - N
Soil Ammonium
NH4+ N
Nitrification
If the NO - is leached away before
3
the plant can take it up it leaves one H+
behind and soil more acid
Figure 7. If NO3-N is leached from the root zone before the plant can take it
up the eco-system is left a little more acid.
CREATION AND DESTRUCTION OF THE 'STABLE
STATE' OF SOIL ACIDITY
In nature most eco-systems have evolved to an almost stable
state where the acidity/alkalinity balance changes slowly towards
becoming more acid. In this natural stable state the eco-system is
producing a small amount of acid that releases sufficient cations
from the parent material to maintain fertile soil (Figure 8). For this
publication the term "stable state pH" will refer to the alkali/acid balance in an eco-system that has evolved to the point where changes
are very slow.
The natural processes that control acid addition in the natural
eco-system:
 The age of the soil.
 Climate, particularly rainfall, affects the intensity of leaching
and of erosion and hence the opportunity for loss of
alkaline materials.
 The vegetation affects acidification through variations in N
fixation capacity, in the ability to absorb and recycle nu-
trients from and in the soil profile, in its ability to minimise
the leaching of nutrients through effects on water use and
nutrient ‘stripping’ from the leached water. In addition different plants contain different concentrations of anions and
this affects the amount of alkali lost when plant material is
removed and alkali returned to the surface from the deeper
soil layers.
 Soil parent materials affect the rate of acidification due to
differences in their acid neutralising capability or buffering
capacity. Basalt, and often alluvium, have a high neutralising capability (also called a pH buffering capacity) compared to previously weathered materials such as a
sandstone which may contain 98% quartz (SiO2).
Figure 8. A diagrammatic depiction of an ‘stable state pH' where a small
amount of leakage of NO3-N and loss of biomass creates enough
acidity to release sufficient nutrients from the parent material to
maintain soil fertility.
In Wagga Wagga the stable state pH(0 to 10 cm) is between 5
and 6, increasing with depth to about 8 at 1 metre for soils that have
not been disturbed by farming. These soils are generally duplex soil
and support open woodland vegetation. This stable state pH has
developed over more than maybe 30,000 years and has as part of
its genesis the regular burning by the aboriginals who inhabited this
land for more than 40,000 years.
When the land is turned over to agricultural production
acidification rates increase and the 'natural' stable state pH is
destroyed.
The normal agricultural practices of fertilising and harvesting
along with the increase in leaching of nitrate nitrogen accelerate the
acidification of agricultural lands. In addition a build up in organic
matter takes organic anions out of the eco-system leaving it a little
more acid but this is reversed if the organic matter is reduced later
(Figure 1). An increase in erosion of the fertile topsoil will remove
this organic matter leaving the soil more acid.
The challenge to farmers and their advisers is to establish a
stable state pH for farming if it is to continue for 100’s and maybe
1,000’s of years. This will mean managing the processes that acidify
the soil such as fertilisers, product removal and increased nitrate
nitrogen leakage away from the root zone and applying an alkaline
material, most likely applying finely ground lime.
In Wagga Wagga the stable state pH that cropping farmers
strive to achieve is pH between 5.0 and 5.5 by applying finely
ground limestone. Most plants and soil biota are not affected by the
acidity at this soil pH and the net movement of H+/OH- down the
profile is in balance. This stable state pH is economically sustainable
as their cropping/pasture rotation is based on plants that are sensitive to acidity and the purchase of lime comes from, and maintains,
the farm profits.
In an experiment at Wagga Wagga the long term effect of
tillage, crop rotations, lime and fertiliser on soils has been running
since 1979. This experiment indicates dramatic effects on pH(0-10cm)
occurred in the first ten years but this has now stabilised
(CONYERS et al., 1996). Acid addition is now thought to be equal
to the ongoing weathering rate seen as a 7% reduction in fine clay
fraction over 20 years (Slattery Pers.Com.).
To the east of Wagga Wagga in the non-arable grazing lands
lime application does not increase profits sufficiently to cover the
cost. The farmers who are establishing a new stable state pH grow
perennial, acid tolerant pastures and their agricultural practices are
less acidifying. These farmers have a stable state pH that is less
than 5.0 and often less than 4.8. While acidification rates are lower
than at 5.5 it is not sustainable as the whole profile is acidifying.
DISTRIBUTION OF SOIL ACIDITY OVER THE SOIL
PROFILE (PH PROFILE)
To understand how to achieve a fertile stable state pH profile
it is necessary to understand the factors that control the distribution
of soil acidity down the profile. Planning a cropping or pasture program that will establish a sustainable, fertile pH profile requires
management of the acid being added to each layer within the profile.
Examples of ways that pH profile influences crops and
pastures in an acidic soil:
 Acid sensitive plants will be affected if the pH is below 4.7
beyond 20 cm. The root of an acid sensitive plant can often
break through 10 cm of inhospitable acidic soil is less likely
to penetrate 20 cm of acidic soil.
 The effect of lime moves down the profile very slowly and
only when the pH in the layer above is greater than 5.5.
 The response to liming is affected by the pH profile below
the limed layers.
 Where sub-surface acidity occurs it may take a number of
years for the economic response to liming to be realised so
the pH profile is important in planning a crop and pasture
rotations in association with a liming program.
DEVELOPMENT OF A PH PROFILE
Different soil profiles develop under different agricultural,
forestry and natural eco-systems. For example in many systems the
surface 5 cm has higher pH than the sub-surface layers (5-20 cm).
In undisturbed soils around Wagga Wagga, for example, the top 5
cm has a pH of 5.5 to 6, a pH of 4.5 to 5.5 in the 5 to 20 cm layer
with the pH rising again in lower root zone due to net alkali addition.
In other systems the reverse may occur. Where net acid addition
occurs on a whole profile basis, the long-term effect is usually a
strongly acid profile to depth, with a slightly less acid layer in the top
2-3 cm. Subsequently leaching, diffusion and soil movement or
mixing, can redistribute hydrogen and hydroxyl ions and their chemical equivalents (aluminium, carbonate and bicarbonate ions)
within the profile.
If it is difficult to avoid the development of a highly acid pH
profile to depth over laying a less acid deep sub-soil. In the natural
eco-system this happens very slowly but where acidification is
accelerated the profiles will become acid to depth quickly unless
finely ground lime is incorporated to at least 10 cm to neutralise any
acid addition.
The factors that affect the development of the soil pH profile
are:
 Leaching of nutrients affect the location of acid and alkaline
reactions in the N and C cycles, and the pattern of excretion of H+ and OH- (or HCO3- and organic anions) by roots
in response to imbalances in cation and anion uptake.
 Net acid addition can occur close to the top of the profile if
nitrate formed from nitrification is leached. Acidity and alkalinity can be redistributed within the soil profile by mass
flow in water, by diffusion or by soil movement. In general
the movement by water will be acid if the pH is below 5.0
and net alkaline if the pH is above 5.5.
 The alkaline effect of applied lime moves slowly down the
profile (pH > 5.5), unless the surface pH is high (pH 7) and
biological activity is high.
 Plant transport of nutrients and organic anions onto the soil
surface.
 Some worms and ants will bring soil up from depth and will
transport lime down the profile both by active movement
and faster leaching down the holes they create (BAKER et
al., 1995).
The locations in the soil profile where N and C cycle
acids and alkalis are added.
The primary processes of acid addition to the soil from the N
and C cycles are:
 Nitrification.
 Dissociation of organic acids in the soil (from organic acids
produced in the soil by microflora or organic acids added to
the soil in plant residues).
 Immobilisation or volatilisation of ammonium ions in the
soil.
 Excretion of hydrogen ions by roots in response to uptake
of more cations than anions.
The primary processes of alkali addition to soils (or hydrogen
ion consumption) are:
 Denitrification, nitrate immobilisation by microflora.
 Mineralisation of organic N to ammonium.
 Adsorption of hydrogen ions by organic anions in organic
matter added to soils with lower pH than the organic
matter.
 Oxidation of organic anions to carbon dioxide and water.
 Excretion of OH or HCO3 by roots in response to greater
uptake of anions than cations.
Therefore the net distribution of acids and alkali addition to
the soil profile depends on the distribution of all the above primary
processes of addition of acids and alkalis.
Among the quantitatively dominant processes, the oxidation
of organic anions (alkaline) and ammonification (alkaline) usually
occur near the soil surface where the organic matter is added. The
location of the dominant acid process, nitrification, depends on the
depth to which ammonium is leached before nitrification occurs.
Finally another quantitatively important process, the pattern of excretion of acid or alkali by roots, is affected by the relative availability
of the nutrient cations and anions in a given soil layer. The most
useful simplification of this balance is that roots usually absorb more
non-N cations than non-N anions so can be expected to excrete
some acid where N availability is low in a soil layer. Where N
availability is high, rates of acid excretion can be expected where
most of the soil N is in the ammonium form. Conversely moderate to
high rates of alkali excretion can be expected where most of the soil
N is in the nitrate form.
Therefore ecological processes such as the leaching of
ammonium after ammonification and the leach of nitrate before
absorption by roots, affect the distribution of acid addition to the soil
profile.
The leaching of NO3-N to depth within the profile before the
plant takes it up is one example of how the leaching of nutrient can
affect the pH profile. It will leave the point where the nitrification took
place more acid and will increase the pH at the point where it is
taken up. While this could be a method to correct the acidity in the
sub soil it is not always certain that the NO3-N will be taken up and
not leached from the root zone.
THE MOVEMENT OF ACID AND ALKALI WITHIN THE SOIL
PROFILE BY LEACHING AND DIFFUSION
The net alkalinity or acidity of the soil solution across the
common range of soil pH values is shown in Figure 9. Therefore the
figure describes the effect of a simultaneous equilibrium between
alumino-silicate minerals, Al(OH)3, H2O, CO2, CaCO3, on the net
alkalinity or acidity of the soil solution (MUNNS et al., 1992).
Net acid balance in soil solution at different soil air
CO2(g) (Al silicate; Al(OH)3/CaCO3/CO2(g)/H2O system)
Moles alkali excess/L
0.0015
0.001
100 mm leachate at 0.001
moles/L contains 1 kmol OHexcess/ha
0.0005
CO2 0.0003 atm.
CO2 0.0015 atm.
CO2 0.003 atm.
0
3.5
-0.0005
4
4.5
5
5.5
6
6.5
7
7.5
-0.001
Soil solution pH
Figure 9. Net alkalinity or acidity in the soil solution as a function of soil
pH, the solubility of Al minerals and calcium carbonate in the
soil, and of the carbon dioxide concentration in the soil solution.
Key features of this figure are that the solution leaching down
the soil profile is net acid below pH 5.2 to 5.6 and alkaline above
that pH range. Furthermore, above about pH 5.5 the net alkalinity of
the soil solution is very sensitive to the carbon dioxide concentration
of the soil air. Thus the mass flow of alkali in leachate is sensitive to
the biological activity in the soil and the pH while the mass flow of
acid below pH 5.5 is sensitive only to the pH. A further feature is that
net movement of acid or alkali by leaching is quite small within the
pH range 4.5 to 6.5.
PLANT TRANSPORT OF NUTRIENTS AND ALKALINITY
ONTO THE SOIL SURFACE
Plants with a high organic anion and associated cation
content have a greater capacity to recycle soil cations and alkalinity
to the surface in plant residues. Thus these species are useful in an
ecosystem to achieve an excess of alkali in the upper part of the soil
profile by increasing the rate of oxidation of organic anions near the
surface. Where this process achieves a pH significantly above 5.5,
the leaching of a net alkaline solution downward (Figure 9) helps to
neutralise acid addition deeper in the profile.
Among the agricultural species clovers and medics are higher
in organic anions (75 to 150 cmol/kg) than high quality grasses (50
to 70 cmol/kg). Cereals and low quality grasses that are relatively
high in cellulose are low (< 60 cmol/kg) in organic anions and cations (SLATTERY et al., 1991). Some dicotyledonous weeds such
as cape weed and Paterson's curse are high in organic anions.
Among the tree species eucalypts and acacias are often low in
organic anions (< 100 and often < 60 cmol/kg leaf litter) while other
species such as poplars, plane trees and white cedars are particularly high (150 to 250 cmol/kg leaf litter). Limited data on Casuarina
spp. indicates they may be in the high end of the eucalypt range and
other species have organic anion concentrations between 100 and
150 (NOBLE et al., 1996). This factor should be taken into account
when trying to design more sustainable agricultural and forestry
ecosystems.
The effect of the major processes affecting the pattern of
acidity in a profile have been modelled (HELYAR, 2001). This model
demonstrates that the equilibrium soil pH profile developed under a
given pattern of acid and alkali production is very sensitive to some
factors that are subject to management. Examples are the organic
anion content of the plant grown, the depth of leaching of ammo-
nium before nitrification and of nitrate before absorption by plants.
Furthermore, where the production system results in net acid
addition to the soil profile, a deep, highly acid profile will develop
over time unless sufficient lime is applied to the surface to neutralise
the acid added. Even in natural ecosystems that achieve very low or
zero net acid addition, (no nitrate leaching and no export of organic
anions from the system), a duplex soil with an acid sub-surface layer
can form where plant recycling of alkalinity from the subsoil is low.
This results in net acid addition to the surface soil and net alkali
addition to the subsoil (Figure 10). The consequences for soil profile
development and for soil fertility are profound.
Eucalyptus – grass vegetation
with:
 low nutrient and alkali
recycling capacity
Net acid
addition
Leaching
Pastures or trees with:
 high nutrient and alkali
recycling capacity
Leaching
 Restores acid balance
 Exaggerates problem of clay
dissolution (A horizon)
precipitation (B horizon)
Net alkali
addition
Leaching
Result
Net
alkali
addition
 Solodic soils, acid surface, heavy
clay subsoil
 Gradational well
structured soil, no
strongly acid layer
Net
acid
addition
Figure 10. Effect on soil pH profiles and soil type of contrasting patterns
of acid and alkali distribution in the soil profile that occur
under different types of vegetation where net acid addition to
the profile is very low or zero.
MANAGING AGRICULTURALLY INDUCED SOIL
ACIDITY
LIME APPLICATION RATES SHOULD AT LEAST EQUAL THE
NET ACID ADDITION
To stop a highly acid layer developing in some section of the
profile lime application must equal alkali removal as the first step to
establishing a fertile pH profile without a strongly acid layer anywhere in the root zone. One exception to this suggestion is where
the acid and alkali additions are such that a highly acid layer is
created at the bottom of the root zone and the pH is above 5.5
elsewhere. In this situation the acid layer is used to mobilise nutrients from parent material and accumulate them in the main root
zone. The soils under Brigalow vegetation may be an example of
such a soil in Australia.
THE SURFACE SOIL SHOULD BE LIMED TO PH 5.5 OR ABOVE
This will ensure that the net movement of alkali to the deeper
layers within the profile (Figure 9). A soil pH greater than 5.5 will
also remove most of the problems associated with soil acidity. Care
should be taken not to increase the pH of the surface 10 cm significantly above 6.0 because this may induce deficiency in other plant
nutrients such as zinc, boron and manganese in well weathered
soils.
Where the soil is acid to depth it may take many years for the
alkaline effect of the lime to move below 20 cm, especially for highly
buffered soils. Changes in pH in the subsoil layers of an acidic soil
have been measured for 12 years in a trial to the east of Wagga
Wagga (HELYAR et al., 1997). This has confirmed that if the pH is
maintained around 5.5 in the top 10 cm that the pH at 15 to 20 will
increase.
There is opportunity to correct acidity in sub-surface layers
where the exchange capacity of the sub-surface soils is positive
(Figure 11). Here gypsum, that is far more soluble than lime, can be
used neutralise acidity at depth.
> Al-OH + SO42> Fe-OH + SO42-
> Al-SO4- + OH> Fe-SO4- + OH-
Figure 11. The liming effect of gypsum on positively charged soils high in
iron and aluminium.
REDUCE THE LEACHING OF NITRATE NITROGEN
In soils with a pH above 4.7 nitrate nitrogen (NO3-N) is the
main form of nitrogen that is available to the plant. Nitrate nitrogen is
formed from ammonium nitrogen (NH4-N) by the process of nitrification. It is easily leached as it is weakly adsorbed by soil materials
and is highly soluble. When NO3-N is leached away from the point of
nitrification it leaves that point more acid. If the NO3-N is taken up
further down the profile this can increase pH at the point of uptake.
When the NO3-N is leached below the root zone and into the sub
soil aquifers it leaves the soil more acid (Figure 7).
Table 2 lists the factors that affect NO3-N leaching in the
Wagga Wagga district roughly in order of importance, qualifies the
effect of each factor and indicates how the effect can be reduced, or
increased. The absolute effect of each factor will depend on rainfall
and is usually periodic. In wetter climates the effect may occur every
year but in drier climates it may occur in some years.
EROSION OF SOIL CONTAINING ORGANIC MATTER OR
ACCUMULATING BIOMASS (EG. FORESTS)
A build up of soil organic matter or of live and dead biomass
above the soil has the same effect on soil acidity as removing
produce from the paddock. Increasing organic matter has many
benefits, for example improving soil structure, but it also makes the
soil more acid. Lime application should be considered to balance
acid added during a phase of increasing soil organic matter if the pH
is low enough to affect plant growth and profits from the enterprise.
This build up in organic matter will stabilise at a new level after some
years.
The higher organic matter level is often associated with
higher levels of available nitrate nitrogen increasing the potential for
further acidification from leaching of nitrate nitrogen. Higher levels of
organic matter in the soil (greater than 6%) in the soil will adsorb
soluble aluminium minerals resulting in less aluminium being available to the plants. In this situation plants sensitive to aluminium can
grow in soils with a low pH.
Table 2. Factors that affect NO3-N leaching in Wagga Wagga district
roughly in order of importance, the nature of the effect of each
factor and some indications of how the effect can be reduced, or
increased.
Factor affecting
NO3 -N leaching
Poor plant growth
Nature of effect
Reducing the effect
Inefficient water use increases
leakage of water containing
NO3-N into sub soil
NO3-N will leach before annual
pastures establish
Efficient water use by healthy well
managed crops and pastures dry
out the soil profile over summer.
Perennial pastures will utilise the
NO3-N as it comes available.
NO3-N will leave the root zone
before being utilised
Deep rooted perennials will ‘catch’
the NO3-N before it leaches below
the root zone. (Ridley et al., 1990).
Pastures
Clover/grass ratio
High N producing pastures
increase NO3-N available to be
leaching
Annual crops
Delay in sowing annual crops
will allow the NO3 to move on
the first water front ahead of
the roots
Where urea and anhydrous
NH4 oxidise before the plant
can use it can be leached away
Reduce the clover in pastures
below 20% but this reduces
production.
Include deep rooted perennials.
Always sow on the rain that breaks
the season.
Annual pastures
Non acidifying N
Fertiliser
Stubble retention
Retaining stubble increase
NO3-N available to be leaching
Improves structure and
therefore drainage
Reduced cultivation
No cultivation and direct drill
seeding improved soil structure
Soil pH
Lower pH inhibits nitrification
thus reducing NO3-N available
Adsorption onto
clay
NO3 is weakly adsorbed by
clays slowing leaching
Apply fertiliser after the crop is up
in split application or in bands if
pre-sowing.
Increases leaching of NO3-N from
top 15 cm by 40%. Most is
absorbed down the profile but
some is lost. (Heenan & Chan,
1992)
Increased leaching of NO3-N by
18%. Most is absorbed down the
profile but some is lost.
Maintain pH below 4.7 but this will
acidify subsurface layers and may
well reduce productivity.
Nothing. Where +ve point of charge
is present the rate of leaching is
reduced.
However if the organic mater or biomass is removed in soil
erosion event then the soil is left a little more acid.
REMOVAL OF PRODUCE
Grain, pasture and animal products are slightly alkaline and
their removal from a paddock leaves the soil more acid. If very little
produce is removed, such as in beef production, then the system
remains almost balanced. If, however, a large quantity of produce is
removed, particularly clover or alfalfa hay, products that are relatively high in organic anions, the soil becomes significantly more
acid.
If the produce is sold off-farm, regular liming is the only way
to maintain pH. The rates of lime required to neutralise the
acidification caused by removal of produce are given in Table 3. The
acidification caused by removing hay or silage is neutralised if it is
fed to livestock in the paddock where it was made and the waste
products are distributed evenly over the paddock.
Neutralisation of the acid added to forest soils as a result of
biomass accumulation and export in logs should be considered as a
component of sustainable management of forests. Otherwise long
term acidification of the forest soil will occur. The rate of lime required can be calculated from the amount of organic anions removed in
the logs plus the amount in the standing biomass that remains after
logging.
Table 3. The amount of lime needed to neutralise the acidification caused
by removal of produce (SLATTERY et al., 1991).
Produce
Lime requirement
(kg/t of produce)
Corn and wheat
9
Soybeans and lupins
20
Meat
17
Grass hay
25
Clover hay
40
Maize silage
40
Lucerne hay
70
USE OF FERTILISERS
The acidification that results from using fertilisers depends on
the fertiliser type. Some nitrogenous fertilisers, for example urea,
have no effect on pH if all the nitrogen is utilised by the plant. Acidity
will only result if the nitrate nitrogen produced from these fertilisers is
leached (Table 4).
Table 4. Acidifying effect of nitrogenous fertilisers and legume-fixed
nitrogen.
Lime required to balance acidification
(kg lime/kg N)
Source of nitrogen
0% nitrate
leached
50% nitrate
leached
100% nitrate
leached
High acidification
Sulfate of ammonia,
Mono-ammonium
phosphate (MAP)
3.7
5.4
7.1
Medium acidification
Di-ammonium
phosphate (DAP)
Low acidification
Urea
Ammonium nitrate
Aqua ammonia
Anhydrous ammonia
Legume fixed N
1.8
3.6
5.3
0
1.8
3.6
-3.61
-1.8
0
Alkalisation
sodium and calcium
nitrate
1
Equivalent to applying 3.6 kg lime/ kg N.
Other nitrogenous fertilisers, for example sulfate of ammonia,
have an acid reaction and their use acidifies the soil even if fully
utilised by plants. Additional acidification results if nitrate nitrogen is
formed and leached. Calcium nitrate, potassium nitrate and sodium
nitrate neutralise soil acidity if utilised by plants and have no effect if
all the nitrate is leached, but they are expensive and have limited
application.
Superphosphate has no direct affect on soil pH, but its use
stimulates growth of clovers and other legumes. In pasture systems
where most of the increased plant growth is returned to the soil,
increases of soil N status and a build-up of soil organic matter occur,
both of which increase soil acidity.
Elemental sulfur is acidifying and requires 3.125 kg of lime for
each kg of sulfur to neutralise its effect. This effect can be avoided
by using products that contain sulfur in the sulfate form such as
gypsum, potassium sulfate and superphosphate.
Acidification of the soil can be reduced by avoiding the use of
highly acidifying fertilisers such as sulfate of ammonia and monoammonium phosphate (MAP). Nitrogen fertiliser (including urea) that
is pre-sown should be drilled into narrow bands to slow nitrification
and subsequent leaching. Surface application of nitrogenous fertiliser for crops before sowing, even if harrowed, can result in nitrate
leaching and consequent acidification. Nitrate nitrogen formed from
post-emergent applications is more likely to be utilised by the crop
and will cause less acidification.
LIME REQUIREMENT TO MAINTAIN A NEW ‘AGRICULTURAL
STABLE STATE’ PH
Based on experience in Australia the following steps may
serve as a guide to calculating lime requirements for a nominated
paddock.
1. Have the soil tested.
a. Take 20 to 30 samples per paddock to 20 cm deep. Split
the samples into 0-10 cm and 10-20cm sub-samples and
thoroughly mix the sub-samples for each depth.
b. Have the soil analysed by a reputable laboratory.
c. If the pH is less than 5 then determine the exchangeable
aluminium. If the exchangeable aluminium is above 5% it
is likely that acidity is already, or will in the future, cost
production. The effect will vary with the aluminium tolerance (acid tolerance) of the crop or pasture to be grown.
2. Estimate of the acidification of your enterprise based on
measurement and experiments from the local district. This
will account for the acidification due to nitrate leaching,
removal of product and organic matter build-up. Add to this
your above average use of fertilisers and adjust for above
average production.
The figures in Table 5 are estimates of acidification based on
known rates that can be used to calculate how much lime is required.
Table 5. Typical rates of acidification to used as a guide to calculating lime
requirement.
Cause of acidification
NO3-N leaching1- District average
Rate of lime to
neutralise acid added
Your lime
requirement
Average rate2
High rainfall - annual crops or grasses
250 kg/year
.kg
High rainfall - perennial crops and
pastures
100 kg/year
.kg
Medium rainfall - annual crops or grasses
150 kg/year
.kg
Medium rainfall - perennial crops and
pastures
50 kg/year
.kg
e.g. Soybeans
20 kg/tonne
.kg
e.g. Corn silage
40 kg/tonne
.kg
36 kg/100 kg DAP
.kg
Above average Crop removal (See
Table 3)
Above average Fertiliser type (See
Table 4)
e.g. DAP
Total annual lime requirement
Time to next liming if pH is above 5.03
1.
.kg/year
2.5/Total ann. lime req
Years
This rate will be based on local experimental data or if not available will need to be
built up with farmer experience over time.
2. This rate is increased by 10% if stubble is retained in 10 years or more rotation.
5% if there is no cultivation in 10 years or more rotation.
This rate is decreased by 20% if the pH is between 4.6 and 5.0.
50% if the pH is between 4.2 and 4.5.
(below 4.2 there is virtually no acidification).
3 If the pH < 5 it may be time to lime now. Experience in Wagga Wagga is that it is
most economical to apply lime when 2 to 2.5 tonne per hectare when required to
neutralise accumulated acidity, usually 10 to 20 years.
CONCLUSION
Nearly every agricultural practice is more acidifying than the
natural eco-systems. To establish an agricultural eco-system that
will continue indefinitely will mean planning. This plan will be based
on an understanding the causes of the acidity. It will mean changing
some practices and most likely applying finely ground limestone as
the source of alkalinity to maintain this balance.
It is difficult to plan without having the soil chemically
analysed. In the Acid Soil Action program in Australia, of which one
author is the coordinator, the first step in developing a culture
sustainable agriculture practice within the farming community is to
sample and analyse the soil to depth.
Those making this plan must remain mindful that the person
who ultimately decides on what happens to the land is the owner of
the land. He needs to be well informed and well advised.
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