Plant Soil
https://doi.org/10.1007/s11104-021-04885-1
REGULAR ARTICLE
How nutrient rich are decaying cocoa pod husks?
The kinetics of nutrient leaching
D.-G. J. M. Hougni & A. G. T. Schut & L. S.Woittiez &
B. Vanlauwe & K. E. Giller
Received: 7 October 2020 /Accepted: 10 February 2021
# The Author(s) 2021
Abstract (less than 15%). Potassium leaching was mainly driven
Aim Recycling of cocoa pod husks has potential to by rainfall frequency (P < 0.05) and reinforced by in-
contribute to mineral nutrition of cocoa. Yet little is tense rainfall, especially at lower frequency. Under
known of the nutrient content and nutrient release pat- water-saturated conditions, 11% of K was leached out
terns from the husks. The potassium (K) rich husks are within 48 h from fresh husks compared with 92% from
usually left in heaps in cocoa plantations in Africa. We partially decayed husks.
aimed to understand and quantify release patterns of K Conclusion Some initial decomposition of cocoa pod
and other nutrients from husks under varying rainfall husks is required to expose K to intense leaching. As
regimes and assessed the effects of partial decomposi- decomposition progresses, abundant K losses are to be
tion and inundation on nutrient leaching rates. expected under frequent and/or intense rainfall events.
Methods We incubated chunks of cocoa pod husks to
assess decomposition rates and we measured nutrient Keywords Nutrient cycling . Potassium . Cocoa pod
leaching rates from two sets of husk chunks: one set was husks . Leaching tubes . Farmer practices
placed in tubes that were submitted to simulated sched-
uled rainfall events while the second set was continu-
ously inundated in beakers.
Results Decomposition of husks followed a second- Introduction
order exponential curve (k: 0.09 day−1; ageing constant:
0.43). Nutrient losses recorded within 25 days were Cocoa (Theobroma cacao L.) is a major source of
larger and more variable for K (33%) than for other income for about 5 million small-scale farmers
macronutrients released in this order: Mg > Ca ≈ P >N (Poelmans and Swinnen 2016). Approximately 74% of
the global production originates from four countries in
Editorial Responsibility: Alfonso Escudero West and Central Africa: Côte d’Ivoire, Ghana, Camer-
oon and Nigeria (ICCO 2020), where the cocoa planta-
D.<G. J. M. Hougni (*) :A. G. T. Schut : L. S. Woittiez : tions are among the least productive in the world
K. E. Giller
Plant Production Systems, Wageningen University and Research, (Oomes et al. 2016). A recent survey in the two coun-
Wageningen, the Netherlands tries that produce the most cocoa - Côte d’Ivoire and
e-mail: shadowhgni@yahoo.fr Ghana - set the average cocoa bean yields at 352 and
e-mail: deo-gratias.hougni@wur.nl 423 kg ha−1 respectively (Bymolt et al. 2018). This is
B. Vanlauwe less than one-tenth of the potential yield of cocoa in
R4D-Central Africa and Natural Resource Management, West Africa (Zuidema et al. 2005). The most often
International Institute of Tropical Agriculture, Nairobi, Kenya reported causes of this poor productivity are high
Plant Soil
incidence of pests and diseases, sub-optimal farming essential to improve plantation management. Al-
practices such as pruning and shade management, age- though previous studies have revealed that rainfall
ing of plantations, and the lack of adequate plant nutri- is the main driver of nutrient leaching from various
ent supply (Wessel and Quist-Wessel 2015). The vast crop residues (Calonego et al. 2005; Rosolem et al.
majority of cocoa plantations in West Africa is planted 2005; Salètes et al. 2004), little is known about cocoa
on cleared forest land whose nutrient capital accumulat- pod husks which have a thick and highly differenti-
ed over a long period (Ruf et al. 2014). The fertility of ated pericarp (Lu et al. 2018). Indeed, most experi-
the forest soil is progressively depleted by continuous mental designs did not differentiate the effect of the
nutrient offtakes in the cocoa beans with inadequate amount of rainfall from that of its frequency. Besides,
recycling or inputs of nutrients. when cocoa is grown in lowlands, heaps of cocoa pod
At cocoa harvest the ripe pods are plucked and taken husks may be exposed to temporary inundation. The
to a work station located on the plantation or nearby speed and magnitude of nutrient leaching under such
where the pods are split to extract the beans. The husks conditions remain poorly understood. Our research
of the cocoa pods are most often simply left to rot in objective was to describe the kinetics of nutrient
heaps at the work station. The cocoa pod husks contain leaching from the husks and evaluate the effects of
substantial amounts of nutrients, especially potassium rainfall amounts and frequency, biomass decay, and
(van Vliet and Giller 2017).With an amount of nutrients inundation on nutrient leaching patterns. We
estimated at 10.6–31.4 kg N ha−1 and 27.2– hypothesised that leaching rates of K depend on
77.2 kg K ha−1 for 1000 kg of cocoa beans harvested water saturation, rainfall frequency and amounts,
(Hartemink 2005), poor management of the husks can but not on biomass decomposition. To test this hy-
cause fairly large losses from the plantations. The fresh pothesis, three laboratory experiments were conduct-
cocoa husks weigh 10 times more than the dry beans ed. The first experiment quantified the decomposi-
they contain (Khanahmadi et al. 2016; Mansur et al. tion rate of cocoa pod husks at ambient temperature,
2014). Based on this ratio and with an estimated 80% high relative humidity, and in absence of biotic ma-
moisture content of the husks (Campos-Vega et al. nipulation; the second experiment assessed the effect
2018), the West African production of 3.5 million tons of the size of husk chunks and the amount and fre-
of dry beans (ICCO 2020) corresponds to 35 million quency of rainfall on nutrient loss rates from the
tons fresh matter (FM) and seven million tons dry matter chunks inserted in leaching tubes; and the third ex-
(DM) of cocoa husk generated in 2019, most of which is periment tested the rate of nutrient leaching from
not actively managed. Active management of the cocoa husks in conditions of permanent water saturation
pod husks thus provides a great opportunity to improve that simulated inundation.
nutrient cycling in cocoa plantations.
Recommended Good Agricultural Practices (GAP)
for cocoa plantations include active husk management Materials and methods
(e.g. composting, mulching). When the pods are infect-
ed with black pod disease due to Phytophthora spp., Thirty freshly-harvested cocoa podswere collected from
they should be incorporated in soil or burnt to avoid a smallholder cocoa farm in Cote d’Ivoire. After split-
inoculant dissemination through aerosols and insects. ting the pods and removing the beans and pulp, the
For healthy pods, it is recommended that farmers should husks were packed and shipped to the Netherlands
use the husks to mulch the plantation (Asare and David where they were stored at 4 °C. All the husks were
2011). Yet we lack a thorough understanding of how delivered as half-pod, broken longitudinally. Before
rapidly nutrients are lost from the husks or how they using any husk, we discarded 2 cm from both proximal
should best be managed as mulch. The initial high (close to the peduncle) and distal ends, as well as 1 cm
moisture content of the husks and humid climate in from the longitudinal edge to reduce any potential gra-
which cocoa is grown are conducive to rapid decompo- dient in nutrient partitioning as observed in pears
sition, and leaching losses from the soil can be further (Saquet et al. 2019) and to avoid bruised areas. We
exacerbated by abundant rain. carried out a preliminary analysis of the husk nutrient
Knowledge of the nutrient value and nutrient re- content and the values were well within indicative
lease patterns from cocoa pod husks is lacking, yet ranges (Table 1).
Plant Soil
Table 1 Composition of cocoa pod husks & Experiment 2: nutrient leaching from leaching tubes
Component Measured values Ranges References
(average±standard reported in The treatments applied in this experiment were the
deviation, %) literature (%) size of the chunks, the frequency of the simulated rain-
fall, and the rainfall amount per rain event. We selected
C 40.9–50.23 1, 3, 8 16 husks of 400–600 g each, which were sliced into
N 1.21±0.38 0.17–2.23 1, 3, 8 chunks of 2 g, 5 g or 25 g (hereafter referred to as small,
P 0.14±0.04 0.15–0.32 2, 3, 4, 8 medium, and large, respectively). Fifty grams fresh
K 2.89±0.84 2.52–3.8 2, 3, 4, 8 matter (FM) of chunks of the same size from each husk
Mg 0.26±0.06 0.11–0.28 2, 3, 4, 8 were used to fill individual PVC cylinders of 10 cm
Ca 0.26±0.10 0.18–0.46 2, 3, 4, 8 diameter to a height of 20 cm. Chunks from a given husk
S 0.14–0.97 1, 3, 8 served to fill entirely one or more tubes. Two leaching
Cellulose 12.9–35 2, 4, 5, 6, 7 treatments with demineralized water were used to sim-
Hemicellulose 8.7–12.8 2, 4, 7 ulate either a heavy (50 mm per event) or a light rain
Lignin 14–38.8 2, 4, 6, 7 shower (12.5 mm per event). The water was sprinkled
(1) Adjin-Tetteh et al. (2018) (2) Campos-Vega et al. (2018) over each tube at 2, 4, or 8 day intervals. A treatment
(3) Fidelis and Rajashekhar Rao (2017) (4) Lu et al. (2018) was a unique combination of either level of the three
(5) Mansur et al. (2014) (6) Thomsen et al. (2014) above-mentioned factors (e.g. ‘2 d, 50 mm, small’ rep-
(7) Titiloye et al. (2013) (8) Tsai and Huang (2018) resented a tube containing small chunks that received
every other day the equivalent of 50 mm of water, see
Appendix Table 3). A blank treatment (leaching tube
with no cocoa pod husk) was also included to detect any
Experimental setup background nutrient contamination. The experiment ran
for 31 days and was arranged in a 3 × 3 × 2 completely
We conducted three laboratory experiments. To analyse randomized design with three replicates. Room temper-
the relation between decomposition and leaching kinet- ature was maintained at 20 °C throughout the experi-
ics, an incubation experiment (Experiment 1) was ment, while a paper-cup placed on the top of the
established to measure the decomposition rate of husks. leaching tube raised the RH inside the tubes to about
We ran two separate leaching experiments which dif- 100%.
fered in the degree of water saturation and duration of The leaching tubes (Fig. 1) were blocked at their base
saturated conditions (Experiments 2 and 3). with a flat rubber-stopper perforated in the centre to
& Experiment 1: decomposition of cocoa pod husks create a small outlet. A filter-paper (20 μm mesh) was
inserted in the tube, and mounted on the top of the
In the first experiment the decomposition rate was rubber-stopper. The top of the tube was loosely covered
analysed. Six replicate husks were sliced into 10 chunks with a perforated paper-cup used as dripper that deliv-
of 25 g, giving a total set of 60 chunks that were used to ered 1–1.6 mL/min. All leachates were collected until
quantify biomass loss over 63 days. At the onset of the 48 h after water had been sprinkled; the volume was
experiment, the weight of each chunk was measured measured for each tube. A leachate sample was taken for
while the initial moisture contents of the husks were each tube and analysed for K content. In addition,
assessed from additional pieces. All the 60 chunks were selected samples (Appendix Table 3) were analysed
incubated in the dark at 20 °C and a relative humidity for other nutrients (N, P, Ca, Mg). At the end of the
(RH) of 100%, in aerobic conditions. To realise these experiment (day 31), residual DMweight of chunks was
conditions, the chunks were placed in individual alu- determined.
minium crucibles, floating in a tray filled with water, the & Experiment 3: potassium leaching under water-
whole apparatus being enclosed in a plastic foil, with an saturated conditions
ample headspace. At 0, 2, 4, 6, 8, 12, 16, 28, 49 and
63 days, one chunk per replicate was randomly selected In this experiment two batches of husks were sub-
and both fresh and dry weight (after 48 h in a 105 °C jected to permanent water-saturation: the first batch had
oven) were recorded. only fresh husks while the second one consisted of
Plant Soil
values at the start of the experiments. The data were
analysed for each experiment, with weight losses in
.
Legend experiments 2 and 3 estimated from curves developed
using data from experiment 1 (explained below). Model
performance was evaluated using the Akaike Informa-
Filter-paper
tion Criterion (AIC), the Schwarz Bayesian Criterion
Individual chunk (BIC), the residual mean square error (RMSE), and the
likelihood ratio test.
& Experiment 1
Dripper
To describe the decomposition of the chunks, we
used a second-order exponential model (Eq. 1, Yang
and Janssen 2000). It expressed the percentage residual
dry weight (C) as a function of time t (days), and was
characterized by a constant decomposition rate k (day−1)
and a rate-modifier termed the ‘ageing coefficient’ (a)
which basically allows for decrease of parameter k over
time [1, 6].
−ktaCi ¼ e for the ith husk; i∈ ð1Þ
Fig. 1 Sketch of an individual leaching tube in Experiment 2
A decomposition curve was fitted to observations
husks that already decayed for three-weeks to reflect the with a non-linear mixed-effects model with pod as the
initial phase of husk decomposition. Three husks served random factor to account for non-linearity of most bio-
as replicates, and were sliced into chunks of 25 g. Indi- logical processes and the expected pod-to-pod variabil-
vidual chunks (one per husk) were maintained at 15 cm ity in initial nutrient concentration. In doing so, we
depth in a beaker containing 2 l of distilled water. Over determined the model’s parameters and estimated the
the first 48 h, a sample of the solution (10 ml) was taken loss in weight of husks over time.
at 20 min, 40 min, 1, 1.5, 2, 3, 4, 6, 10, 20, 36, and 48 h
and each sample was analysed for its K content. The & Experiment 2
same procedure was repeated 3 weeks later with the
second batch, using chunks from the same pods that We first compared losses of the different nutrients, by
had been kept in the incubator at 20 °C at a relative plotting the average residual amounts observed in the
humidity of 100%. husks. We found that K losses followed curvilinear
Nutrient contents of leachates and of the initial husks patterns for most leaching tubes, whereas trends were
were estimated. Potassium (K), Ca, and Mg were deter- unclear for the other nutrients. Since we did not observe
mined using Atomic Absorption Spectroscopy (AAS), losses larger than 20% for the structurally-bound ele-
whereas N-NO −, N-NH +3 4 and P-HPO
2−
4 were measured ments (N, P, Ca, Mg), we only compared their respec-
spectrophotometrically with a segmented-flow analyser tive losses on day 25. For these elements we ran an
as described by Houba et al. (2008). Due to the relatively- analysis of variance (ANOVA) based on a linear mixed-
limited presence of ionic forms of N, P and Ca in the cell, effect model, considering only four contrasted treat-
we refer to these as “structurally-bound” elements in ments: ‘2 d, 12 mm, large’, ‘2 d, 50 mm, small’, ‘8 d,
contrast with K and to a lesser extent Mg. 12 mm, large’, and ‘8 d, 50 mm, small’ (see Appendix
Table 3 for detail of treatments).
Data analysis Potassium (K) was lost in larger and more variable
amounts than the other nutrients. All the treatments were
We expressed the residual amount of nutrients in husks thus taken into account in the following analyses. We
and the residual weight (DM) as percentage of the used a non-linear mixed-effect regression to describe K
Plant Soil
loss kinetics over a period of 50 days and to analyse the Xmidabcj j ¼ β0 þ β1X a þ β2Xb þ β3X c
variations among treatments. This model (see details
below) described residual K as a logistic function of þ β4X aX b þ β5X bX c þ β6X aX c
time, but it also involved a multiple linear regression of þ β7X aX bX c ð4Þ
the estimated parameters on the predictors (rainfall
amount, frequency, size of chunks). Because the effect and
of chunk size was minimal in the full model that ex- RL ¼ β þ β X a þ β Xb þ β X aX b ð5Þ
plained variation of one of the parameters, it was ex- abj j 8 9 10 11
cluded from the set of explanatory variables for that as sub-models,
parameter.
We compared the residual amount of the structurally- where Y was the percentage residual K measured
bound nutrients on the 25th day of experiment 2, based on the jth tube at X days, j ∈ [1, 54];
on a linear model (Eq. 2). The explanatory variables Asym was the coefficient describing the
were the type of nutrient (X1) and the treatment (X2). asymptotic value of initial K content
Yk;ij j ¼ β0 þ β1X 1 þ β2X 2 þ ð Þ (unitless);β3X 1X 2 2 Xmid was the coefficient describing the
inflexion point or time at 50% K loss
(days);
where was the residual amount of the kth nutrient, RL was the coefficient describing the
−1
Yk,i|j measured on the jth tube on which the ith leaching rate (day );
treatment was imposed, with j ∈ [1, 12], k ∈ Xa was the frequency at which rainfall
[1, 4], and i ∈ [1, 4]; events were scheduled, with a ∈ [1, 2];
X1 was the macronutrient analysed; Xb was the amount for individual rainfall
X2 was the treatment imposed on the chunks events, with b ∈ [1, 2];
in the jth tube; Xc was the size of the chunks, with c ∈ [1, 3];
β was the global intercept of the model; β0 and β8 were the global intercepts of the sub-0
β1, β2, and were the estimated coefficients associated models;
β to the individual effects of the β1-β7,and β9- were coefficients associated to the3
macronutrient, the treatment and their β11 individual and combined effects of either
interaction, respectively. predictor.
In order to describe patterns and evaluate variance of & Experiment 3
K loss through leaching, a non-linear regression (includ-
ing a logistic function) was fitted to data from experi- In inundated husks, K loss through leaching was
ment 2. The logistic function (Eq. 3) was required to fit expected as result of the strong concentration gradient
the general model which included two imbricated sub- between the immersed husks and the surrounding water,
models (Eq. 4 and Eq. 5) that were multiple linear in line with Fick’s first law of solute diffusion. To test if
regressions. The parameters Xmid (time to 50% K loss, different rates of cocoa pod husk decay resulted in
days) and RL (leaching rate, day−1) were regressed on similar trends of K loss, we compared K leaching kinet-
rainfall, rain frequency, and size, while the parameter ics from fresh chunks with a set of chunks that was
Asym (unitless) was kept constant throughout with a allowed to decompose for a 3-week period in an incu-
unique estimated value of 1.16 ± 0.02 (P < 10−4). bator at 20 °C. Loss of K under water-saturated condi-
Asym tions was described using Eq. 1, where C now repre-
Yabcj j ¼ ð ð3Þþ RL X−Xmid Þ sented the residual K (%) in the chunk at time t (hours).1 e abj j abcj j Similar to decomposition, the second-order decay curve
with was fitted to the data, using a non-linear mixed effects
Plant Soil
model. The estimated coefficients were compared using Effects of the size of chunks, amounts and frequency
Welch’s modified t-test (Welch 1947). of rainfall on potassium leaching
All the models were fitted in R (R Core Team 2013)
via restricted maximum likelihood (REML) using the Leaching curves
‘nlme’ package (Pinheiro et al. 2019) whereas t-tests
were run with the ‘BSDA’ package (Arnholt and Evans For most combinations of rainfall amount (12.5 and
2017). The mixed-effects models included a random- 50 mm) and frequency (2, 4 or 8 day intervals) the size
intercept (husks for Experiments 1 and 3, and tubes for of the chunks did not consistently influence K leaching
Experiment 2) with unconstrained variance-covariance patterns over the simulated period (0–50 days, Fig. 4),
matrices obtained via Log-Cholesky parametrization with exception of 12.5 mm rainfall at an 8-day interval.
(Pinheiro and Bates 1996). For this latter treatment, leaching curves were very
dissimilar across chunk sizes, with the smallest chunks
losing 33% more K than the largest chunks on day 50
Results (Fig. 4c).
The effect of the interaction between frequency of
Decomposition of cocoa pod husks rainfall and rainfall amount on K leaching was signifi-
cant (P < 0.05, Table 2). The rainfall frequency did not
Cocoa pod husk decay was described satisfactorily by a affect K loss patterns when 50 mm rainfall events were
second-order exponential function (R2 = 0.77, Fig. 2). provided. Treatments with 50 mm and 2, 4 and 8-day
The fitted second-order exponential model showed a rainfall frequency intervals did not differ more than 18%
sharp decrease with 24% and 32% DMweight loss after in the total amount of K lost on day 50 (Fig. 4b and c).
14 and 30 days respectively, gradually followed by a The estimated effect of rainfall frequency was larger at
phase of slower weight loss and reduced decomposition 12.5 mm rainfall. Treatments with 12.5 mm and 8-day
rates. Extrapolating this model to a year suggested that intervals lost 12–45% K on day 50 while the treatments
more than 33% of the initial DM weight of the husks with 12.5 mm and 2 day intervals lost 80–82% K (Fig.
would remain in absence of exogenous factors such as 4a and c).
human manipulation, macrofauna activity or fluctuating The amount of rainfall per rainfall event intensified K
environmental conditions. The second order model was losses, more so for less frequent rainfall events (Fig. 4).
more accurate (RMSE of 0.05%) than the first-order The differences between the treatments with 12.5 mm
model (RMSE = 0.11%) which had a smaller coefficient and 50 mm per rainfall event at 2, 4 and 8-day intervals
of determination (R2 = 0.71, Appendix Table 4). were 4–5%, 10–23% and 22–56% respectively.
We also compared rainfall regimes, 12.5 mm every
Overview of nutrient leaching under scheduled rain 2 days with 50 mm every 8 days, with the same cumu-
simulations lative amount of rainfall (200mm at day 31). The total K
lost did not differ between rainfall regimes, with largest
Of all the nutrients tested, K losses were strongest and differences about 13% on day 47 (Fig. 4). Based on the
most variable across the treatments (e.g. 33 ± 21% on model, it was expected that these differences would
day 25).More structurally-bound nutrients N, P, Ca, Mg become smaller if the experiment were run for a longer
recorded average losses of less than 15% at the end of period of time, to less than 5% by day 83 (not shown).
the experiment (day 31, Fig. 3). Nutrient losses on day
25 significantly differed among the more structurally- Model parameters
bound nutrients (P < 0.05), with Mg being the most
leachable among these (10 ± 9%). On the contrary, N The rainfall frequency had the largest effect on the
was hardly released throughout the experiment, with estimated values of time to 50% K loss (Xmid) and K
less than 1% lost from any of the treatments, suggesting leaching rate (RL, Fig. 5). Potassium leaching from
that the inorganic N content of the husk cells was either cocoa pod husks increased with rainfall frequency. Re-
very small or that any N released was strongly ducing rainfall frequency from 2 to 8-day intervals,
immobilized. Treatments did not affect nutrient losses resulted in increased Xmid value of 19 days (P <
(P > 0.05, Appendix Table 5). 0.001), decreasing leaching rates by 54% (P < 0.001).
Plant Soil
Fig. 2 Fitted decomposition
model to observations from
chunks of cocoa pod husks left at
20 °C, 100% RH. Each replicate
is represented by a different
symbol
Overall, quadrupling the amount of water did not have a compared to the 12.5 mm treatments. The effect of the
significant effect on the parameters Xmid and RL size of chunks was not significant, except for Xmid at a
(Table 2). However, at 8-day rainfall intervals, Xmid rainfall frequency of 8 days with 12.5 mm, with the
decreased by 15 days (P < 0.05), and RL increased from largest chunks retaining more K than the smallest (P <
0.03 to 0.05 day−1 (P < 0.05) for the 50 mm when 0.05, not shown).
Fig. 3 Average (± standard deviation) nutrient losses from cocoa labelled after the treatment with, in this order: rain frequency (2-
pod husks (n = 3) for selected treatments in a scheduled leaching day versus 8-day intervals), rainfall amount (12.5 versus 50 mm
experiment (Experiment 2). For each row, the minor y-axis was per event), and size of the chunks (small versus large)
Plant Soil
Fig. 4 Simulated effects of the chunk size of cocoa pod husks 50 days. Treatments are indicated in the legend in this order:
(line type) and rainfall (line colour) on K leaching patterns at rainfall frequency (day intervals), rainfall amount (mm), and size
different scheduled rain frequencies (subplots) over a period of of the chunks (small, medium or large)
Effect of decomposition on K leaching from cocoa pod water, temporarily preventing husks from further wa-
husks under water-saturated conditions ter absorption. In comparison, K leaching from husks
was much faster, with 81% and 92% losses recorded
Fresh husks under water retained most of their K with within 24 and 48 h respectively when inundated after a
less than 11% of K lost after 48 h (Fig. 6), partially period of 3 weeks of decay (Fig. 6), corresponding to a
accounting for nutrient release due to bruising during weight loss of only 28% in aerobic conditions (Fig. 2).
chunk slicing. The observed K loss followed an expo- The estimated values for both the k and a coefficients
nential pattern, with a small leaching constant (k = were significantly larger (P < 0.001, not shown) for
0.03 day−1). A protective jellified layer of hydrated these partly decayed husks when compared with fresh
pectates was formed around the chunks immersed in husks.
Table 2 Significance (probability ofWald F-tests) of the effects of rainfall frequency, rainfall amount, and size of the chunks on K leaching
patterns from cocoa pod husks in a scheduled leaching (experiment 2)
Main and Interaction terms of regression Estimated parameters
Asymptotic leachability Time to half K (Xmid) Leaching rate (RL)
Intercept <0.001 < 0.001 < 0.001
Rain frequency (‘Frequency’) < 0.001 < 0.001
Rainfall NS NS
Size NS
Frequency x Rainfall <0.05 <0.05
Frequency x Size <0.01
Rainfall x Size NS
Frequency x Rainfall x Size <0.01
NS non-significant. Empty cells indicate terms for which selected coefficients were not available as a consequence of constraints imposed on
the model structure
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Fig. 5 Estimated main effects (±95% confidence intervals) of pod husks in a scheduled leaching experiment (experiment2). All
rainfall frequency (a, d), rainfall amount (b, e), and size of chunks interaction effects are displayed in Appendix Table 6
(c) on time to 50%K loss (a, b, c) and K loss rate (d, e) from cocoa
Fig. 6 Changes in the residual K under saturated conditions for in grey-scale) represent fresh and partially decayed (3-week)
fresh and partially decomposed cocoa pod husks after a 3-week husks, respectively. Fitted models are shown with dashed and
incubation at 20 °C and 100% humidity. The changes in the solid lines for fresh and partially decayed husks, respectively.
percentage of residual K as a function of time were described with Coefficients are estimated in Appendix Table 7
a second-order exponential model. Circles and triangles (replicates
Plant Soil
Discussion weeds, household compost, fruit and vegetable wastes
(Ranjbar and Jalali 2012), wood chips and pruned ma-
To gain insights into the temporal availability of nutri- terials (Ordóñez-Fernández et al. 2014) have been stud-
ents in cocoa pod husks, we analysed the losses of K, ied in the past. The associated loss kinetics varied
Mg, Ca, N and P in leaching experiments. The husks strongly, depending on the crop residues and the exper-
contain large amounts of K with concentrations of about imental conditions. Potassium loss curves from cereal
3%. Potassium is a yield-limiting nutrient inmany cocoa straw, legume residues and prunings under intermittent
plantations, especially in more demanding environ- rainfall events were described by power or exponential
ments with limited shade conditions (Ahenkorah et al. functions (Lupwayi et al. 2006). Similar patterns were
1987). We observed that K leached rapidly from the proposed for empty fruit bunches, the major residues
husks, with leaching rates varying as a function of from oil palm milling (Caliman et al. 2001; Lim and
rainfall amount and frequency (Fig. 4). Under frequent Zaharah 2000). However, these functions do not ac-
and abundant rainfall events (2-day interval, 50 mm per count for the limited initial K loss that was observed in
event), we recorded less than 11%K losses after 10 days, the very first days of the second experiment we ran.
and up to 45% K losses after 31 days. Our fitted regres- We hypothesised that waterlogging provoked fast K
sion model predicted that 78% K will be lost after losses from fresh and decayed husks. Water saturation
50 days. By contrast, more structurally-bound nutrients provoked substantial K release for partially-
in the husks (such as Ca, P and N) did not leach in large decomposed husks, but not for fresh husks (Fig. 6)
amounts during the same period (less than 15% after suggesting that some decomposition is required to allow
31 days for all treatments, Fig. 3). Under inundated release of K. This is in contrast to Vanlauwe et al. (1995)
conditions, K leaching from decomposed husks oc- and Lupwayi et al. (2006) who found that K leaching is
curred rapidly when compared to fresh husks (92% independent of decomposition. The findings align with
versus 11% K respectively lost within 2 days, Fig. 6). observations from Calonego et al. (2005) that K losses
Therefore, inundation occurring during the first days from leaves increase with senescence. Similarly, Li et al.
after pod breaking would likely not deplete a husk pile (2014) recorded K losses of 80% in fully senesced rice
of K. This is likely because the cocoa pod has a waxy, straw in the first two hours and 90% after 3 days of
water repellent epicarp which needs to be broken down inundation.
before K can leach rapidly. Although the artificial con- Potassium leaching is not restricted to pod husks in a
ditions imposed in our experiments limit extrapolation cocoa plantation, but also occurs from canopies in forest
of the leaching rates to on-farm contexts, they show the environments (Moslehi et al. 2019). Rain droplets col-
relative importance of the factors tested when assessing lected at the forest floor contain 10–40 times more K
kinetics of nutrient leaching. Our results highlight the than rain droplets above the canopy (McDowell et al.
need for a proper on-farm management of fresh husks to 2020). Foliar K leaching (rain-wash minus direct
reduceK losses from cocoa farms, evenmore if there are rainfall) in 15–30 year old cocoa plantations was esti-
risks of inundation. mated at 23.1–39.7 kg ha−1 year−1, two to three times
To describe K leaching patterns under successive more than in sole rainfall (13.6 kg ha−1 year−1, Dawoe
rainfall events, we used a logistic function (S-shape) et al. 2017). Although the quantity of K leached from
since the exponential function (with initial drastic de- canopies is larger than from cocoa pod husks, it remains
crease) poorly fitted our data. We only observed an in the system in contrast to poorly managed husks.
exponential K release from cocoa pod husks under As hypothesised, the amount and frequency of rainfall
water-saturated conditions, reflecting a rapid initial loss altogether determined K loss patterns in our scheduled
of K. Extremely rapid release of K has been reported for rainfall simulations (Table 2). The effect of rainfall fre-
other crop residues (Talgre et al. 2014). We did not find quency was more prominent than that of rainfall amount,
literature reporting onK leaching from cocoa pod husks. and their interaction was significant. In general, rainfall has
Nutrient leaching from various organic materials such as a large effect on K leaching from crop residues (Cavalli
cereal straw (Li et al. 2014; Lupwayi et al. 2006; et al. 2018; Schreiber 1999). In a single rain event, K losses
Rosolem et al. 2005), legume residues (Lupwayi et al. from millet straw gradually increased with rainfall amount
2006); oilseeds, roots and tubers, and farm manure and reached a plateau as from 40 mm (Rosolem et al.
(Kolahchi and Jalali 2012; Ranjbar and Jalali 2012), 2005). Our results show that rainfall amount interacted
Plant Soil
with rainfall frequency, with significant differences (P < Uexküll and Cohen 1980) and husk recycling alone is
0.05) between large and small rainfall amounts at low but insufficient to meet the K needs of cocoa, highlighting
not high rainfall frequencies. At times of recurrent rainfall, the importance of K fertilizer additions. Yet, over the
delayed management of cocoa pod husks would result in life time of a plantation, poor husk management can
rapid K release from the residues. Likewise, when rainfall remove large amounts of K. Indeed, when heaps of
amounts are high, regardless of the frequency, cocoa husks cocoa pod husks are piled repeatedly at the same loca-
need to be actively managed to avoid large K losses. tion, the local soil K reserves will progressively be
We did not find significant differences in huskweight at concentrated and lost, benefiting only a few
the end of the scheduled leaching experiment (P > 0.05, neighbouring trees. Unfavourable drainage will worsen
not shown), suggesting that microbial decomposition was the K losses from the production system. Ideally, the
not affected by the amount or frequency of rainfall, con- husks should be redistributed over the whole plantation,
trasting with the findings of Joly et al. (2019). This simi- although yield benefits are expected to be small while
larity in decomposition rates was likely due to the con- the labour required would be substantial. Instead, rotat-
stantly moist environment in the leaching tubes which Joly ing the pod breaking station and sequential mulching of
et al. (2019) did not observe. Since water content of the small field patches would reduce labour requirements
husks is high (80%, Lu et al. 2018), the conditions in heaps while contributing to improved K recycling and reduce
of cocoa pod husks are likely to be similar to our tubeswith K fertilizer requirements in these resource-constrained
a high humidity in a moist environment, and we expect smallholdings. In addition to the spatial aspects of cocoa
therefore that decomposition of heaped husks is not strong- pod husk management, asynchrony between nutrient
ly affected by rainfall frequency and amounts. release from residues and uptake by trees also needs
Beside K, the husks also contain other nutrients consideration. Despite the lack of reports on the tempo-
which are relevant to cocoa nutrition. We found that ral dynamics of K demand in cocoa, uptake can be
more structurally-bound nutrients that are less abundant thought of as a continual process, characterized by
as free ions in the vacuole and cytosol were lost at much steady uptake when soil moisture is satisfactory and
slower rates than K (P < 0.05). Macronutrients were lost the tree is growing actively, increased rates during pod
in this relative order K > >Mg > Ca ≈ P >N, aligning enlargement, and declines under water stress. K supply
with the findings of Ranjbar and Jalali (2012). However, from cocoa pod husks is primarily governed by the
these authors found faster loss rates for Mg and Ca than seasonality of production through the major and minor
we observed, which may be related to tissue composi- crops. The major crop coincides with the dry season in
tion and the state of the residues used (dryness, stage of most production zones in West Africa, and therefore
decomposition). For cocoa pod husks, limited losses desiccation followed by rapid rewetting of the cocoa
were observed forMg, and even less for Ca. The relative pod husks will likely provoke a fast release of K. The
immobility of Mg and Ca in the husks can be related to minor crop harvest coincides with the rainy season, and
their abundance in cell wall pectins and sparingly solu- our analysis would be relevant to describe kinetics of
ble salts (Gerendás and Führs 2013). However, Mg nutrient release. As K is released from the cocoa pod
appeared more mobile than Ca probably because it is husks, its availability for absorption by the root system
also stored in ionic form in the vacuoles. Together, Mg will depend on several soil characteristics, including
and K also play an important role in osmotic regulation hydrology, sorption capacity, texture and mineralogy
and the cation-anion charge balance (Marschner 2012), (Alfaro et al. 2004; Freitas et al. 2018; Najafi-Ghiri
therefore their abundance in the vacuoles makes them et al. 2017; Rosolem and Steiner 2017). Of these prop-
more leachable than the other nutrients. In addition, K+ erties, the cation exchange capacity will play a strong
has the lowest valence and thus moves more freely role in buffering nutrient fluxes.
through disintegrating plant tissue.
To what extent does husk management contribute to
K recycling and cocoa nutrition? For current regional Conclusion
mean cocoa bean yields of 400 kg ha−1 in west Africa,
about 17–31 kg K ha−1 can be expected in cocoa pod Cocoa pod husks are rich in potassium and their poten-
husks (Hartemink 2005). These are modest amounts in tial contribution to tree nutrition is significant. Since
relation to crop requirements (up to 300 kg K ha−1, Von risks of K losses through leaching are high, optimizing
Plant Soil
nutrient cycling in cocoa requires improved manage- husks under diverse climatic conditions and manage-
ment compared with current practices of heaping and ment practices is needed to better inform field manage-
abandonment. We found that early-stage decomposition ment. Further, there is a paucity of knowledge about the
of the husks, water-saturation, and rainfall regime fate of K as it leaches from the husks into the topsoil.
(amount and frequency) significantly altered K leaching Effective leaching study designs are required to capture
patterns and the resulting nutrient losses. Knowledge the effect of the root system, soil characteristics, and pod
about the kinetics of nutrient leaching from decaying management on K spatial distribution in soil and its
husks can guide timely management of the husks, pro- temporal availability to cocoa trees.
vided that accurate meteorological predictions are ac-
cessible to farmers.
We have briefly explored innovative methods of Acknowledgements This research was funded through a grant
handling and disposing the husks to improve nutrient from NORAD to the CocoaSoils project (www.CocoaSoils.org).
We are sincerely grateful to Hennie Halm, Bert Rijk, Wandji
cycling, but more insight is needed both in terms of the Njankoua, Lucette Adet, Ambra Tosto, an anonymous farmer,
outcomes on nutrient budgets, technical requirements, and many others who helped us to design and run the lab trials.
and farm resource utilization. For instance, we suggest We thank Joost van Heerwaarden for advice on the statistical
that on-farm estimation of nutrient leaching from the analysis of the experimental data.
Appendix
Table 3 Cumulative amount of water (mm) sprinkled over tubes for each treatment in Experiment 2
Treatments Days
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31
2d, 12 mm, small 12.5* 25 37.5 50 62.5* 75 87.5 100 112.5* 125 137.5 150 162.5* 175 187.5 200
4d, 12 mm, small 12.5* 12.5 25 25 37.5* 37.5 50 50 62.5* 62.5 75 75 87.5* 87.5 100 100
8d, 12 mm, small 12.5* 12.5 12.5 12.5 25* 25 25 25 37.5* 37.5 37.5 37.5 50* 50 50 50
2d, 50 mm, small 50* 100* 150* 200* 250* 300* 350* 400* 450* 500* 550* 600* 650* 700* 750* 800*
4d, 50 mm, small 50* 50 100 100 150* 150 200 200 250* 250 300 300 350* 350 400 400
8d, 50 mm, small 50* 50 50 50 100* 100 100 100 150* 150 150 150 200* 200 200 200
2d, 12 mm, medium 12.5* 25 37.5 50 62.5* 75 87.5 100 112.5* 125 137.5 150 162.5* 175 187.5 200
4d, 12 mm, medium 12.5* 12.5 25 25 37.5* 37.5 50 50 62.5* 62.5 75 75 87.5* 87.5 100 100
8d, 12 mm, medium 12.5* 12.5 12.5 12.5 25* 25 25 25 37.5* 37.5 37.5 37.5 50* 50 50 50
2d, 50 mm, medium 50* 100 150 200 250* 300 350 400 450* 500 550 600 650* 700 750 800
4d, 50 mm, medium 50* 50 100 100 150* 150 200 200 250* 250 300 300 350* 350 400 400
8d, 50 mm, medium 50* 50 50 50 100* 100 100 100 150* 150 150 150 200* 200 200 200
2d, 12 mm, large 12.5* 25* 37.5* 50* 62.5* 75* 87.5* 100* 112.5* 125* 137.5* 150* 162.5* 175* 187.5* 200*
4d, 12 mm, large 12.5* 12.5 25 25 37.5* 37.5 50 50 62.5* 62.5 75 75 87.5* 87.5 100 100
8d, 12 mm, large 12.5* 12.5 12.5 12.5 25* 25 25 25 37.5* 37.5 37.5 37.5 50* 50 50 50
2d, 50 mm, large 50* 100 150 200 250* 300 350 400 450* 500 550 600 650* 700 750 800
4d, 50 mm, large 50* 50 100 100 150* 150 200 200 250* 250 300 300 350* 350 400 400
8d, 50 mm, large 50* 50 50 50 100* 100 100 100 150* 150 150 150 200* 200 200 200
Treatment code: For size, small, medium and large refer to chunks of 2, 5, and 25 g respectively. For rainfall amount, 12mm and 50mm refer
to 12.5 mm/leaching event and 50mm/leaching event, respectively. For rainfall frequency, 2d, 4d, and 8 8d refer to regular (every other day),
intermittent (every 4 days), and irregular rainfall events (every 8 days), respectively. Cells with figures in bold correspond to effective days of
leaching for a given Treatment. An asterisk (*) represents a day at which the leachate of a given treatment is analysed for other nutrients (N,
P, Ca, Mg)
Plant Soil
Table 4 Comparison of 1st and
2nd order exponential models Predictor/Model evaluation criterion 1st order exponential 2nd order exponential
used to describe cocoa pod husk model (Olson) model (Yang and Janssen)
decomposition with estimates (±
95% confidence intervals) Decomposition rate (k), day
−1 0.01±0.00 0.09±0.01
Ageing constant (a), unitless NA 0.42±0.04
AIC −69 −131
BIC −60 −119
RMSE 0.11 0.05
R2 0.71 0.77
NA not applicable
Table 5 Effects of treatments (combination of rainfall frequency, on day 25 of the scheduled leaching experiment (experiment 2).
rainfall amount, and size of chunks) and type of structurally-bound Note that the probability indicates the P value for differences
nutrients on the total losses (±95% confidence intervals) estimated between treatments based on the Wald F-test
Treatments Nutrient losses (% of initial value in the husks) Probability across treatments
N P Ca Mg
2d,12 mm, large 0.02±5.22 5.27±5.22 4.91±5.22 11.12±5.22 0.99
2d,50 mm, small 0.06±5.22 5.72±5.22 8.89±5.22 13.36±5.22
8d,12 mm, large 0.02±5.22 1.56±5.22 1.92±5.22 1.92±5.22
8d,50 mm, small 0.3±5.22 14.09±5.22 6.78±5.22 15.47±5.22
Probability across nutrients 0.03 0.17
Table 6 Effect of rainfall amount, rainfall frequency, and size of chunks on K leaching model’s parameters (see Eq.3 in main text). The
intercept of the model were set for ‘2d, 12mm, small’ treatment
Parameter Rainfall Frequency Size Treatment label Estimate(±95% confidence intervals)
Asym All All All All 1.16±0.04
Xmid 12.5 mm 2 days Small 2d, 12 mm, small 27.68±5.6
Xmid 12.5 mm 2 days Medium 2d, 12 mm, medium 30.35±10.14
Xmid 12.5 mm 2 days Large 2d, 12 mm, large 25.95±10.13
Xmid 50 mm 2 days Small 2d, 50 mm, small 29.11±13.58
Xmid 50 mm 2 days Medium 2d, 50 mm, medium 28.46±24.68
Xmid 50 mm 2 days Large 2d, 50 mm, large 28.04±24.79
Xmid 12.5 mm 4 days Small 4d, 12 mm, small 32.67±13.52
Xmid 12.5 mm 4 days Medium 4d, 12 mm, medium 31.01±24.68
Xmid 12.5 mm 4 days Large 4d, 12 mm, large 36.71±24.54
Xmid 50 mm 4 days Small 4d, 50 mm, small 24.25±32.82
Xmid 50 mm 4 days Medium 4d, 50 mm, medium 27.51±60.21
Xmid 50 mm 4 days Large 4d, 50 mm, large 25.57±60.09
Xmid 12.5 mm 8 days Small 8d, 12 mm, small 46.77±14.27
Xmid 12.5 mm 8 days Medium 8d, 12 mm, medium 59.86±31.22
Xmid 12.5 mm 8 days Large 8d, 12 mm, large 83.87±50.57
Xmid 50 mm 8 days Small 8d, 50 mm, small 33.18±34.63
Xmid 50 mm 8 days Medium 8d, 50 mm, medium 32.7±72.53
Xmid 50 mm 8 days Large 8d, 50 mm, large 32.13±110.3
RL 12 mm 2 days All 2d, 12 mm 0.07±0.01
RL 50 mm 2 days All 2d, 50 mm 0.07±0.03
RL 12 mm 4 days All 4d, 12 mm 0.06±0.03
RL 50 mm 4 days All 4d, 50 mm 0.07±0.06
RL 12 mm 8 days All 8d, 12 mm 0.03±0.03
RL 50 mm 8 days All 8d, 50 mm 0.05±0.06
Plant Soil
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Attribution 4.0 International License, which permits use, sharing, (Theobroma cacao L.) pod husk: renewable source of bioac-
adaptation, distribution and reproduction in anymedium or format, tive compounds. Trends Food Sci Technol 81:172–184.
as long as you give appropriate credit to the original author(s) and https://doi.org/10.1016/j.tifs.2018.09.022
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indicate if changes were made. The images or other third party and release of nutrients from crop residues on soybean-maize
material in this article are included in the article's Creative Com- cropping systems. Rev Bras Cienc Agra 13:8. https://doi.
mons licence, unless indicated otherwise in a credit line to the org/10.5039/agraria.v13i2a5527
material. If material is not included in the article's Creative Com- Dawoe EK, Barnes VR, Oppong SK (2017) Spatio-temporal
mons licence and your intended use is not permitted by statutory dynamics of gross rainfall partitioning and nutrient fluxes in
regulation or exceeds the permitted use, you will need to obtain shaded-cocoa (Theobroma cocoa) systems in a tropical semi-
permission directly from the copyright holder. To view a copy of deciduous forest. Agrofor Syst. https://doi.org/10.1007
this licence, visit http://creativecommons.org/licenses/by/4.0/. /s10457-017-0108-3
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