Biology and Fertility of Soils

, Volume 41, Issue 1, pp 15–21

Manioc peel and charcoal: a potential organic amendment for sustainable soil fertility in the tropics


  • Stéphanie Topoliantz
    • Museum National d’Histoire Naturelle
    • Museum National d’Histoire Naturelle
  • Sylvain Ballof
    • Office National des Forêts
Original Paper

DOI: 10.1007/s00374-004-0804-9

Cite this article as:
Topoliantz, S., Ponge, J. & Ballof, S. Biol Fertil Soils (2005) 41: 15. doi:10.1007/s00374-004-0804-9


In tropical areas, where crop production is limited by low soil quality, the development of techniques improving soil fertility without damage to the environment is a priority. In French Guiana, we used subsistence farmer plots on poor acidic soils to test the effect of different organic amendments, bitter manioc peel (M), sawdust (Sw) and charcoal (Ch), on soil nutrient content, earthworm abundance and yard-long bean (Vigna unguiculata sesquipedalis) production. The peregrine Pontoscolex corethrurus was the only earthworm species found. Pod production and plant growth were lowest in unamended soil. The application of a mixture of manioc peel and charcoal (M + Ch) improved legume production compared with other organic mixtures. It combined the favourable effects of manioc peel and charcoal. Manioc peel improved soil fertility through its low C:N ratio and its high P content, while charcoal decreased soil acidity and exchangeable Al and increased Ca and Mg availability, thus alleviating the possible toxic effects of Al on plant growth. The M + Ch treatment was favourable to P. corethrurus, the juvenile population of which reached a size comparable to that of the nearby uncultivated soil. The application of a mixture of manioc peel and charcoal, by improving crop production and soil fertility and enhancing earthworm activity, could be a potentially efficient organic manure for legume production in tropical areas where manioc is cultivated under slash-and-burn shifting agriculture.


Organic farmingSlash-and-burn cultivationEarthworm densityLegumeSoil nutrient content


In the tropics, traditional shifting cultivation still supplies food to 300–500 million families (FAO 1985). However, in some areas, under increasing population pressure and lack of cultivable land, farmers usually shorten the fallow period (Seiler and Crutzen 1980; Brady 1996). Biological soil functioning is rapidly perturbed when the balance between plant needs, nutrient availability and decomposer activity is disrupted by the shift from traditional slash-and-burn agriculture towards permanent cultivation (Myers and De Pauw 1995; Lavelle 1997). Without cheap, reliable techniques to maintain the fertility of acid soils under permanent cultivation, crop production rapidly decreases and poverty increases among farmers.

Considering the cost of inorganic, synthetic fertilizers for poor local farmers, and significant leaching losses in sandy soils poor in organic matter, new fertilising techniques in the tropics increasingly rely on the management of soil biota, a key factor in soil fertility (Lavelle et al. 1989; Beare et al. 1997). Methods using green waste and soil fauna activity were developed to improve crop production by stabilising nutrients and organic matter within soil biogenic structures (Sparovek 1998; Senapati et al. 1999).

Senapati et al. (1999) improved tomato production in Peru with composted sawdust, reaching the same level as with inorganic fertilizers. Several studies, reviewed by Glaser et al. (2002), showed that charcoal, through its positive effects on soil physical and chemical properties, can improve microbial activity and crop production. In the presence of the endogeic earthworm Pontoscolex corethrurus (Glossoscolecidae, Oligochaeta), a species adapted to tropical cultivated soils (Lavelle et al. 1987), we previously found that yard-long bean (Vigna unguiculata sesquipedalis L.) production was improved by amendment with manioc peel and to a lesser extent by charcoal. These two amendments, respectively, increased P availability and decreased soil acidity (Topoliantz, unpublished doctorate thesis). In Latin America and Africa, bitter manioc (Manihot esculenta ultilissima Crantz) is widely cultivated for its tubers (Guillaumet 1996), and their peel, produced in large quantity, provides a potentially valuable green waste that can be used for vermicomposting despite its cyanide toxicity (Mba 1983, 1996).

Tian et al. (1997) found that combining mulches of different quality could help to synchronize nutrient supply with plant needs. The goal of our present study was to test the combined effects of manioc peel, charcoal and sawdust on yard-long bean production, in the true conditions of subsistence farming. Considering the whole agroecosystem, plant-soil-fauna, we also determined how mulches affected the soil nutrient content and the earthworm populations as compared with unamended soil. The trial was carried out in the local farming context in order to provide new cheap solutions to improve crop yields in the tropics.

Materials and methods

Site and soil amendment

The experimental study was conducted in French Guiana at Maripasoula (3°39′N; 54°2′W) in a local farmer’s field (Topoliantz et al. 2002). The soil was an Oxisol with 58% sand, 14% silt, 24% clay, and 3% organic matter in the top 10 cm. Soil preparation for legume culture was carried out according to local practices. In April 2001 the natural herb layer was cut and the soil was hoe-harrowed to 20 cm depth. At the soil surface, rows of different organic mixtures composed of weakly composted manioc peel (M), local home-made charcoal (Ch) and fresh sawdust from a local sawer (Sw) were applied at the rate of 100 l per row (67 l m−2). The chemical composition of M, Ch and Sw is presented in Table 1. Equal volumes of organic wastes were mixed together and were covered with soil to form mounds, as in traditional practice. There were four mounds amended with M + Ch, M + Sw, Ch + Sw, and M + Ch + Sw and a control mound made of non-amended soil (NoA), each 25 cm wide and 6 m long, placed at 1 m distance from each other, parallel to the gentle land slope. Lack of space and allowance for traditional methods of cultivation prevented us imposing a randomized block design on the local farmer who kindly allowed us to perform the experiment on his own land.
Table 1

Chemical composition of the different amendments (Topoliantz et al. 2002)

Manioc peels

Wood charcoal


pH (H2O)




Ashes (%)




Total C (g kg−1)




Total N (g kg−1)




C:N ratio (g kg−1)




Total P (g kg−1)




Total K (g kg−1)




Crop production and plant sampling

Four months after incorporating organic mixtures to the soil, i.e. in October 2001, seeds of yard-long bean (V. unguiculata sesquipedalis) were sown in the mounds, with pairs of seeds in holes about 40 cm apart. Two months later, i.e. in December 2001, plants were harvested by hand, paying attention to the root system, and the survival rate was measured. Aerial parts were separated from root systems, shoots were dried in newspaper and root systems were preserved in ethyl alcohol until laboratory analysis. Legume nodules were isolated from roots, then all plant parts (shoots, pods, roots and nodules) were dried at 105°C and weighed. Aphids present on root systems were collected from the fixative and counted.

Earthworm sampling

Earthworms were sampled in the mounds at the time of crop harvesting (December 2001) according to the TSBF method (Anderson and Ingram 1993). Four soil blocks 25×25×30 cm were collected 2 m apart on the ridge of each mound, thus totalling 20 samples. Earthworm individuals and cocoons were sorted by hand and counted. Earthworms were also sampled with the same method under adjacent uncultivated herb vegetation. Twelve samples were taken in four rows 3 m long and 90 cm apart. All earthworm individuals were identified as P. corethrurus. They were sorted according to their size, as juveniles (newly hatched individuals) and adults (clitellate as well as non-clitellate), before being replaced in the soil.

Soil sampling and analyses

In the gaps between the five mounds, a total of 18 (4×4) soil samples were taken for determining the composition of the original soil at the time of soil preparation (April) and for checking gradients of soil fertility in the experimental field (Table 2). At the time of harvest (December) 11 soil blocks per experimental mound, each 7×7 cm area from 0- to 10-cm depth, were collected at different places on the ridges, thus totalling 55 samples. The soil was air-dried before transport to the laboratory for chemical analyses: organic C and N according to ISO 10694 and 13878, respectively (AFNOR 1999), soil acidity (pHwater and pHKCl), total P (Ptot) and available P (Pav, Olsen method), total K (Ktot), cation exchange capacity (hexamine cobalt extraction), exchangeable K (Kex), Mg (Mgex), Ca (Caex), Na (Naex) and Al (Alex) according to Baize (2000). The base saturation of the sorption complex (S/CEC, S = sum of exchangeable K, Mg, Ca and Na) was calculated. Cyanide contents were measured according to NF T90-107 (AFNOR 2001) in soil samples as well as in three samples of weakly composted manioc peel which had been dried for 48 h at 105°C.
Table 2

Chemical soil composition of the initial soil in the upper 10 cm (means of 16 replicates ± SE)

pH H2O


pH KCl


Total C (g kg−1)


Total N (g kg−1)


C:N ratio


Total P (mg kg−1)


Available P (mg kg−1)


Total K (mg kg−1)


Exchangeable K (mg kg−1)


Exchangeable Ca (mg kg−1)


Exchangeable Mg (mg kg−1)


Exchangeable Na (mg kg−1)


Exchangeable Al (mg kg−1)


CEC (cmolc kg−1)


Base saturation (%)


Statistical analysis

Crop, earthworm and soil chemical data were statistically analysed (Glantz 1997). Differences between rows (six) and slope positions (three) in chemical properties of the original soil were tested by two-way ANOVA without replication. The effects of treatments (M + Ch, M + Sw, Ch + Sw, M + Ch + Sw and NoA) on crop, earthworm and soil data were tested by one-way ANOVA (data were log-transformed when necessary) or Kruskall-Wallis rank tests when assumptions of ANOVA were not fully satisfied even after log transformation. Multiple comparisons among means or between groups were done a posteriori using Tukey’s test or Dunn’s test following ANOVA or Kruskall-Wallis rank test, respectively. The effect of treatments on plant survival was tested by χ2. Relationships between nodules, pods, shoot, root and aphids were tested by Spearman rank correlation coefficients. Comparisons of cocoon and earthworm abundances between natural fallow and other treatments were made using Dunnett’s method. The chemical composition of the initial soil (between mounds) and that of the non-amended mound (NoA) were compared by t-test.


There were no detectable differences between the six rows (four rows between mounds and two outside) in the 15 soil parameters which were measured on the initial (uncultivated) soil (Table 2). Only 2 out of 15 parameters showed a slope effect (weak, although significant at P <0.05 level): the P content decreased from 198 to 160 mg kg−1 from up- to downslope and the base saturation was maximum downslope (67%) and minimum at mid slope (48%). Given the absence of differences between rows and because replication was achieved along the slope within each treatment, we considered that treatment effects were not biased by spatial heterogeneity, despite the absence of block design.

The survival rate of bean (number of shoots living at the time of harvest against number of seeds planted) was only weakly affected by treatments (χ2 =9.38, P =0.052), although in NoA the observed survival rate was higher than in other treatments. Pod production (per shoot) was higher in M + Ch than in Ch + Sw and NoA treatments where it was nil or near nil (Fig. 1). The effects of treatments on other plant parameters are presented in Table 3. The dry weight of shoots and the shoot:root ratio were higher in the presence of amendments (M + Ch, M + Sw, Ch + Sw, M + Ch + Sw) but did not differ among amendments. The shoot:nodule ratio was higher in M + Ch than in NoA and the root:nodule ratio was not affected by treatments.
Fig. 1

Mean pod production of Vigna unguiculata sesquipedalis (yard-long bean) according to treatments (M manioc peel, Ch charcoal, Sw sawdust, NoA non-amended soil) with standard error. Numbers of replicates are shown in Table 2. Groups significantly different are denoted by different letters. Kruskall-Wallis rank analysis (H =18.5, P <0.001) was followed by a posteriori Dunn’s tests

Table 3

Biological parameters of Vigna unguiculata sesquipedalis according to treatments. Effects of treatments were tested by one-way ANOVA (F) or Kruskall-Wallis rank test (H). All weights and ratios are based on dry weights of plant parts. Groups significantly different according to a posteriori multiple comparisons are followed by different letters (M manioc peels, Ch charcoal, Sw sawdust, NoA non-amended soil)

M + Ch

M + Sw

Ch + Sw

M + Ch + Sw


Tested statistics

Number of shoots






Shoot weight (g dry wt)

7.08±1.92 a

7.43±1.51 a

3.92±0.83 a

5.76±2.42 a

0.64±0.08 b

F =18.6***

Shoot:root ratio (wt:wt)

7.92±0.52 a

8.12±0.69 a

7.78±1.34 a

8.73±1.17 a

3.84±0.35 b

F =9***

Pod production:shoot ratio (wt:wt)

0.31±0.08 a

0.11±0.08 ab

0±0 b

0.07±0.03 ab

0.08±0.04 ab

H =14.3**

Pod production:root ratio (wt:wt)

2.39±0.61 a

0.95±0.63 ab

0±0 b

0.67±0.28 ab

0.24±0.13 b

H =17.9***

Shoot:nodule ratio (wt:wt)

2,409±823 a

1,046±373 ab

1,300±891 ab

712±395 ab

384±82 b

H =13.9**

Root:nodule ratio (wt:wt)






H =9 N.S.

* P <0.05, ** P <0.01, *** P <0.001, N.S. not significant

Aphid density (number of individuals per gram of dry root, 4.2 in average) was not affected by treatments (H =0.574, P =0.97).

Figure 2 shows the distribution of cocoons, juvenile and adult/subadult P. corethrurus under each treatment and the natural fallow, at the time of harvest (December). As a consequence of cultivation, a decrease was observed in juvenile densities compared to the natural fallow, except under M + Ch amendment. Only Ch + Sw decreased cocoon densities compared to the natural fallow. No general decrease and no differential effect of treatments was observed in the adult/subadult population.
Fig. 2

Mean densities of cocoons, juveniles and adults/subadults of the earthworm Pontoscolex corethrurus according to treatments (4 replicates each) and in the natural fallow (12 replicates), with standard errors (M manioc peels, Ch charcoal, Sw sawdust). Treatment effects were compared by one-way ANOVA (F). Significance of F for each developmental category is indicated by: * P <0.05, ***P <0.001. Groups significantly different are labelled with different letters. One-way ANOVA was followed by comparisons between amendments and natural fallow using Dunnett contrasts

At the time of harvest, all soil nutrients except Na were significantly influenced by treatments (Table 4). A higher pHwater was found in treatments using charcoal (M + Ch, M + Ch + Sw and Ch + Sw) and the highest pHKCl was observed in the M + Ch treatment. Whatever the method used to measure pH, NoA was the most acidic. The C:N ratio was highest in amendments containing sawdust (M + Sw, Ch + Sw, M + Ch + Sw). Cation exchange capacity (CEC) was lowest in Sw + Ch and M + Sw + Ch treatments and highest in NoA. In this latter treatment, we found the highest rate of exchangeable Al and the lowest rate of exchangeable Mg, Ca and base saturation, the reverse being observed in the M + Ch treatment.
Table 4

Nutrient contents of the soil at the end of the trial according to treatments (means of 11 samples ± SEM). Otherwise as for Table 3

M + Ch

M + Sw

Ch + Sw

M + Ch + Sw


Tested statistics

pH H2O

4.88±0.03 a

4.62±0.05 b

4.81±0.06 a

4.88±0.04 a

4.40±0.04 c

F =21.3***

pH KCl

4.00±0.04 a

3.74±0.02 cd

3.84±0.03 bc

3.87±0.02 b

3.67±0.02 d

F =21.3***

Total C (g kg−1)

21.4±1.2 bc

29.1±2.2 ab

26.0±2.8 abc

32.5±3.2 a

19.6±0.6 c

F =5.8***

Total N (g kg−1)

1.27±0.08 ab

1.45±0.06 a

1.07±0.05 b

1.36±0.07 a

1.46±0.03 a

F =7.7***

C:N ratio

16.6±0.4 bc

19.9±1.1 ab

23.8±1.7 a

23.5±1.5 a

13.4±0.1 c

H =37.2***

Total P (mg kg−1)

171±11 ab

175±7 ab

159±21 b

167±8 ab

212±15 a

H =16.8**

Available P (mg kg−1)

14.9±0.7 bc

17.6±0.8 ab

13.0±0.7 c

18.1±0.8 a

15.0±0.5 bc

F =9***

Total K (mg kg−1)

736±31 bc

836±39 b

673±41 c

891±58 b

1,091±28 a

F =15.7***

Exchangeable K (mg kg−1)

2.67±0.20 b

5.05±0.33 a

2.59±0.30 b

4.66±0.45 a

2.20±0.11 b

F =21.2***

Exchangeable Ca (mg kg−1)

24.4±1.3 a

15.5±0.6 b

17.7±0.6 b

16.5±1.0 b

16.4±1.6 b

F =10.3***

Exchangeable Mg (mg kg−1)

4.59±0.37 ab

4.99±0.36 a

3.49±0.27 b

4.31±0.26 ab

2.23±0.14 c

F =14***

Exchangeable Na (mg kg−1)






F =1.9 N.S.

Exchangeable Al (mg kg−1)

3.84±0.23 c

6.34±0.41 b

4.98±0.32 bc

5.79±0.61 b

12.7±0.6 a

F =60.2***

CEC (cmolc kg−1)

2.42±0.11 ab

2.53±0.08 ab

2.17±0.09 b

2.31±0.13 b

2.71±0.08 a

F =4.2**

Base saturation (%)

70.0±1.6 a

53.6±1.9 b

58.6±1.8 b

58.8±3.3 b

39.6±2.3 c

F =23.3***

When compared to the composition of the original soil (Table 1), NoA exhibited a lower C:N ratio (t =4.7, P <0.001), pHwater (t =5, P <0.001) and pHKCl (t =3.5, P =0.001) and a higher exchangeable Al level (t =−3.7, P <0.001). Most cyanide levels were below the detection limit (<0.1 mg kg−1) except for three samples in the M + Sw + Ch treatment which showed a very low cyanide content (0.11, 0.11 and 0.12 mg kg−1) as compared with manioc peel (3.6 mg kg−1).


The development of aerial parts of V. unguiculata sesquipedalis L. was improved by amending mixtures as compared with non-amended soil. When nutrients are scarce, plants develop their root system at the expense of shoot growth (Taiz and Zeiger 1998). Nodulation was reduced by amendment, suggesting a poorer mineral N availability in the unamended soil (Salisbury and Ross 1985). In the absence of amendment we also observed a higher soil acidity compared with amended as well as initial soil. A high level of Al, often observed at low pH in the soil solution (Nair and Prenzel 1978), may be detrimental to cultivated plants by reducing root growth and uptake of P, Mg and Ca by roots (Foy 1974). The amendments did not exert any depressive effect on aphids, confirming that in this trial these sucking insects drain the sap flow without affecting plant growth (Salisbury and Ross 1985).

Pod production was highest in treatments with manioc peel, especially when supplemented with charcoal rather than sawdust. This result confirmed those obtained in our previous bioassay (Topoliantz et al. 2002), in which manioc peel amendment alone was the most productive treatment compared with charcoal, sawdust and non-amended soil. We attribute the beneficial effect of manioc peel to a higher P availability, the effect being reinforced in the presence of P. corethrurus, which stimulates the mineralisation of microbial P (Barois et al. 1987; Brown 1995). Phosphorus and K levels in manioc peel were estimated at 2.26 and 10.3 g kg−1, respectively, thus higher than in charcoal (0.268 and 5.05 g kg−1) and far higher than in sawdust (0.0633 and 0.347 g kg−1). However, at harvest time we did not detect any increase in available P in the soil under manioc peel, except when manioc peel was added together with charcoal and sawdust (Table 4). Given that pod production was maximized under manioc peel amendment (Table 3) and that soil collection for chemical analyses (Table 4) was done at harvest, we cannot rule out that at least part of the added P (38 g m−2) was mineralized then taken up by crop plants, and thus could not be detected in the soil. The negative effect of sawdust on pod production (Table 3) could be due to N immobilization, which has been repeatedly reported to occur in decaying wood (Swift 1977; Edmonds 1987).

At the time of harvest Al availability in the soil was lowest in the presence of a mixture of manioc peel and charcoal (Table 4). Charcoal, by increasing pH, diminishes mobile forms of Al (Tan 1982). We found that the soil amended with charcoal also presented higher levels of exchangeable Ca and Mg. Several studies previously showed that charcoal as well as its ashes increase the availability of P, K, Mg and Ca and decrease that of toxic elements such as heavy metals (Fe, Mn and Zn; Tryon 1948; Kishimoto and Sugiura 1985; Voundi Nkana et al. 1998). This effect can be attributed to the porous nature of charcoal, rather than to a liming effect, given its scarcity in nutrients when compared to manioc peel (Table 1). Charcoal exhibits a high internal surface (Carcaillet and Thinon 1996) which increases in the course of time and becomes progressively oxidised (Chan et al. 1999). Once chemically activated, charcoal acts as a filter and retains positively as well as negatively charged mineral ions in an exchangeable form (Holl and Horst 1997). Kishimoto and Sugiura (1985) showed that charcoal also improved soil porosity and moisture holding capacity, which led to better plant and root growth. In our study, lower soil acidity and toxic metals and a higher rate of macro-nutrients were observed when charcoal was combined with manioc peel (with a high phosphorus content); this also led to better plant growth. We suspect a synergistic effect of these two amendments due to the high surface area of charcoal that confers a high sorptive capacity for chemical compounds (Titoff 1910; Zackrisson et al. 1996; Wardle et al. 1998) and microbial communities (Pietikaïnen et al. 2000). Given the low content of swelling clays and organic matter in tropical Oxisols (Brady and Weil 1999), the existence of a stable, solid component with strong sorptive properties, such as charcoal (Skjemstad et al. 1996), would help to retain mineral nutrients liberated during the decomposition of organic amendments.

In contrast, at harvest soils from treatments with a mixture of manioc and sawdust exhibited the lowest pH and the same low base saturation as the complete mixture of manioc, sawdust and charcoal. Thus the addition of sawdust to a mixture of manioc and charcoal did not provide any benefit to legume production, even suggesting a negative effect of the three amendments when combined. Fresh sawdust is not palatable to saprophagous biota because of its low nutrient and high phenolic content (Tian et al. 1993). Thus the addition of non-composted sawdust may inhibit manioc peel decomposition. It seems that the positive effect of sawdust amendment on soil nutrient content may appear later than that of manioc peel because of much slower decomposition.

Only the mixture of manioc and charcoal did not depress earthworm densities. In the natural fallow, P. corethrurus juveniles and cocoons were mostly present in the upper 20 cm of the soil and adults tended to live more deeply (Topoliantz, unpublished doctorate thesis). Fragoso (1985) had already observed the same vertical distribution of P. corethrurus during the dry season. Under cultivation, adults and juveniles were mostly present at greater depths than in the fallow (Topoliantz, unpublished doctorate thesis), suggesting some avoidance effect of mounded soil, which was lessened under mixed manioc-charcoal amendment. Presumably earthworms burrowed deeper in order to avoid desiccation of the superficial soil layers, especially in non-amended cultivated soil. The manioc-charcoal mixture presented as many cocoons and juveniles as the natural fallow, indicating that this amendment did not interfere with P. corethrurus reproduction.

The cyanide content of our manioc peel was very low (3.6 mg kg−1) compared with that reported by Mba (1983) in decomposed peel (168.5 mg kg−1). Thus in our study this amendment did not produce cyanide toxicity in the soil. We also observed that soil aggregates 0.2–0.6 cm (mostly earthworm faeces) were negatively correlated with soil acidity (Topoliantz, unpublished doctorate thesis), indicating that charcoal, which increased pH, favoured juvenile earthworm activity. Although P. corethrurus is known to tolerate soil acidity (Lavelle et al. 1987), juveniles are apparently more sensitive to this factor, and thus any treatment increasing pH will favour this peregrine species which is able to ingest charcoal and mix it with the original soil (Topoliantz and Ponge 2003).

Under conditions that can be managed by subsistence farmers the combination of charcoal and manioc peel offers a cheap, reliable method to alleviate Al toxicity and to increase the nutrient status of acid tropical soils, through the buffer and filter power of charcoal and the input of P by manioc, thus improving legume production. It also provides favourable conditions for earthworm reproduction, allowing the maintenance of the fragile equilibrium between producers (plants) and decomposers (microbes, soil fauna) under tropical agriculture. As manioc peel is rapidly decomposed by soil biota, this amendment will have to be regularly added, whereas charcoal does not need to be supplied repeatedly, being persistent in the soil (Glaser et al. 2001). Furthermore, an excess of charcoal could even depress plant growth due to too high an alkalinity (Kishimoto and Sugiura 1985).


We thank the Mission pour la Création du Parc du Sud de la Guyane, the PPF Guyane of the Museum National d’Histoire Naturelle and the GIS Silvolab of French Guiana for financial support and commodities. We are grateful to Mr. Lobbini and his son, local farmers, for their technical assistance in the field. We also thank the Institut Pasteur (Lille, France) for cyanide analyses and the Institut National de la Recherche Agronomique (Arras, France) for mineral soil analyses.

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© Springer-Verlag 2004