Short communication

Effect of wood ash application on soil solution chemistryof tropical acid soils: incubation study

J.C. Voundi Nkana a,b,*, A. Demeyer c, M.G. Verloo c

a Institut f€uur Bodenkunde, Universit€aat Bonn, Nussallee 13, D-53115 Bonn, Germanyb IRAD, B.P. 2123 Yaound�ee, Cameroon

c Laboratory of Analytical Chemistry and Applied Ecochemistry, University of Gent, Coupure Links 653, B-9000 Gent, Belgium

Received 22 April 2002; received in revised form 15 May 2002; accepted 20 May 2002

Abstract

The objective of this study was to determine the effect of wood ash application on soil solution composition of three tropical acid

soils. Calcium carbonate was used as a reference amendment. Amended soils and control were incubated for 60 days. To assess

soluble nutrients, saturation extracts were analysed at 15 days intervals. Wood ash application affects the soil solution chemistry in

two ways, as a liming agent and as a supplier of nutrients. As a liming agent, wood ash application induced increases in soil solution

pH, Ca, Mg, inorganic C, SO4 and DOC. As a supplier of elements, the increase in the soil solution pH was partly due to ligand

exchange between wood ash SO4 and OH� ions. Large increases in concentrations of inorganic C, SO4, Ca and Mg with wood ash

relative to lime and especially increases in K reflected the supply of these elements by wood ash. Wood ash application could

represent increased availability of nutrients for the plant. However, large concentrations of basic cations, SO4 and NO3 obtained

with higher application rates could be a concern because of potential solute transport to surface waters and groundwater. Wood ash

must be applied at reasonable rates to avoid any risk for the environment.

� 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Wood ash; Lime; Soil solution; Tropical acid soils; Cameroon

1. Introduction

Wood ash is a by-product of the wood industry re-sulting from burning of wood residues for either energyproduction or waste reduction. As an alternative tolandfilling, land application of wood ash on agriculturalsoils appears a promising way of valorisation. Previousresearch has shown that wood ash is a significant sourceof lime and nutrients P, K, Mg and Ca (Etiegni andCampbell, 1991). The effectiveness of wood ash as aliming agent has been established (Clapham and Zi-bilske, 1992; Meiwes, 1995; Muse and Mitchell, 1995)and many studies showed that wood ash may also in-crease soil nutrients and enhance plant growth (Ohnoand Erich, 1990; Krejsl and Scanlon, 1996; VoundiNkana et al., 1998).

Knowledge of the chemical composition of the soilsolution can be a useful tool for evaluating the feasibilityof wood ash amendments. The soil solution is the mobilephase in soils from which plants derive their supply ofnutrients and the medium where soil chemical reactionsoccur (Curtin and Smillie, 1995). There is evidence thatthe chemistry of the soil solution of wood ash amendedsoils is a good indicator for solubility, mobility andavailability of nutrients and in general for the nutrientstatus of the soil. Unfortunately, few data on the effectsof wood ash application on soil solution chemistry areavailable for acid soils of the tropics. Such data areimportant to predict nutrient mobility and availabilityto plants and losses by leaching and may help to elab-orate a careful strategy including the use of wood ashamendment in management of chemical fertility oftropical acid soils. This study was undertaken to inves-tigate the effects of wood ash application on the soilsolution chemistry of three tropical acid soils of theforest zone of Central Cameroon. Calcium carbonatewas used as a reference amendment.

Bioresource Technology 85 (2002) 323–325

*Corresponding author. Tel.: +49-228-73-29-79.

E-mail address: jcvnkana@hotmail.com (J.C.V. Nkana).

0960-8524/02/$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.

PII: S0960-8524 (02 )00140-2

2. Methods

Surface samples (0–20 cm) from three strongly acidsoils (Kandiudult) in the forest zone of Central Cam-eroon were used in this study (Table 1). They are locatedat three sites, Mbalmayo (3�310N 11�300E), Mengang(Akonolinga) (3�460N 12�150E) and Nkolbisson (Yao-und�ee) (3�510N 11�300E), designated Mb, Me and Nk,respectively. Wood ash used was obtained directly fromburning wood residuals of CORON S. A. industry inYaound�ee and had an alkaline pH of 10.2, a calciumcarbonate equivalence (CCE) of 27.3% and was com-posed of 7.8% Ca, 0.8% Mg, 1.7% K, 0.14% P and 1.8%SO4.Two rates of wood ash and lime were applied plus

an unamended control. Each treatment was replicatedthree times. Based on the CCE of each material,amendments were applied at the amount correspond-ing to 1 and 2 times the level of exchangeable Al (incmol (1/3 Al3þ) kg�1). The incubation study was initi-ated by mixing the amendment with 0.5 kg of air-driedsoil. Deionised water was added to attain field capacityand the soils were homogenised. Samples were incu-bated in plastic pots at constant room temperature.After 0, 15, 30, 45 and 60 days of incubation, subs-amples corresponding to 100 g dry soil mixture, weretaken from each pot, brought to saturation and thenequilibrated overnight. The soil solutions (saturation

extracts) were displaced by centrifugation at 1300 g for20 min.

3. Results and discussion

Soil solution pH and EC were significantly higherwith wood ash than lime and increased significantly withincreasing application rates. The increase in EC wasassociated with increased concentrations of inorganiccarbon, SO4, Ca and Mg in wood ash and lime treat-ments and of K in wood ash treatments (Fig. 1). Nitrateappeared at 15–30 days and increased with increasingrates of amendments (data not shown). Ionic concen-trations were higher in wood ash treatments than in limetreatments.It was obvious that wood ash application affects the

soil solution chemistry in two ways, as a liming agentand as a supplier of nutrients. As a liming agent, woodash application induced increases in soil solution pH,Ca, Mg, inorganic C, SO4 and DOC as suggested bymany authors in the case of lime application (Bakkeret al., 1999; Derome and Saarsalmi, 1999; Hildebrandand Schack-Kirchner, 2000).As a supplier of elements, the increase in the soil

solution pH with wood ash was partly due to ligandexchange between wood ash SO4 and OH

� ions (Alvaand Sumner, 1990). Large increases in concentrations ofinorganic C, SO4, Ca and Mg with wood ash relative tolime and especially increases in K reflected the supply ofthese elements by wood ash and the high solubility ofwood ash (Ohno, 1992). The SO4 additions with woodash probably resulted in higher DOC concentrationsand the sorbing SO4 seemed to displace organic poly-electrolyte molecules from the soils (Kaiser and Ka-upenjohann, 1998). The relative higher NO3 levels in soilsolution from wood ash amended soils suggest thatwood ash enhanced nitrification more efficiently thanlime. The improvement of nitrification seemed to be theresult of induced output of DOC since, according toHildebrand and Schack-Kirchner (2000), this output is apotential energy source for heterotrophic nitrifiers.Besides additional input of other elements (SO4, Ca

and Mg) with wood ash, increased soil solution K ap-peared as the most important effect of wood ash on thesoil solution composition of tropical acid soils. There isevidence that wood ash application could represent in-creased in availability of nutrients for plants, particu-larly the nutrient base cations, SO4 and NO3. However,large concentrations of basic cations, SO4 and NO3obtained with higher application rates could be a con-cern because of potential solute transport to surfacewaters and groundwater. Wood ash must therefore be

Table 1

Selected properties of the soils used in this study

Characteristics Mb Me Nk

pH-H2O (1:5) 4.78 4.87 5.05

Organic carbon (%) 0.90 1.41 1.01

Total N (%) 0.13 0.18 0.14

Exchangeable Al

(cmolc kg�1)

3.42 2.72 1.10

ECEC (cmolc kg�1) 4.25 4.19 2.79

Particle size

distribution (%)

Sand 47 32 44

Silt 12 9 11

Clay 41 59 46

Moisture content

(% weight)

Field capacity 23 30 25

Saturation 41 48 37

Mineralogy

Fe2O3 (% clay) 7.66 10.36 9.54

Al2O3 (% clay) 2.14 3.25 1.76

Sand Qa Qa Qa

Silt F, K, M, Qa F, K, M, Qa K, M, Qa

Clay Go, Ka, M F, Go, Ka,M Go, Ka, Q

F: feldspar, Go: goethite, K: kaolinite, M: mica, Q: quartz.aDominant mineral.

324 J.C.V. Nkana et al. / Bioresource Technology 85 (2002) 323–325

applied at reasonable rates to avoid any risk for theenvironment.

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Fig. 1. Effect of time on Ca, Mg, K and SO4 in soil solution of soil Me amended with wood ash and lime. W1 andW2, L1 and L2 represent lower and

higher rates of application of wood ash and lime, respectively.

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