ALLEVIATING SOIL ACIDITY THROUGH ORGANIC MATTER
MANAGEMENT
Malcolm E. Sumner
Department of Crop and Soil Sciences, University of Georgia, Athens, GA 30602
Marcos A. Pavan
IAPAR, Londrina, Parana
Introduction
Most of the literature dealing with soil acidity amelioration has focused mainly on the
liming of topsoils under conventional tillage (Kamprath and Foy, 1985; Helyar, 1991;
Foy, 1992). Relatively little research effort has been devoted to subsoil acidity and
strategies for correcting acidification under minimum or no-till systems. Acidity in
topsoils is easily neutralized under conventional cropping systems by mechanical lime
incorporation. However, because of the very high costs involved, it is not practical to
incorporate lime into subsoils and under reduced tillage, little opportunity presents itself
for mechanical mixing of the required lime with the soil. In the case of conservation
tillage, lime cannot even be thoroughly mixed with the topsoil except at the initiation of
the conservation tillage regime. Consequently, other strategies must be developed to
achieve these ends. Acidity in both top- and subsoils often produces toxic levels of Al
and Mn and deficiencies of Ca. Root extension and proliferation is limited by toxic levels
of Al and/or deficient levels of Ca with the result that the crop suffers both drought and
nutrient stresses because the limited root system is no longer efficient in taking up water
and essential elements for growth. Manganese has no effect on the roots but negatively
impacts the above ground portions of the crop. These stresses lead to reduced crop yield
and quality. Thus the problem that requires solution is the precipitation of soluble Al and
the introduction of Ca throughout the profile.
The discussion to follow initially will analyze the problem from a theoretical point of
view followed by an evaluation of the field experimental results available in the light of
the most likely mechanisms involved.
The Problem of Acidity Conservation Tillage Systems
The main problem encountered in conservation tillage systems as far as soil acidity is
concerned is the inability to mix amendments with the soil. Under this regimen, subsoils
are often sufficiently acid under natural conditions to contain toxic levels of Al and Mn.
In addition, applications of soluble ammoniacal sources of N create acidity in the topsoil
which under severe conditions can move down the profile to cause acidification in the
subsoil. Because under reduced tillage conditions, opportunities are limited for the
incorporation of lime into the soil, the problem of acidity in both top- and subsoils is
essentially the same, namely, one cannot place the lime at the site of the acidity.
Consequently, strategies must be developed to move alkalinity into the soil and down the
profile without soil disturbance.
Most of the soils in Brazil on which conservation tillage is practiced fall into the Oxisol
and Ultisol Orders in Soil Taxonomy. These soils contain mainly sesquioxides and
kaolinite both of which are essentially variable charge minerals (charge depends on pH
and salt content). Consequently, acidification of such soils causes a reduction in cation
exchange capacity (CEC) and an increase in anion exchange capacity (AEC) in addition
to the normal consequences of acidification, namely, lower pH, toxic levels of Al and Mn
and deficient levels of Ca. These changes in CEC and AEC are very important in the
amelioration of subsoil acidity as will be seen later. The changes that take place in CEC
and AEC with pH are illustrated in Figure 1. The increase in exchangeable Al as pH
decreases displaces essential basic cations such as Ca, Mg and K on an already reduced
CECB responsible for holding these cations in an exchangeable (available) form.
Consequently, Al saturation increases and Ca, Mg and K saturations decrease very
markedly as pH decreases. Many of the Cerrado soils of Brazil are naturally at or near
their zero point of charge (ZPC) which means that they have very limited capacity to hold
essential cations which under natural conditions are mainly comprised of Al and H.
Therefore, amelioration involves both the neutralization of exchangeable Al and Mn and
restoration of higher levels of exchangeable basic cations throughout the profile. In
addition the increase in organic matter content under conservation tillage substantially
decreases the ZPC by increasing CEC making the soils more resilient. This means that
the pH value of the soil can be maintained at a lower value (5.2-5.8) than in permanent
charge soils without compromising the nutrition of the crop.
A comprehensive review of the world literature on lime movement in soils in the absence
of tillage (Sumner, 1994) shows that lime does not move readily down the profile,
particularly in variable charge soils and in the absence of acid inputs to the topsoil. The
nature of acidity in variable charge soils has been studied sufficiently (Uehara and
Gillman, 1981; Adams, 1981, 1984; Gillman, 1991; Ulrich and Sumner, 1991) to be able
to formulate rules to predict the behavior of lime within the soil profile. For lime applied
on the surface to have an influence on the acidity below, alkalinity in the form of HCO3or OH- ions must move by mass flow (movement with water) in a downward direction.
Of course, acidity in the form of H+, Al3+ or Mn2+ can also be transported downward
under appropriate conditions. At pH 5.2-5.4, mass flow of acidity and alkalinity are
roughly equal (Helyar, 1991). Therefore, for significant alkalinity to be mobile, topsoil
pH must be above 5.4 at which the concentrations of HCO3-, OH- and CO32- begin to
increase logarithmically. However, in variable charge soils, the advancing alkaline front
is retarded by the requirement that part of the alkalinity must be used to develop pH
dependent charge (CEC) which can be quite sizeable in such soils (Figure 1). The
processes taking place are reflected in the following reactions:
Precipitation of Al3+ or Mn2+
2 Exch-Al3+ + 3 Ca(HCO3)2  3 Exch-Ca2+ + 2 Al(OH)3 + 6 CO2
Exch-Mn2+ + Ca(HCO3)2  Exch-Ca2+ + MnCO3 + H2O + CO2
[1]
[2]
Decrease in AEC
Solid-OHH+Cl- + ½ Ca(HCO3)2  Solid-OH + H2O + CO2 + ½ CaCl2
[3]
Increase in CEC
Solid-OH + ½ Ca(HCO3)2  Solid-O- ½Ca2+ + H2O + CO2
[4]
The magnitude of reactions [3] and [4] depend on the nature and extent of the variable
charges in the soil. Thus, downward movement of lime is slower in variable charge soils
than permanent (temperate regions) soils in which only reactions [1] and [2] would be
operative. When soils contain positively charged sites as frequently occurs in acid
subsoils, salt (CaCl2), which is free to move downward, is generated. This is the reason
for the often observed phenomenon of rapid downward movement of Ca2+ with little or
no change in pH. Porter and Helyar (1992) have estimated that it would take 20 to 100
years for the pH to increase by 0.5 in the 10 cm layer underlying a limed topsoil. Based
on this discussion, one can conclude that lime will not move far from the site of
application without the help of other factors.
The following factors have been shown to effectively increase the movement of alkalinity
down the profile in the absence of mechanical disturbance:
Soil Fauna
Earthworms (Springett, 1983) and other burrowing soil animals such are termites and
ants (Lobry de Bruyn and Conacher, 1990) have a mixing action on the soil, and
consequently, can incorporate and mix lime with the soil. Earthworms mix soil both
laterally (Allolobophora calignosa, Lumbricus rubellus) and vertically (Allolobophora
longa) (Figure 2) while ants and termites mix mainly vertically. Lime is more effectively
distributed in the profile if it is first mixed with the topsoil. Earthworms and other soil
fauna are usually better maintained in the soil under conservation tillage production
systems, and therefore, should be encouraged wherever possible to maximize their
benefits. Care should be taken to avoid pesticides and herbicides that negatively impact
these animals.
Acid Inputs
In most highly productive agronomic systems, ammoniacal fertilizers or legumes are used
as sources of N both of which introduce acid into the soil. Although at first glance this
may appear to be potentially deleterious, benefits can accrue in appropriately managed
systems. Provided that sufficient lime is present at or in the surface layer and in the
presence of an actively growing crop, alkalinity can be transferred downward by the
addition of acidifying fertilizers at the surface. When ammoniacal fertilizers are applied,
much of the added NH4+ is converted by the process of nitrification to NO3- which
produces acidity and dissolves lime as follows:
Nitrification
NH4X + 2 O2  HNO3 + HX + H2O
where X can be NO3-, ½SO42-, H2PO4- or HPO42Dissolution of Lime
[5]
CaCO3 + 2 HNO3  Ca(NO3)2 + H2O + CO2
[6]
The neutral salt Ca(NO3)2 formed in this reaction is free to move down the profile. When
roots take up cations, they excrete H+, and for anions, they liberate OH-. Because plants
usually take up more N than Ca, net alkalinity results in the root zone as follows:
Differential Uptake of NO3- and Ca2+
Root  4 OH- 

+ 2 Ca(NO3)2 
Root  4 NO3- 

Root  2 H+
Root  Ca2+

+ Ca(OH)2 + 2
H2O [7]

Thus, at the root surface where uptake is going on, Ca(OH)2 (lime) is produced which
will neutralize any acidity present. The efficacy of this strategy has been clearly
demonstrated (Percival et al., 1955; Pearson et al., 1962; Abruna et al., 1964; Adams et
al., 1967; Weir, 1975, Adams, 1981) and is illustrated in Figure 3. Without adequate lime
in the topsoil (No Lime), application of NH4NO3 resulted in acidification of the entire
profile. However, when adequate lime was present in the topsoil (6.7 Mg/ha annually and
20.2 Mg/ha), the pH down the entire profile was raised above the initial pH. On the other
hand, leaching soils treated with Ca(NO3)2 in the absence of plant roots is was not very
effective (Kotze and Deist, 1975; Pleysier and Juo, 1981). With crops that are very acid
sensitive, this strategy may not work because of insufficient root growth, but with acid
tolerant tropical grasses, the effect is likely to be great.
Use of Gypsum
A voluminous literature from South Africa, Brazil and Georgia and summarized by
Sumner (1990, 1993, 1995) has incontravertibly shown that surface applications of
gypsum are effective in ameliorating subsoil acidity in highly weathered soils. Because
gypsum is a sparingly soluble salt (more soluble than lime), it is readily leached down the
profile over time. When it reaches the acid subsoil, the level of exchangeable Ca is
enhanced and the toxicity of exchangeable Al3+ is reduced both of which allow better root
development and exploitation of water from the subsoil resulting in higher yields.
Reduced Al3+ toxicity is achieved by one or more of the following mechanisms:
Ion Pairing
Al3+ + CaSO4  AlSO4+ + Ca2+
[8]
The AlSO4+ ion pair is much less toxic to roots than Al3+. In cases where single
superphosphate (contains 50% gypsum) has been used, ion pairing of Al with fluoride
will further detoxify the soil.
“Self-liming” Effect
2 HO-[Fe,Al]-OH + 2CaSO4  (HO-[Fe,Al])-SO4-)2 Ca2+
[9]
Al3+ + Ca(OH)2  Al(OH)3 + Ca2+
+ Ca(OH)2
[10]
Formation of Basic Aluminum Sulfate Minerals
3 Al3+ + K+ + CaSO4 + 3H2O  KAl3(OH)6(SO4)2 + 3 H+ + Ca2+
[11]
Alunite
The alunite formed is insoluble in the pH range 4-5 and is thus not readily dissolved by
the H+ liberated.
Movement of Organic Compounds
Experiments in solution culture (Hue et al., 1986) have demonstrated that short chain
carboxylic acids can detoxify Al3+ to varying extents depending on the relative positions
of the OH/COOH groups on the main C chain. When cotton roots were grown in soil
solutions extracted from acid Ultisols, root growth was correlated with monomeric Al
and not with total Al in solution supporting the view that soluble organic compounds are
mobile and capable of detoxifying acid soils. Hern et al. (1982) demonstrated that, if an
acid soil was leached with EDTA in the presence of lime, nearly all the exchangeable Al
below was precipitated whereas lime in the absence of EDTA had no effect. Application
of animal manures (Lund and Doss, 1980; Wright et al., 1985) and/or complexing agents
such as EDTA in the presence of lime (Wright et al., 1985) have increased pH and
exchangeable K and Mg and decreased exchangeable Al in the subsoil also supporting
the view that complexation by organic compounds is important in the transfer of
alkalinity from top- to subsoils. The mechanism involved is illustrated below:
Al Complexation
Al3+ + 3 KOOC-R  Al(OOC-R)3 + 3 K+
[12]
If the pH is above 5, the Al-OM complexes precipitate as Al(OH)3.
Pocknee and Sumner (1997) have shown that the basic cation and N contents of organic
matter are important in the neutralization of soil acidity. Plant residues with high contents
of basic cations (cotton, tobacco, peach, alfalfa, soybean) are more effective in
neutralizing soil acidity than those with low contents (maize, wheat straw, barley grain)
when applied at the same rates (200 Mg/ha) (Table 1). In fact, there is a linear relation
between soil pH after incubation and crop residue basic cation content (Figure 4). This
means that the quality of the organic matter measured as its acid neutralizing capacity
(sum of basic cations) is very important in the amelioration of soil acidity. For example,
wheat straw is no match for cotton leaves (Table 1). Higher N contents tend to have a
negative effect due to the acidity formed when the N is oxidized during decomposition.
When a crop residue decomposes, lime is liberated in proportion to the basic cation
content as illustrated in the following reactions for Ca oxalate and Ca gluconate, both
major organic compounds present in plants:
Organic Matter as Lime
Ca(OOC)2 + ½ O2 
CaCO3 + CO2
[13]
Ca(C6H11O7)2.H2O + 11 O2  CaCO3 + 11 CO2 + 12 H2O
[14]
These reactions are essentially the same as those for the thermal decomposition of these
compounds to form CaO which on exposure to the atmosphere would become carbonated
to form CaCO3 as illustrated below:
Ca(OOC)2 + heat + ½ O2
CaO + CO2  CaCO3
CaO + 2CO2 (
[15]
[16]
Thus when the basic cations are calculated on an equivalent basis, organic matter after
decomposition has the same neutralizing capacity as lime except for the small correction
required to account for the acidifying effect of the N content (Pocknee and Sumner,
1997). Similar results were obtained by Franchini et al. (1999) on three Brazilian soils
with three plant materials but at a lower rate (80 Mg/ha) (Table 2).
In addition to the liming and Al3+ complexing abilities of organic matter, it can also
generate alkalinity by ligand exchange for OH- on mineral surfaces as illustrated below:
Ligand Exchange
2 [Fe,Al]-OH + Ca(OOC)2C2H4  Ca(OH)2 + [Fe,Al]2(OOC)2C2H4
[17]
Thus, low molecular weight compounds moving downward have the potential to
neutralize acidity by any or all of the above mechanisms.
Having explored the more theoretical aspects of soil acidity amelioration by organic
compounds, one needs to evaluate the results of field experiments to establish whether
they are consistent with and support the reactions discussed above.
Conservation Tillage Brazilian Experiences
Long term tillage and crop rotation experiments on acid Brazilian soils have shown that
no-tillage increased pH-CaCl2, KCl-exchangeable Ca and Mg, and Mehlich-1 P, and
decreased KCl-exchangeable Al (Figures 5, 6, 7, 8) as compared with conventional
tillage (Muzilli, 1983; Sidiras and Pavan, 1985; Machado and Gerzabek, 1993). Oliveira
et al. (2000) reported the results of a field experiment established in 1976 on a Typic
Haplorthox in which the crop rotation wheat/soybean (W/S) was more effective in
decreasing soil acidity than wheat/maize (W/M) and wheat/cotton (W/C) rotations.
Franchini et al. (2000) found that wheat/soybean rotation maintained soil pH and
exchangeable Al near their initial values (Figures 9, 12) as compared with other
combinations involving blue lupin, maize and black oats. While the wheat soybean
rotation was effective in keeping exchangeable Ca at higher levels than the other
rotations, there was some decrease relative to the initial value particularly at depth
(Figure 10). The rotations had little differential effect on exchangeable Mg (Figure 12).
Soil acidification observed was largely due to the N fertilization of the maize.
Recent studies have shown that plant residues left on the soil surface to serve as a mulch
play a major role in alleviating soil acidity (Franchini et al., 1999ab; Meda et al., 1999).
They reported that black oats, oil seed radish, and blue lupin were the best plant materials
used in crop rotation to alleviate soil acidity; millet and pigeon pea had little effect on soil
acidity, while wheat and rice straw had no effect at all. Velvet bean, red and white clover,
rye grass, Crotalaria spectabilis, and Crotalaria breviflora showed effects intermediate
between wheat straw and black oats (Figures 13, 14, 15). Franchini et al. (2000) reported
that the best time to cut black oats to improve soil fertility was about 60-70 days after
sowing (Figure 16). Cassiolato et al. (2000) evaluated several black and white oat
cultivars and found that SI 83400 and UPF 90H400-2 were the most efficient as Ca
carriers into an acid soil profile (Figure 17). Miyazawa et al. (2000) reported that the
effect of plant material in alleviating soil acidity was associated with water soluble
organic compounds present in the plant tissues and not those produced by microbial
activity.
Cassiolato et al. (2000) tested several laboratory methods to estimate the efficiency of
plant residues in neutralizing soil acidity. They found that the sum of Ca+Mg+K,
electrical conductivity, the titre with NaOH between pH 3 and 7, and Cu2+ measured by
selective ion electrode were all related to the effect of plant material on soil acidity. They
showed that electrical conductivity by the plant extract, which can be easily and cheaply
measured under routine conditions, could be used to estimate the efficiency of plant
extracts to neutralize soil acidity.
All lime applied to no-tillage systems is applied on the surface of soil. It is well known
that lime applied to the surface only slowly penetrates into the subsurface. Studies
conducted in Paraná have demonstrated that lime applied on soil surface under no-tillage
with high amounts of plant residues moves downward into the subsoil increasing pH and
Ca and decreasing Al (Oliveira and Pavan, 1996). Miyazawa at al. (1998) found that
black oats was highly efficient as a Ca carrier into Ca deficient subsoils (Figure 18). They
also reported that oil seed radish was more efficient than black oats in complexing
exchangeable Al (Figure 19) and that wheat residue had no effect on the mobility of lime
into the soil (Figure 20). Their results, indeed support the view that soluble organic
compounds from plant residues used in crop rotation are mobile in soil and capable of
detoxifying acid subsoils.
For maximum yields in Brazil, the soil should be kept covered. This can be achieved by
ensuring that the soil surface is always covered by a large amount of mulching material
not only to prevent soil crusting and erosion (Sumner and Stewart, 1992) but also to
alleviate soil acidity. The data obtained suggest that, in order to improve rooting
throughout the soil profile, greater emphasis should be placed on agronomic practices
that maintain good quality soil cover by appropriate crop rotation under no tillage.
Conclusions
Reduced tillage systems promote the accumulation of organic matter which protects the
soil surface from the energy of impacting raindrops. As a result, crusting, runoff and
erosion are reduced. The accumulate organic matter provides soluble organic compounds
which carry basic cations into the subsoil where they contribute to the reduction in
acidity. The quality of the residue measured in terms of its basic cation content is of
prime importance with residues of dicotylenenous plants being superior to those of
monocotylenenous plants.
Profile acidity can be reduced by the incorporation of lime and organic matter in the
surface layers by soil fauna which distribute these materials to deeper layers. Leaching of
CaNO3 formed in the topsoil as a result of nitrification and the reaction of the HNO3
produced with lime, reduces subsoil acidity as a result of the differential uptake of Ca and
NO3 by roots. In addition, the leaching of gypsum into the subsoil causes the precipitation
of Al3+ as a basic Al sulfate. The movement of basic cations in organic complexes into
the subsoil also reduces acidity when the organic matter either complexes Al 3+ or
decomposes forming CaCO3. Results confirm that at least the last three mechanisms are
operative in Brazil.
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Table 1. Basic cation and N contents of various organic materials and soil pH (KCl) after
574 days of incubation with a Cecil soil at 30oC at a rate of 200 Mg of material/ha
(Pocknee and Sumner, 1997)
Material
Basic cation content
Ca
Mg
N
content
Sum of
Ca+Mg+K
g/kg
mmolc/kg
PH after
574 days
K
g/kg
Cotton leaves
37.9
5.8
22.5
26.9
2944
7.60
Tobacco leaves
16.1
5.2
32.6
28.7
2065
7.79
Peach leaves
22.4
4.0
16.1
19.7
1859
7.12
Alfalfa
142
3.9
24.4
32.4
1653
6.84
Soybean leaves
10.1
3.3
19.2
28.9
1267
5.71
Compost
20.6
1.8
1.6
7.1
1217
5.93
Maize leaves
6.4
1.8
19.2
10.0
958
5.44
Wheat
1.3
1.0
7.7
4.6
344
4.85
Barley grain
0.9
1.7
2.8
27.8
256
4.26
Control
0.0
0.0
0.0
0.0
0.0
4.01
Table 2. Basic cation content or organic materials incubated with three Brazilian soils at a
rate of 80 Mg/ha (Franchini et al., 1999)
Material
Basic cation content
Ca
Mg
Sum of
Ca+Mg+K
K
Average pH of incubations
for 15, 30, 60 and 90 days
LV soil§ LR soil LE soil
mmolc/kg
Oil seed radish
38.4
12.7
33.6
84.7
6.5
6.7
5.7
Soybean
10.6
19.3
10.8
40.8
6.0
5.8
4.9
Wheat
2.2
1.2
2.0
5.4
4.3
5.2
4.3
Control
0.0
0.0
0.0
0.0
4.0
4.6
4.4
§
LV = Latossolo Vermelho-Amarelo, LR = Latossols Roxo, LE = Latossols VermelhoEscuro
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Changes in cation exchange capacity (CEC) and anion exchange capacity
(AEC) for (a) Typic Rhodudult, (b) Oxisol, and (c) Oxic Humitropept
subsoils. CECT is the sum of exchangeable Al+Ca+Mg+K+Na and CECB
is the sum of Ca+Mg+K+Na (From Sumner, 1995).
Effect of the introduction of Allolobophora longa on pH profiles where
lime (5 Mg/ha) was either applied on the surface or incorporated to a
depth of 10 cm (From Springett, 1983)
Effect of rates of calcitic limestone on soil profile acidity after 4 years
under a Coastal bermudagrass (Cynodon dactylon [L.} Pers.) Sod
fertilized with NH4NO3 at an annual rate of 900 kg N/ha. Single lime
applications were made at the beginning of the experiment except for the
treatment labeled “6.7 Mg/ha annually”. (From Adams et al., 1967)
Relationship between basic cation content of organic materials added to an
acid Cecil soil (Typic Kanhapludult) after 574 days of incubation at 30oC
(From Pocknee and Sumner, 1977)
Long term effect of conventional tillage, no-tillage, and permanent cover
on soil pH. Crop rotation soybean/wheat/soybean/green manure (Adapted
from Sidiras & Pavan, 1985)
Long term effect of conventional tillage, no-tillage, and permanent cover
on
KCl-exchangeable
Ca
+
Mg.
Crop
rotation
soybean/wheat/soybean/wheat/green manure (Adapted from Sidiras &
Pavan, 1985)
Long term effect t of conventional tillage, no-tillage, and permanent cover
on effective Al saturation (Al/Al+Ca+Mg+K). Crop rotation
soybean/wheat/soybean/wheat/green manure (Adapted from Sidiras &
Pavan, 1985).
Long term effect of conventional tillage, no-tillage, and permanent cover
on
extractable
P-Mehlich.
Crop
rotation
soybean/wheat/soybean/wheat/green manure (Adapted from Sidiras &
Pavan, 1985).
Long term effect of crop rotation on pH (WS = wheat/soybean; LSO =
blue lupin/soybean/black oats; LMO = blue lupin/maize/black oats; = blue
lupin/maize; ----- = Initial value) (Adapted from Franchini et al., 2000)
Long term effect of crop rotation on Ca (WS = wheat/soybean; LSO =
blue lupin/soybean/black oats; LMO = blue lupin/maize/black oats; = blue
lupin/maize; ----- = Initial value) (Adapted from Franchini et al., 2000).
Long term effect of crop rotation on Mg (WS = wheat/soybean; LSO =
blue lupin/soybean/black oats; LMO = blue lupin/maize/black oats; = blue
lupin/maize; ----- = Initial value) (Adapted from Franchini et al., 2000).
Long term effect of crop rotation on Al (WS = wheat/soybean; LSO = blue
lupin/soybean/black oats; LMO = blue lupin/maize/black oats; = blue
lupin/maize; ----- = Initial value) (Adapted from Franchini et al., 2000).
Effect of plant material on soil pH CaCl2 (Adapted from Meda et al.,
2000).
Effect of plant material on soil KCl-exchangeable Ca (Adapted from Meda
et al., 2000).
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
Effect of plant materials on soil KCl-exchangeable Al (Adapted from
Meda et al., 2000).
Effect of Black Oat cutting time on soil pH (J.C. Franchini, Personal
information-not published).
Effect of oat cultivars on soil pHCaCl2 (Adapted from Cassiolato et al.,
(2000).
Effect of oat cultivars on KCl-exchangeable Ca (Adapted from Cassiolato
et al., 2000).
Effect of oat cultivars on KCl-exchangeable Al (Adapted from Cassiolato
et al., 2000).
Soil pH distribution with depth following surface lime and plant material
applications (Adapted from Miyazawa et al., 1998).
Exchangeable Al distribution with depth following surface lime and plant
material application (Adapted from Miyazawa et al., 1998).
Exchangeable Ca distribution with depth following surface lime and plant
material application (Adapted from Miyazawa et al., 1998).