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Abstract Rapid turnover of organic matter leads to a
low efficiency of organic fertilizers applied to increase
and sequester C in soils of the humid tropics. Charcoal
was reported to be responsible for high soil organic mat-
ter contents and soil fertility of anthropogenic soils
(Terra Preta) found in central Amazonia. Therefore, we
reviewed the available information about the physical
and chemical properties of charcoal as affected by differ-
ent combustion procedures, and the effects of its applica-
tion in agricultural fields on nutrient retention and crop
production. Higher nutrient retention and nutrient avail-
ability were found after charcoal additions to soil, related
to higher exchange capacity, surface area and direct nu-
trient additions. Higher charring temperatures generally
improved exchange properties and surface area of the
charcoal. Additionally, charcoal is relatively recalcitrant
and can therefore be used as a long-term sink for atmo-
spheric CO2. Several aspects of a charcoal management
system remain unclear, such as the role of microorgan-
isms in oxidizing charcoal surfaces and releasing nutri-
ents and the possibilities to improve charcoal properties
during production under field conditions. Several re-
search needs were identified, such as field testing of
charcoal production in tropical agroecosystems, the in-
vestigation of surface properties of the carbonized mate-
rials in the soil environment, and the evaluation of the
agronomic and economic effectiveness of soil manage-
ment with charcoal.
Keywords Carbon sequestration · Charcoal addition to
soil · Nutrient leaching · Soil amelioration · Sustainable
landuse
Introduction
An intensification of agricultural production on a global
scale is necessary in order to secure the food supply for
an increasing world population. As a result, fallow peri-
ods are often reduced in shifting cultivation in the humid
tropics leading to irreversible soil degradation and in-
creased destruction of remaining natural forests due to
cultivation of new areas after slash-and-burn (Vosti et al.
2001). In most tropical environments, sustainable agri-
culture faces large constraints due to low nutrient con-
tents and accelerated mineralization of soil organic mat-
ter (SOM) (Tiessen et al. 1994; Zech et al. 1997). As a
consequence, the cation exchange capacity (CEC) of the
soils, which is often low due to their clay mineralogy,
decreases further. Under such circumstances, the effi-
ciency of applied mineral fertilizers is very low when the
loss of mobile nutrients such as NO3–or K from the top-
soil is enhanced by high rainfall (Melgar et al. 1992;
Cahn et al. 1993). Additionally, many farmers cannot af-
ford the costs of regular applications of mineral fertiliz-
ers. Therefore, nutrient deficiency is prevalent in many
crop production systems of the tropics.
The most common form of landuse in the tropics is
shifting cultivation using slash-and-burn techniques.
During burning of the aboveground biomass the nutri-
ents are rapidly released into the soil. These nutrient ad-
ditions have positive effects on soil fertility only for a
short period (Cochrane and Sanchez 1980; Kauffman et
al. 1995; Kleinman et al. 1995). Additionally, burning re-
leases large amounts of the greenhouse gases CO2and
NOx which lead to global warming (Fearnside et al.
1999). Applications of mulches, composts, and manures
have frequently been shown to increase soil fertility.
However, under tropical conditions organic matter is
usually mineralized very rapidly (Tiessen et al. 1994)
and only a small portion of the applied organic com-
pounds will be stabilized in the soil in the long term,
but successively released to the atmosphere as CO2
(Fearnside 2000). An alternative is the use of more sta-
ble compounds such as carbonized materials or their ex-
B. Glaser (✉) · W. Zech
Institute of Soil Science and Soil Geography,
University of Bayreuth, 95440 Bayreuth, Germany
Fax: +49-921-552246
J. Lehmann
Department of Crop and Soil Science, Cornell University,
Ithaca, NY 14853, USA
Biol Fertil Soils (2002) 35:219–230
DOI 10.1007/s00374-002-0466-4
REVIEW ARTICLE
Bruno Glaser · Johannes Lehmann · Wolfgang Zech
Ameliorating physical and chemical properties
of highly weathered soils in the tropics with charcoal – a review
Received: 24 April 2001 / Accepted: 5 March 2002 / Published online: 18 April 2002
© Springer-Verlag 2002
tracts. Recent investigations (Glaser 1999; Glaser et al.
2000, 2001a) showed that carbonized materials from the
incomplete combustion of organic material (i.e. black C,
pyrogenic C, charcoal) are responsible for maintaining
high levels of SOM and available nutrients in anthropo-
genic soils of the Brazilian Amazon basin. These so-
called Terra Preta do Indio (Terra Preta) characterize the
settlements of pre-Columbian Indios. In Terra Preta soils
large amounts of black C indicate a high and prolonged
input of carbonized organic matter probably due to the
production of charcoal in hearths, whereas only low
amounts of charcoal are added to soils as a result of for-
est fires and slash-and-burn techniques (Fearnside et al.
1999; Fearnside 2000).
Coal from geological deposits (“coal”) and from vari-
ous specialized procedures were successfully used for
soil amelioration (Berkowitz et al. 1970; Radlein et al.
1996; Rakishev et al. 1996). As carbonized materials
comprise a wide range of materials from partly charred
material to graphite and soot particles, with no general
agreement on clear-cut boundaries according to their dif-
ferent physical and chemical properties (Schmidt and
Noack 2000), we will focus in this review on the influ-
ence of highly aromatic “charcoal” amendments to soil.
The objectives were to discuss: (1) the effects of char-
coal on nutrient availability and nutrient retention in soil;
(2) the possibility to increase C sequestration with char-
coal amendments; and (3) to evaluate the economic fea-
sibility of using charcoal amendments in smallholdings
in the humid tropics.
Charcoal amendments to soil and crop production
Improving crop yields with charcoal amendments
Adding charcoal to soil can significantly increase seed
germination, plant growth, and crop yields (Table 1).
Chidumayo (1994) reported generally better seed germi-
nation (30% enhancement), shoot heights (24%) and
biomass production (13%) among seven indigenous
woody plants on soils under charcoal kilns compared to
the undisturbed Zambian Alfisols and Ultisols (Table 1).
Kishimoto and Sugiura (1985) found that the heights of
sugi trees (Cryptomeria japonica) increased by a factor
of 1.26–1.35, and the biomass production increased by a
factor of 2.31–2.36, five years after application of
0.5 Mg charcoal ha–1. Not only trees but also annual
220
Table 1 Relation between charcoal amendments to soil and crop response
Treatment Amendment Biomass Plant Root Shoot Plant type Soil type Reference
(Mg ha–-1) production height biomass biomass
(%) (%) (%) (%)
Control – 100 100 – – Bauhinia wood Alfisol/Ultisol Chidumayo (1994)
Charcoal Unknown 113 124 – – Bauhinia wood Alfisol/Ultisol
Control – 100 – – – Soybean Volcanic ash soil, Kishimoto and
loam Sugiura (1985)
Charcoal 0.5 151 – – – Soybean Volcanic ash soil, Iswaran et al. (1980)
loam
Charcoal 5.0 63 – – – Soybean Volcanic ash soil, Kishimoto and
loam Sugiura (1985)
Charcoal 15.0 29 – – – Soybean Volcanic ash soil,
loam
Control – 100 – – – Pea Dehli soil Iswaran et al. (1980)
Charcoal 0.5 160 – – – Pea Dehli soil
Control – 100 – – – Moong Dehli soil
Charcoal 0.5 122 – – – Moong Dehli soil
Control – 100 – 100 – Cowpea Xanthic Ferralsol Glaser et al.
(2002a, 2002b)
Charcoal 33.6 127 – – – Oats Sand
Charcoal 67.2 120 – – – Rice Xanthic Ferralsol
Charcoal 67.2 150 – 140 – Cowpea Xanthic Ferralsol
Charcoal 135.2 200 – 190 – Cowpea Xanthic Ferralsol
Control – 100 100 100 100 Maize Alfisol Mbagwu and
Piccolo (1997)
Coal humic acid 0.2 118 114 122 114 Maize Alfisol
Coal humic acid 2.0 176 145 186 166 Maize Alfisol
Coal humic acid 20.0 132 125 144 120 Maize Alfisol
Control – 100 100 100 100 Maize Inceptisol
Coal humic acid 0.2 125 119 122 127 Maize Inceptisol
Coal humic acid 2.0 186 148 198 173 Maize Inceptisol
Coal humic acid 20.0 139 131 147 130 Maize Inceptisol
Control – 100 100 100 – Sugi trees Clay loam Kishimoto and
Sugiura (1985)
Wood charcoal 0.5 249 126 130 – Sugi trees Clay loam
Bark charcoal 0.5 324 132 115 – Sugi trees Clay loam
Activated charcoal 0.5 244 135 136 – Sugi trees Clay loam
crops were found to have higher yields after applications
of carbonized organic matter. Crop yields could be in-
creased by up to 200% upon higher charcoal additions
(Table 1). Similar observations were made after addi-
tions of humic acids from coal deposits, which increased
maize growth by up to 1 g kg–1 on Nigerian Alfisols and
Inceptisols (Mbagwu and Piccolo 1997). In addition to
supplying essential plant nutrients, humic acids were re-
ported to have hormone-like properties that enable a
stimulation of plant growth (Nardi et al. 2000). On the
other hand, large additions of charcoal or coal-derived
humic acids may also have detrimental effects on crop
growth. Yield declines of soybeans and maize were ob-
served with an addition of 5 Mg charcoal ha–1 and 15 Mg
charcoal ha–1 (Kishimoto and Sugiura 1985; Table 1)
and >1 g coal-derived humic acids kg–1, respectively
(Mbagwu and Piccolo 1997). The reason for these reduc-
tions can be attributed to an increase in pH for pH-sensi-
tive plants, such as observed for pine (Tryon 1948)
or due to pH-induced micro-nutrient deficiencies
(Kishimoto and Sugiura 1985). However, crop yields did
not generally decline after additions of large amounts of
charcoal. From the few data available, no general opti-
mum range can be deduced. Instead, for optimum plant
growth, the amount of added charcoal may have to be de-
termined for each type of soil and plant. Additionally, re-
cent investigations showed that crop yields can be en-
hanced even more compared to control soils if charcoal
amendments are applied together with inorganic or organ-
ic fertilizers (Glaser et al. 2002a; Lehmann et al. 2002).
Charcoal amendments and nutrient availability
The application of charcoal can increase the pH and de-
crease the Al saturation of acid soils, which often are
major constraints for productive cropping in highly
weathered soils of the humid tropics (Cochrane and
Sanchez 1980; Mbagwu and Piccolo 1997). The accumu-
lation of the ashes from burned biomass and its effect on
soil pH is a well-documented mechanism for improving
soil fertility (e.g. Sanchez et al. 1983). In the Peruvian
Amazon, topsoil pH was 0.4 pH units higher after slash-
and-burn (Kauffman et al. 1995). The addition of coal
ash (110 Mg ha–1) was shown to increase the pH of an
eroded Palouse soil from 6.0 to 6.8 (Cox et al. 2001).
Also, the addition of charcoal increased the pH of soils
with various textures by up to 1.2 pH units from pH 5.4
to pH 6.6 (Mbagwu and Piccolo 1997). This effect was
still detectable 3 years after charcoal application where
the pH values were 5.8 and 6.3 in the control and the
charcoal plots, respectively (Kishimoto and Sugiura
1985). As expected, the pH increase was larger in sandy
and loamy soils than in clayey soils (Tryon 1948). Con-
sequently, also the base saturation increased and was ten-
fold higher after charcoal amendment (Table 2).
Tryon (1948) also found increasing amounts of ex-
changeable bases after additions of 45% hardwood and
conifer charcoals to sandy and loamy soils (Table 2).
With respect to the cations, he used the term “available”
rather than “exchangeable” because the sum of the deter-
mined cations exceeded the CEC by a factor of about 3.
This observation could be explained by the fact that most
of the cations in the ash contained in the charcoal were
not bound by electrostatic forces but present as dissolv-
able salts and, therefore, readily available for plant up-
take. From these results it might be concluded that char-
coal is not only a soil conditioner which increases the
CEC (Glaser 1999; Glaser et al. 2000, 2001a) but may
act as a fertilizer itself. Applications of charcoal which
inevitably contain ash add free bases such as K, Ca, and
Mg to the soil solution, increasing the pH value of the
soil and providing readily available nutrients for plant
growth. However, despite the positive effects of charcoal
additions on CEC and available nutrients, high charcoal
applications of >100 Mg ha–1 are certainly inadequate
for practical use. But estimates from this study allow the
conclusion to be drawn that also lower amounts of added
charcoal will significantly increase the availability of
base cations in soil. Equally total N and available P
forms were higher after charcoal amendments to soil
(Table 2).
Charcoal amendments and nutrient retention
Not only the nutrient contents but also the nutrient reten-
tion can be improved with charcoal additions to soil, but
not with the addition of coal degradation products such
as coal ash or fly ash. This is especially important in
highly weathered soils with low ion-retention capacities.
Mixing relatively large amounts of hardwood charcoal
with soil increased the CEC by 50% compared to the un-
amended soil (Tryon 1948; Mbagwu and Piccolo 1997;
Table 2). But even low amounts of weathered charcoal
(Glaser 1999) could increase the CEC of soil (Table 2),
whereas the additions of coal ash (Cox et al. 2001) or fly
ash were ineffective in increasing the nutrient retention
in soil. Fujita et al. (1991) estimated an anion sorption
capacity of pure wood charcoal of 88.2 cmolckg–1. In
comparison, soils of Amazonia have 1.0–6.0 cmolckg–1
(Sombroek 1966), and a higher retention of ions may be
expected from charcoal applications. NH4+leaching
from an unfertilized Ferralsol was reduced when char-
coal was applied and resembled the low values found in
an Anthrosol (Terra Preta) with a high pyrogenic C con-
tent (Lehmann et al. 2002), indicating that NH4+was ad-
sorbed by the charcoal, and the elevated N uptake by rice
after the combined application of charcoal and fertilizer
was partly an effect of NH4+retention. This retention
could not be found for other cations or anions, because
K, Ca, and Mg were in higher supply with charcoal addi-
tions (Lehmann et al. 2002).
Additionally, charcoal has the potential to form or-
gano-mineral complexes (Ma et al. 1979), which was
also observed in charcoal-containing soils (Glaser et al.
2000). It is assumed that slow oxidation (biotic and/or
abiotic) on the edges of the aromatic backbone of char-
221
222
Table 2 Effective cation ex-
change capacity (ECEC), base
saturation, and nutrients of
soils amended with charcoal.
Oxi Oxisol, TP Terra Preta
Charcoal ECEC BS Available K Available Ca Available Mg Total N Available P
(g kg–-1) (cmolckg–-1) (%) (cmolckg–-1) (cmolckg–-1) (cmolckg–-1) (g kg–-1) (mg kg–-1)
Sand, hardwood charcoal (Tryon 1948)
0 3.4 35 0.03 1.00 0.17 0.7 7.0
150 4.2 155 0.22 6.01 0.29 1.2 23.0
300 5.1 281 0.46 13.46 0.41 2.4 37.4
450 5.9 336 0.57 18.56 0.71 2.6 37.7
Sand, conifer charcoal (Tryon 1948)
0 3.4 35 0.03 1.00 0.17 0.7 7.0
150 3.0 44 0.12 1.10 0.11 0.8 7.0
300 3.3 85 0.24 2.26 0.30 0.7 16.1
450 3.3 102 0.22 2.80 0.36 0.8 17.2
Loam, hardwood charcoal (Tryon 1948)
0 4.4 53 0.16 1.78 0.38 0.6 3.6
150 5.4 128 0.37 6.16 0.36 0.6 6.6
300 6.6 212 0.60 12.80 0.56 1.9 19.3
450 6.9 310 0.82 19.81 0.74 2.5 27.0
Loam, conifer charcoal (Tryon 1948)
0 4.4 53 0.16 1.78 0.38 0.6 3.6
150 4.3 55 0.22 1.82 0.33 0.6 4.4
300 4.3 89 0.29 3.03 0.52 0.7 5.7
450 4.2 140 0.37 4.90 0.62 0.7 9.0
Sandy soil, activated charcoal (Rajput and Sastry 1984)
0 85.6 – – – – 0.67 –
1 97.6 – – – – 0.68 –
Sandy soil, activated charcoal (Rajput and Sastry 1984)
0 10.0 – – – – 0.78 –
1 10.3 – – – – 0.78 –
Oxi, TP 30–40 cm (Glaser 1999)
0.1 (Oxi) 0.79 4 0.004 0.00 0.06 0.27 2
1.4 (TP) 3.20 97 0.239 2.58 0.33 0.45 183
0.1 (Oxi) 1.64 2 0.004 0.02 0.02 0.50 3
5.4 (TP) 2.54 89 0.026 1.62 0.63 0.74 285
0.1 (Oxi) 1.64 2 0.004 0.02 0.02 0.50 3
6.0 (TP) 7.93 99 0.034 6.51 1.30 0.70 199
0.2 (Oxi) 0.25 71 0.048 0.08 0.05 0.41 1
5.3 (TP) 2.00 25 0.014 0.32 0.17 1.03 70
0.1 (Oxi) 7.6 4 0.014 0.19 0.07 0.90 2
11.9 (TP) 28.9 60 0.026 15.43 1.71 2.31 98
Ferralsol, secondary forest charcoal (Lehmann et al. 2002)
0 5.4 96 2.8 1.5 0.9 3.2 8
135 29.0 98 25.8 1.7 1.0 4.0 11
Alfisol, coal humic acid (Mbagwu and Piccolo 1997)
0 5.6 97 0.2 2.0 1.0 – –
0.2 8.0 100 – – – – –
2.0 16.3 100 – – – – –
20.0 20.8 100 – – – – –
Inceptisol, coal humic acid (Mbagwu and Piccolo 1997)
0 6.6 69 0.2 2.7 3.5 – –
0.2 10.3 72 – – – – –
2.0 15.9 100 – – – – –
20.0 26.7 100 – – – – –
coal forming carboxylic groups is responsible for both
the potential of forming organo-mineral complexes and
the sustainably increased CEC (Glaser 1999; Glaser et
al. 2000, 2001a).
Not only metal ions but also dissolved organic matter
and dissolved organic nutrients will be retained through
charcoal additions to soil. There are numerous applica-
tions of charcoal and activated C in industrial processes,
but discussion of this would divert us from the reviewed
topic. However, it should be mentioned at this point that
charcoal can effectively sorb both polar compounds, such
as polar organic pesticides (Sudhakar and Dikshit 1999),
and hydrophobic molecules, such as polycyclic aromatic
hydrocarbons (Kleineidam et al. 1999; Schmidt and
Noack 2000), lignin and tannin (Mohan and Karthikeyan
1997). On the other hand, charcoal does not contain sig-
nificant amounts of (available) harmful substances such
as polycyclic aromatic hydrocarbons (Glaser 1999).
A higher nutrient retention can also be achieved
merely by a retention of soil water in micro- and meso-
pores (see next section). If water percolation through soil
can be reduced, nutrient leaching will also decrease. By
this mechanism nutrients can be retained which are nor-
mally not sorbed to soil and are very mobile and suscep-
tible to leaching, such as NO3–at high pH, or base cat-
ions at low pH. These results show that charcoal may
contribute to an increase in ion retention of soil and to a
decrease in leaching of dissolved organic matter and or-
ganic nutrients.
Charcoal amendments and water retention
and structural stability of soils
Low SOM contents may be responsible for the low
available water capacity and the weak structure of many
agricultural soils (Bembridge 1989; Mbagwu 1989;
McRae and Mehyus 1985; Piccolo et al. 1996; Rose
1991). Charcoal may not only change soil chemical
properties, but also affect soil physical properties such
as soil water retention and aggregation (Piccolo and
Mbagwu 1990; Piccolo et al. 1996). These effects may
enhance the water availability to crops and decrease ero-
sion (Mbagwu and Piccolo 1997; Piccolo et al. 1997).
Intensive mechanized cultivation heavily contributes to
the decline of SOM contents in soils in the tropics. At-
tempts have been made to improve the physico-chemical
properties of such soils through soil management prac-
tices based on the incorporation of organic residues like
green manure, organic wastes, and coal-derived humic
substances (Mbagwu et al. 1991; Piccolo and Mbagwu
1990). Some studies showed that the water holding
capacity of the amended soils increased relative to the
controls (Bembridge 1989; Mbagwu 1989; Rose 1991;
McRae and Mehyus 1985). The effectiveness of the
organic residues depended, however, on the type and
stage of decomposition as well as on soil characteristics
(Mbagwu 1989). An important disadvantage of using or-
ganic residues is that large amounts, between 50 and
200 Mg ha–1, were required to obtain substantial im-
provements in both soil water retention capacity and
structural stability. For practical field applications, these
rates are not realistic (Piccolo et al. 1996). In addition,
organic wastes may be rich in inorganic and organic pol-
lutants.
Mbagwu and Piccolo (1997) compared the behaviour
of undecomposed organic residues and coal-derived
humic acids with respect to their potential for improving
the aggregate stability of four soil orders from southern
Nigeria. The authors found substantial improvements of
the macro-aggregate stability, varying from 20% to
130%, following additions of low amounts (1.5 Mg ha–1)
of coal-derived humic acids, whereas very large amounts
(50–200 Mg ha–1) of undecomposed organic residues
were needed to obtain significantly higher aggregate sta-
bility. Reduced soil bulk density following humic acid
amendments was reflected by the increased total porosity
and macroporosity as well as water infiltration rates.
Piccolo et al. (1996) studied the potential of coal-derived
humic substances to improve the available water holding
capacity and aggregate stability of typical Mediterranean
soils in the laboratory using agricultural surface soils (0–
20 cm) from three regions (Sicily, Tuscany, and Venetia)
and five rates of humic acid additions (0, 0.05, 0.10,
0.50, 1.0 g kg–1 on a dry matter basis). The field capacity
and the available water holding capacity were signifi-
cantly (P<0.05) higher when only 0.05 g kg–1 humic ac-
ids was applied to soil. At an application rate of 1.0 g
kg–1, the relative improvements in available water hold-
ing capacity over the control were 30%, 10%, and 26%
for soils of the three different regions, respectively. Low
rates (0.05–0.10 g kg–1) of humic acids were also needed
to obtain a 40–120% increase in aggregate stability rela-
tive to the control. The possible mechanisms by which
coal-derived humic acids improve soil physical proper-
ties are the formation of organo-mineral complexes by
functional groups of the humic acids. The hydrophobic
polyaromatic backbone reduces the entry of water into
the aggregate pores leading to an increased aggregate
stability and water availability.
On the other hand, it is well known that charcoal has
a high surface area due to its porous structure. Kishimoto
and Sugiura (1985) estimated inner surface areas of
200–400 m2g–1 of charcoal formed between 400°C and
1,000°C. As a result soil water retention increased by
18% upon addition of 45% (by volume) charcoal to a
sandy soil (Tryon 1948). Glaser et al. (2002b) reported
that charcoal-rich Anthrosols whose surface areas were
3 times higher than those of surrounding soils increased
the field capacity by 18%. Tryon (1948) also studied the
effect of charcoal on the percentage of available mois-
ture in soils of different textures. Only in sandy soil did
the addition of charcoal increase the available moisture
(Table 3). In loamy soil, no changes were observed, and
in clayey soil the available soil moisture even decreased
with increasing coal additions, probably due to hydroph-
223
Table 3 Effect of charcoal on percentage of available moisture in
soils on a volume basis (Tryon 1948)
Soil 0% 15% 30% 45%
Charcoal Charcoal Charcoal Charcoal
Sand 6.7 7.1 7.5 7.9
Loam 10.6 10.6 10.6 10.6
Clay 17.8 16.6 15.4 14.2
obicity of the charcoal. Therefore, improvements of soil
water retention by charcoal additions may only be ex-
pected in coarse-textured soils or soils with large
amounts of macropores.
Controlling the chemical properties of charcoal
Information on the physico-chemical properties of char-
coal in soils is limited (Golchin et al. 1997a, 1997b).
Scientists assume that charcoal consists of a range of
combustion-produced materials with high amounts of
aromatic, elemental, or graphitic C (Goldberg 1985;
Schmidt and Noack 2000). The polynuclear aromatic and
heteroaromatic ring systems as structural units have been
unambiguously shown for coals (Hayatsu et al. 1981)
and charcoals (Haumaier and Zech 1995; Glaser et al.
1998). Due to this structure, a chemical and microbiolog-
ical inertia is attributed to charcoal.
Three main factors influence the properties of char-
coal: (1) the type of organic matter used for charring, (2)
the charring environment (e.g. temperature, air), and (3)
additions during the charring process. The source of
charcoal material strongly influences the direct effects of
charcoal amendments on nutrient contents and availabili-
ty. The pH of various soils was higher after applications
of hardwood (pH 6.15) than of conifer charcoals
(pH 5.15) probably due to their different ash contents of
6.38% and 1.48%, respectively (Table 2). Thus, hard-
wood charcoals are more effective in reducing soil acidity
than conifer charcoals. Consequently, also the Ca, Mg,
and K contents were higher with hardwood than with co-
nifer charcoals. Even more pronounced was the different
effect of both types of charcoal on CEC. While hardwood
charcoal increased the CEC compared to the original soils
it decreased when conifer charcoal was used (Table 2).
The charring conditions were shown to influence
the degree of aromaticity and, therefore, the adsorption
characteristics of charcoal. The degree of aromaticity
increased with increasing charring temperature
(Shafizadeh and Sekiguchi 1983) and charring time
(Glaser et al. 1998). Charcoals obtained by heating at
200–700°C with exclusion of air had fewer ion-exchang-
ing functional groups than when oxidized with air
(Table 4).
The properties of charcoal can additionally be
influenced by additions of N compounds. For coal,
Aleksandrov et al. (1988) reported that the oxidation of
coal to humic acids was significantly enhanced by a 1-
month incubation in the presence of 5% diammonium
phosphate. Consequently, the sorption capacity increased
by 55–147%. Added N compounds chemically reacted
with coal forming carboxyl and phenol groups. Radlein
et al. (1996) produced an organic slow-release N fertiliz-
er by pyrolysing NH3or urea with organic wastes.
Knicker et al. (1996) showed that during coalification,
amide-N is progressively converted to stable pyrrolic N.
Charring at high temperature and pressure in the pres-
ence of a weak oxidizing reagent seems to be the best
way to produce carbonized materials with a high CEC
but requires specialized equipment. On the other hand,
charring of rape straw and reed under low pressure and
temperature without oxidants yielded CEC values of
80–130 cmolckg–1 which are up to 3 times lower than
the CEC values of SOM which ranged between 200 and
400 cmolckg–1 (AG Boden 1994; Russell 1973).
In addition to these three factors, microbes may also
have an important impact on the properties of charcoal
(e.g. surface oxidation). Some information exists about
the effect of coal flay ash on microbial activity in soil
and in turn on the degradation of coal by microorgan-
isms. This information may only be applied with caution
to charcoal, since different coals and lignites span a large
range of chemical structures which are only rudimentari-
ly known (Fakoussa and Hofrichter 1999; Schmidt and
Noack 2000). Microbial respiration, populations and en-
224
Table 4 CEC at natural
pH value (effective CEC,
ECEC; cmolckg–-1) of
charcoals formed under
different conditions
Charcoal Formation conditions ECEC Reference
Hardwood Unknown 5.5aTryon (1948)
Conifer Unknown 0a
Activated charcoal Unknown 1.2bRajput and Sastry (1984)
Maple Flash pyrolysis 480°C 490cRadlein et al. (1996)
Wheat straw Flash pyrolysis 500°C 440c
Poplar aspen Flash pyrolysis 450°C 490c
Poplar aspen Flash pyrolysis 500°C 440c
Poplar aspen Flash pyrolysis 550°C 450c
Peat moss Flash pyrolysis 520°C 300c
Flash pyrolysis 480°C 490c
Rape straw 4 h, 300°C 80cB. Glaser, unpublished data
Rape+HNO3+CaCO34 h, 300°C 120c
Reed 4 h, 300°C 100c
Reed+HNO34 h, 300°C 130c
Lignite 4 h, 165°C under O2pressure 1,230dGürüz (1980)
Lignite 4 h, 165°C under O2pressure 1,310e
Lignite 4 h, 165°C under O2pressure 1,200f
Lignite 4 h, 165°C under O2pressure 1,020g
a Calculated from the difference
between soil–charcoal mixtures
and soil alone (Table 5)
b CEC was determined with so-
dium acetate
c Calculated as the sum of car-
boxylic and phenolic groups
(Table 2)
d Base exchange capacity by
shaking with 1 N HCl for 16 h
e Base exchange capacity by
shaking with 1 N HNO3for 16 h
f Base exchange capacity by
shaking with 1 N H2SO4for 16 h
g Base exchange capacity by
shaking with 1 N H3PO4for 16 h
zyme activity decreased with increasing applications of
fly ash to a loamy soil (Pichtel and Hayes 1990), which
may also be decreased by charcoal applications. On the
other hand bacteria and fungi are able to liquefy low-
ranking coal (Fakoussa and Hofrichter 1999) and possi-
bly change its surface properties. Tschech (1989) ob-
served that hydroxylation of aromatic rings is one of the
primary steps in the microbial degradation of aromatic
compounds under aerobic conditions. Van Krevelen
(1961) stated that oxidation is the most important pro-
cess of coal weathering leading to an increase in func-
tional groups such as carbonyl and carboxyl in the coal
structure (Kister et al. 1988; Martinez and Escobar
1995). These observations support the hypothesis that in
soil oxidation of charcoal produces carboxylic groups
providing cation exchange sites (Glaser 1999; Glaser et
al. 2000, 2001a, 2002b). Possibly, manganese peroxi-
dase, laccase or hydrolase may also oxidize charcoal sur-
faces as described for coal (Fakoussa and Hofrichter
1999). This is an important research topic which should
be focused on in the future.
Effective C sequestration by soil amendments
of charcoal
Transformation of biomass C to stable soil organic C
The role of soils as a sink for atmospheric CO2is ambig-
uous. Some processes have been reviewed which may
lead to higher C sequestration in soils such as in im-
proved grasslands or agroforests as well as tropical pas-
tures through redistribution in deeper soil horizons or the
omission of burning (Batjes 1998). Much of the C is re-
leased as CO2upon the application of rapidly decompos-
ing organic fertilizers such as slurry (Glaser et al. 2001b)
or manure (Amelung et al. 1999; Bol et al. 2000) within
a short period, even in temperate climate ecosystems.
Therefore, such manures have to be frequently applied to
maintain high SOM and nutrient levels.
Also in common slash-and-burn systems, most of the
biomass C is rapidly released into the atmosphere upon
burning, and only small amounts of C are transformed
into charcoal (Table 5). In their pioneering work on bio-
mass burning, Seiler and Crutzen (1980) estimated char-
coal formation of about 25% in shifting cultivation fields
based on published photographs. The published data av-
erage at about 3% charcoal formation of the original bio-
mass C (Table 5). Biomass C which is not converted to
charcoal or elemental C in the smoke is gradually re-
leased through combustion and decay. Reburning may
affect the transformation of charcoal into slow-cycling
pools in either direction: by oxidizing charcoal formed in
the initial burning of primary forest or by creating new
charcoal (Fearnside et al. 1993; Graca 1997). On a glob-
al basis, an estimated 4–8 Gt of biomass C is annually
exposed to burning, of which 1.3–7.5 Gt is emitted to the
atmosphere through combustion and 0.5–1.7 Gt is con-
verted to charcoal (Seiler and Crutzen 1980). Therefore,
C entering the soil as charcoal is a significant sink for at-
mospheric CO2and may be important for global C se-
questration.
In comparison to burning, controlled carbonization,
converts even larger quantities of biomass organic matter
into stable C pools which are assumed to persist in the
environment over centuries (Glaser et al. 1998; Haumaier
and Zech 1995; Schmidt and Noack 2000; Seiler and
Crutzen 1980; Glaser et al. 2001a). The amount of char-
coal which can be produced from different forest vegeta-
tion primarily depends on the woody biomass available,
and additionally on the production procedure, such as the
charring environment (e.g. O2), temperature and time
(Table 5). The average recovery of charcoal mass from
woody biomass is 29% according to the published data
compiled in Table 5. The effect of different charcoal pro-
duction methods on its recovery in laboratory experi-
ments varies tremendously, depending on the charring
conditions, and even under field conditions charcoal and
C yields varied by a factor of up to 3, although it is
known that charcoal production is an exothermic process
taking place at between 350°C and 400°C (Falbe and
Regnitz 1992). The weighted average C recovery from
charred woody biomass is relatively high at 50% com-
pared to only 3% after conventional slash and burn tech-
niques (Table 5). Therefore, applications of charcoal to
soil could serve as a long-term CO2sink.
Recalcitrance of charcoal in soil
Theoretically, charcoal can disappear from soil in three
different ways: (1) by erosion from the surface, (2) by
abiotic, and (3) biotic degradation. Under soil erosion,
superficial charcoal accumulations in depressions have
been observed in the Amazon (Bassini and Becker
1990). Thermodynamically, the abiotic oxidation of ele-
mental C to CO2is a strong exothermic reaction
(∆H=–94,052 kJ). However, under environmental condi-
tions, this process is extremely slow (Shneour 1966).
Several studies showed a relationship between a deep
black soil colour and the presence of charred organic
matter (Schmidt and Noack 2000). These observations
support the concept that charcoal is an important source
of the chemically stable aromatic components of SOM.
The question arises whether the oxidation of charcoal is
significantly increased by the soil microorganisms. It is
well known that fungi and bacteria are capable of de-
grading low-rank coals such as brown coal (Fakoussa
and Hofrichter 1999). It was clearly shown by Hofrichter
et al. (1999) that extracellular manganese peroxidase is
the crucial enzyme of wood-rotting and leaflitter-decay-
ing basidiomycetes capable of degrading macromolecu-
lar fractions of brown coal (lignite). As a result of such
decay, reactive products such as phenoxy, peroxyl and
C-centred radicals are formed which subsequently under-
go non-enzymatic reactions leading to the cleavage of
covalent bonds, including the fission of aromatic rings.
If such reactions significantly contribute to the degrada-
225
tion of highly aromatic charcoal in natural environments
such as soil remains to be shown, because the presence
of charcoal from forest burning in soils and sediments
even after thousands of years indicates the high persis-
tence of this C species under natural conditions (Glaser
et al. 2000, 2001a; Saldarriaga and West 1986). This
point of view was corroborated by Shindo (1991) who
amended a natural soil with charred and uncharred plant
materials. The author found no evidence that the charred
material had been significantly utilized by the microbial
population after 40 weeks of incubation. On the other
hand, Shneour (1966) was successful in oxidizing artifi-
cial graphitic 14C to 14CO2in the presence of soils with
high microbial activity. In comparison to a sterile soil,
14CO2evolution which was 3–5 times higher was mea-
sured in the nonsterile soils. Therefore, charcoal is also
mineralized in soil and there is no doubt that charcoal is
not a permanent sink of atmospheric CO2. Otherwise the
earth’s surface would be converted into charcoal within
a period of time of <100,000 years (Kuhlbusch and
Crutzen 1995). However, it can be assumed that the turn-
over time of charcoal is much lower than that of plant lit-
ter applied to soil (Glaser et al. 2001a; Haumaier and
Zech 1995; Schmidt and Noack 2000; Seiler and Crutzen
1980; Shindo 1991). Therefore, the application of char-
coal will lead to higher C sequestration in comparison to
the application of equal amounts of non-charred organic
matter. Compared to slash-and-burn techniques, “slash-
and-char” significantly increases the C sequestration to
soil by a mean factor of 17 (Table 5).
The Terra Preta phenomenon – a model
for sustainable agriculture in the humid tropics
As mentioned above, Terra Preta, a black earth-like an-
thropogenic soil shows an enhanced fertility due to high
levels of SOM and nutrients such as N, P, and Ca (Glaser
1999; Glaser et al. 2000, 2001a; Smith 1999; Sombroek
1966; Zech et al. 1990; Table 2). Terra Preta soils are
typically embedded in a landscape of infertile soils oc-
curring in small patches averaging 20 ha, but 350-ha
sites have also been reported (Smith 1980, 1999). These
anthropogenic soils which are slightly older than
226
Table 5 Biomass conversion into charcoal. n.d. Not determined
Tree species Charring Production Charcoal nCharcoal C Source
temperature method recovery C content yield
(°C) by weight (%) (%)
(%)
Charring
Cellulose 300 Laboratory 89.4 2 44.0 92 Shafizadeh and Sekuguchi (1983)
325 furnace 63.3 2 47.9 71
350 31.8 2 59.9 45
400 16.6 2 76.5 30
450 10.5 2 78.8 19
500 8.7 2 80.4 16
Pinus sylvestris (saw dust)a300 43.3 2 68.5 59.2 Glaser et al. (1998)
Robinia pseudoacacia 350 37.9 60 76.4 64.4 Lehmann et al. (2002)
Acacia mangium 450 33.2 65 71.3 52.6 Lelles et al. (1996)
Eucalyptus camaldulensis 450 32.4 25 46.3 54.9 Vital et al. (1994)
Eucalyptus grandis 470 33.8 60 80.7 60.6 Vital et al. (1986)
Deciduous trees 500 30.2 8 84.7 56.8 Zhurinsh (1997)
Leucaena leucocephala 350–400bMetal kiln 27.4 – 83.1 50.6 San Luis et al. (1984)
Coconut trunk 350–400b25.0 – 77.8 43.2
Mixed tropical wood, Manaus, Brazil 350–400bBrick kiln 41 – 74.8 68.2 Correa (1988)
Miombo woodland 350–400bEarth kiln 23.3 – n.d. – Chidumayo (1991)
Mixed tropical hardwood 350–400bEarth pit – – 69.0 – FAO (1983)
Secondary forest, fruit orchard 350–400bEarth mound 14.3 98 90.0 32.0 Coomes and Burt (2001)
Weighted averagec28.5 79.6 49.9
Charring
Secondary forest – Slash and – 15–23 Seiler and Crutzen (1980)
Secondary forest – burn – 5–10 Crutzen and Andreae (1990)
Primary forest, Manaus, Brazil – 3.5 Fearnside et al. (1993)
Primary forest, Manaus, Brazil – 4.7 Graca (1997)
Secondary forest, Altamira, Brazil – 1.6 Fearnside et al. (1999)
Secondary forest, Manaus, Brazil – 1.8
Average 3.0
a Weight and C yields decreased with increasing charring time,
values after 1 h and 2 h of charring
b It is known that charcoal production is an exothermic smoulder-
ing process where temperature increases to 350–-400°C after ini-
tial burning of the woody material (Falbe and Regnitz 1992)
c Due to the tremendous differences among different charring con-
ditions the average was weighted with the number of replicates
from each study
2000 years can be found throughout the Brazilian Ama-
zon basin (Glaser 1999; Glaser et al. 2000, 2001a;
Sombroek 1966; Sombroek et al. 1993; Woods et al.
1999; Zech et al. 1990) and other regions of South
America such as Ecuador and Peru (W. Zech, unpub-
lished data) but also in West Africa such as in Benin and
Liberia (Zech et al. 1990) and in the savannas of South
Africa (Blackmore et al. 1990).
Terra Preta soils are very popular among local farm-
ers and are preferably used to produce cash crops such as
fruits and vegetables, which have higher yields and more
rapid plant development than on surrounding infertile
soils (German and Cravo 1999; Lehmann et al. 2002).
Fallow periods on Oxisols usually last 8–10 years,
whereas fallow periods on Terra Preta soils which lead to
the effective restoration of their fertility can be as short
as 6 months (German and Cravo 1999). The cropping pe-
riod on Terra Preta, however, is generally shorter than on
adjacent Oxisols, i.e. 0.3–2 years in comparison to
2–3 years, respectively, due to weed invasion on the fer-
tile soils (German and Cravo 1999). Weed management
has to be considered in continuous cropping on Terra
Preta and deserves more attention.
Terra Preta soils not only contain higher levels of
available nutrients but also higher amounts of stable
SOM (Glaser 1999). The total organic C stocks can be as
high as 250 Mg ha–1 in the agronomically important soil
depth of 0–0.3 m and as high as ca. 500 Mg ha–1 up to
1 m soil depth (Glaser 1999). In comparison, adjacent
Oxisols/Ferralsols may contain only 100 Mg C ha–1 and
149 Mg C ha–1 in the 0–0.3 m and up to 1 m soil depth,
respectively. Therefore, C sequestration was 3.4 times
higher in the Terra Preta than in adjacent soils. The total
C sequestration on a landscape level is unclear and
should be the subject of future research.
Additionally, the stable C is very resistant to microbi-
al decay. Recent investigations showed that Terra Preta
soils contained up to 70 times more pyrogenic C (char-
coal) than the surrounding soils (Glaser 1999; Glaser et
al. 2001a). It is assumed that pyrogenic C persists in this
environment over centuries due to its chemical stability
caused by the aromatic structure. The complex chemical
structure makes the compound also resistant to microbial
degradation (Goldberg 1985; Schmidt et al. 1999; Seiler
and Crutzen 1980). This assumption was emphasized by
14C dating results of 1,000–2,000 years for this C type
(Glaser et al. 2000). Oxidation during this time produced
carboxylic groups on the edges of the aromatic back-
bone, which increased the nutrient retention capacity. It
was concluded that pyrogenic C found in these anthropo-
genic soils could act as a significant C sink and is a key
factor for maintaining fertile soils, especially in the tro-
pics. On the other hand, according to Ladd et al. (1993)
every compound is degraded in soil and only physical
protection can make C compounds resistant to microbial
degradation.
Economic and ethnologic viability
With regular fertilizer and lime applications, crop pro-
duction can be considerably extended beyond typical
cropping cycles for shifting cultivation (Smyth and
Cassel 1995). However, the high amounts of required
mineral fertilizers cannot be afforded by subsistence
farmers, especially in remote areas. Charcoal and other
carbonized organic matter can be easily produced by lo-
cal farmers in most regions of the world. The procedures
of charcoal production are well known and the required
tools and resources (organic materials) are readily avail-
able (Graca 1997). However, charcoal is a valuable cash
product in most developing countries (e.g. Coomes and
Burt 2001). Therefore, it should be emphasized that
charcoal production for fertilization purposes will only
be economically feasible if organic waste products are
charred and applied as fertilizer, whereas slashed timber
is certainly more valuable when used as construction ma-
terials or as charcoal sold on the local market.
Although charcoal-enriched Terra Preta soils typically
contain between 15 and 60 Mg ha–1 of charcoal in the
agronomically important depth of 0–0.3 m, charcoal ad-
ditions in the range of 1–3 Mg ha–1 might be sufficient
for significantly increasing crop production as shown be-
fore. To produce 1 Mg charcoal, a kiln containing
9.45 m3wood-equivalent and a labour investment of
about 26 days for one person would be necessary
(Coomes and Burt 2001). Sold on the local market,
this amount of high quality charcoal would yield about
80 US dollars. Low-quality charcoal (waste charcoal
from high-quality charcoal production or using organic
wastes as kiln loads) could be used for soil fertilization
purposes without additional labour investments. Thus
money could be saved which would otherwise be spent
on buying commercial fertilizers. Additionally, charcoal
additions to soil sustainably increase soil fertility which
saves labour and investment costs as mineral fertilizers
have to be applied more frequently. Both from an eco-
logical and economic point of view, it seems most prom-
ising to replace slash-and-burn systems by slash-and-
char techniques. To what extent charcoal amendments
may substitute mineral and organic fertilizers and if they
are economically feasible for cash crop production have
not been clarified, and warrant further research.
Conclusions
The use of charcoal as a soil conditioner for sustainable
agriculture in the humid tropics presents the following
advantages:
1. High nutrient contents and nutrient retention capacity
lead to improved nutrient supply for plants and re-
duced nutrient losses by leaching. We assume that
two processes are responsible for this: (1) nutrients
are physically trapped in the fine pores of amorphous
carbonized materials, and (2) slow biological oxida-
227
tion produces carboxylic units on the edges of the
condensed aromatic backbone of the charcoal which
increases the CEC.
2. Transformation of labile plant organic matter into sta-
ble C pools can reduce the release of the greenhouse
gas CO2into the atmosphere during land clearing and
can increase C sequestration in the soil. There are
strong indications that charcoal is very slowly miner-
alized in the soil environment.
3. Charcoal from fallow vegetation and/or organic
wastes can be easily produced by local farmers and
also by those with a low income. Charcoal production
is a well-known technique and the required tools and
resources are readily available.
Using charcoal as a tool for improving soil fertility while
at the same time increasing C sequestration in soil is far
from being a well-recognized technology. However, the
demonstrated positive effects of charcoal additions on
soil properties and productivity should stimulate further
investigations to test the possibilities for developing a
slash-and-char technique as an alternative to traditional
slash-and-burn or slash-and-mulch systems. Within such
a framework the charcoal would be generated from the
same field to which it is applied, and a slash-and-char
technique does not therefore bear the danger of addition-
al forest destruction. Future research needs to focus on
testing charcoal amendments in experimental plots and
under field conditions and achieving a better understand-
ing of chemical and physical properties of charcoal sur-
faces. Finally, an evaluation of the agronomic effective-
ness and the economic viability of charcoal as a soil
amendment under field conditions is needed.
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