INTRODUCTION
Cocoa
production in Ghana is limited by soil nutrient depletion after many years of
cultivation. The establishment of cocoa farms normally begins with the raising
of seedlings in nurseries using topsoil from refuse dumps known as ‘Black
soil’. This type of soil cannot effectively support cocoa seedling growth in
the nursery currently, because it is basically made up of non-degradable
materials. An alternative attempt is to use topsoil but such soils are mostly deficient
in P, K, Ca, Mg and ECEC (Rhodes, 1995). Poor growth
of cocoa seedlings in the nursery has been ascribed to the use of unsuitable
potting media and also the problem of getting adequate quantities of fertile
topsoil for potting cocoa (Donkor et al., 1991; Ofori-Frimpong
et al., 2006). Under the current
cocoa rehabilitation programme in Ghana, most farmers
who prepare nurseries from seed pods collected from the Seed Production
Division of COCOBOD use different types of soils from their farms for filling
the polybags because of scarcity of fertile topsoil. These soils differ in
their fertility status, which also impact negatively on the growth of cocoa
seedlings. There is, therefore, the need to look for ways of improving the
growth of cocoa seedlings in the nursery, and young cocoa after transplanting
in the field. One such way is to apply foliar
fertilizers to seedlings in the nursery and after transplanting. Foliar
fertilizers have multiple advantages. These include efficient and timely
method of applying and enhanced nutrient utilization. Moreover, the application rates are low, and their
effects are usually independent of soil conditions (Kolota
and Osinska, 2001). Cocoa pod husk-based
compost was used to raise cocoa seedlings at the nursery in Ghana (Ofori-Frimpong et al.,
2010; Mensah-Brako, 2011) and Nigeria (Ayeni, 2008). The use of organic materials in potting media
will not only improve the growth performance of cocoa seedlings but also
improve the quality of soil used for raising the seedlings (Adejobi,
et al., 2013). The use of inorganic
fertilizers is the most effective and convenient way to improve the fertility
of nutrients poor soils. Inorganic fertilizers are known to contain readily
available plant nutrients which are released to plants rapidly after
application. The work by Gockowski et al.
(2004) indicated that cocoa seedlings need nitrogen, phosphorus, potassium and
metabolites such as proteins, lipids, carbohydrates for their growth. Thus it
is important that young nursery seedlings and transplanted seedlings are in
optimal condition as far as their nutrient and energy status are concerned.
Though, there are studies reported on utilization of organic fertilizers
especially cocoa pod husk for raising cocoa seedlings (Odedina
et al., 2007; Ayeni,
2008; Ofori-Frimpong et al., 2010) there is limited research information on how
different fertilizers especially foliar fertilizers can effectively be utilized
to boost the growth of cocoa seedlings in nutrients poor soils in Ghana (Oppong et al.,
2008). The objective of this study was therefore, to determine the effect of
different fertilizer sources on the growth performance of cocoa seedlings raised in soils of different fertility status in the
nursery.
2.0 MATERIALS AND METHODS
2.1
Study site
The experiment was
conducted at the main nursery of the Cocoa Research Institute of Ghana, New Tafo (latitude 6°13’ N, longitude 0°22’ W, altitude 222 m
above sea level) with a total land area
of 4,3027 km.
2.2 soil
sampling analyses
Two soils of different fertility status were
sampled and used for this investigation. Topsoil (0-15 cm depth) collected from
an old cocoa plot (R5) and the other soil known as ‘Black soil’ collected from
a refuse dump were used for the experiment. The soil from the old cocoa plot
has been classified as Rhodic-Lixic Ferrasol (WRB, 2014) and belongs to the Wacri
soil series according to the Ghanaian system of classification (Dwomo and Dedzoe, 2010). Samples
of the soils were air-dried and sieved through a 2 mm mesh and stored for
analysis of their chemical properties. Soil pH was determined using the glass
electrode at soil: water ratio of 1:2.5 (McLean, 1982), organic carbon was by
the Walkley and Black wet oxidation method (Nelson
and Sommers, 1982) and total N by the Kjeldahl digestion and distillation method (Bremner and Mulvaney, 1982).
Available phosphorus was determined by the Troug
method (Troug, 1930). Exchangeable K, Ca and Mg were extracted with 1N ammonium acetate solution and the leachate analyzed by the Atomic
Absorption Spectrophotometer (Thomas, 1982).
2.3 Nursery studies
Bulk soil samples
were passed through 8 mm mesh. Sub samples of each soil type were used to fill the standard polybags with dimensions
18 cm x 25 cm (Ofori-Frimpong et al.,2006; Oppong et al.,2008) which had earlier
been perforated at the bottom
edges to enable perfect
drainage of water. The soils were
either mixed with organic or inorganic fertilizer. Samples of the
soil-fertilizer mixtures were sent to the laboratory for analyses. The
treatments tested were two soil types (Black soil and Wacri
series) and five fertilizer sources (control, conventional NPK 3:5:5, Green
grow organic compost, Foliar NPK 22:16:18+TE and Foliar NPK 15:8:33+TE). The
compost was mixed with the soils at the ratio of 3:1 while the foliar
fertilizers were applied at 12 g/ 15 litres of water.
The inorganic fertilizer was applied at 33.5 g/ 3 kg soil. Mixed hybrid cocoa
seeds were sown at a seeding rate of two per polybags which were thinned to one
seedling per polybags one month after sowing. The foliar fertilizers were
sprayed on the seedlings at bi-weekly interval using pneumatic knapsack
sprayer. Each treatment had thirty seedlings and the experiment was laid out as
a 2 x 5 factorial arranged in Completely Randomized Design (CRD) with four
replications. Seedling girth, height and dry matter production were measured at
bi-weekly intervals for 14 weeks.
2.4
Data analysis
Data collected were
subjected to analysis of variance (ANOVA). Treatment means were compared using
the least significant difference (LSD) at α = 0.05. All statistics were
performed using GenStat Statistical Package (GenStat, 2008).
3.0 RESULTS
3.1 Initial properties of the soils and
fertilizers
The Wacri soil
series had a pH
of 6.1 which is slightly acidic
while the Black
soil was slightly basic with pH of 7.8. Percent organic carbon in the
Black soil (1.75%) was five times higher than the Wacri
series (0.31 %). Nitrogen content in the Black soil (0.19%) was higher than the
Wacri series (0.04%). The available phosphorus was
higher in the Black soil than the Wacri series. The
concentration of the available phosphorus in the Black soil was sixty-four
times higher than the concentration in the Wacri
series. Basal cations (K, Ca
and Mg) were all higher in the Black soil than the Wacri
series. The pH of Green grow (7.3) was neutral. Total N content was low (0.01%)
but phosphorus (5380.7 mg kg-1) and exchangeable Mg (6.56 cmol kg-1) contents were very high. The two
foliar fertilizers have high nitrogen, phosphorus and potassium contents (Table
1). In addition to
the macro nutrient elements, boron,
iron, manganese, molybdenum
and zinc were trace elements
included in the formulations. The
inorganic (N:P:K 3:5:5) was
formulated using the
following primary
fertilizers.; Sulphate of Ammonia, Triple Super phosphate and Muriate of potash.
4.0 DISCUSSION
Proper
seedling growth is usually dependent on the potting media in which the
seedlings are raised. The organic carbon contents of the Black soil could be
described as medium whilst that of the Wacri series
as very low. This is because the values obtained were below the 3% considered
adequate for good cocoa growth ( Ahenkorah
1981). The low organic carbon of the Wacri series
meant that nutrient supply to the seedlings would be difficult. Nitrogen
content of the Wacri series was substantially lower
than the critical level of 0.09 % required for cocoa cultivation (Ahenkorah, 1981; Egbe et al., 1989). Wacri
soil series had the least mean available phosphorous concentration and was
lower than the optimum level of above 20 ppm required for cocoa cultivation (Egbe et al.,
1989; Ogunlade et al., 2006). Mechanical
analyses indicated that the soils have high sand content and this has
implication on their ability to retain moisture and the release of plant
nutrients. The pH of the soils changed
with
the incorporation of the soil
amendments. The addition of Green grow
organic media improved the fertility status of the less fertile Wacri series. This finding was similar to that of Moyin-Jesu (2007) who reported the nutrient superiority of
organically amended fertilizers compared to the ordinary forms of the
materials. Arthur et al. (2019) also
found an improvement in soil chemical properties following the application of
Municipal Solid Waste compost to less fertile topsoil. Application of organic
fertilizer mineralizes and releases nutrients to the soil. The reduction in
soil pH following the application of the NPK fertilizer could be attributed to
the acidifying effect of sulphate of ammonia in the
formulation (McCauley et al., 2017).
The low
initial seedling emergence recorded in the two soil types treated with NPK
fertilizer could be ascribed to the acidic condition created by the sulphate of ammonia fertilizer in the formulation. Although Wacri soil
series treated with Green grow gave
very fertile medium, there was generation of heat in the growth medium and this
affected the early seedling emergence. The rest of the treatments performed
better because the conditions in the growth media were optimal for the seedlings to germinate and emerge.
Considering the dry matter
production, the greatest proportion
was in the leaves, followed by
the stem and then the roots.
This is somehow contrary to the findings of Owusu-Aduomi and
Frimpong (1985), who found the greatest dry matter yield
in
the stem followed by the
leaves and then the roots. The differences
in the dry matter yield distribution observed were attributed to the differences in the formulations of the
foliar fertilizers. The foliar with high potassium concentration (NPK
15-8-33+TE) increased the root dry weights
over
the foliar with high
nitrogen concentration (NPK 22-16-18+TE). As noted by (Egbe
et al., 1989) high potassium affects plant metabolism, carbohydrate
formation and translocation of starch to all parts of the seedlings. Under
water stress, K-treated plants had increased root and shoot biomass, higher
leaf water content, higher chlorophyll fluorescence and reduced electrolyte
leakage from leaves leading to improved vigour (Djan et al.,
2017). On the contrary, foliar fertilizer with high
nitrogen concentration increased the leaf
dry weights of the seedlings.
This is in line with the findings
of Lockwood and Asomaning (1963), who also found
increase leaf dry matter yield with the application of fertilizer with high nitrogen source. The very good
performances of seedling growth
recorded on soils with the foliar fertilizers may be due to the fact
that the optimum soil conditions
were
supported with nutrients from the foliar fertilizers. With the addition of NPK fertilizer, the changes in the fertility status
did
not
increase
much
but this soil
performed worse than
the
soil without any fertilizer added. This may be attributed partly to the
changes in pH.
Similar observation was made by Ofori-Frimpong et al. (2010) who found a depressive
effect of cocoa seedling growth following the application of inorganic NPK
fertilizer. Although the incorporation
of Green grow improved the initial
fertility status of the soils,
the C:N ratio narrowed
to 3:1 which was not
conducive for soil microbial activities.
5.0
CONCLUSION
It is concluded
that, the soil from the refuse dump remains
the best soil for the nursery work. This could be supported by the application of foliar
fertilizers to boost the growth
of seedlings and also reduce the time seedling spent in the nursery before they are planted
out in the field. In the absence of soils from the refuse
dump which is well decomposed, the
surface soils
from any secondary forest
could
be used and supplemented with foliar fertilizers to increase dry matter production and growth
performance of cocoa seedlings. Soils
with the incorporation of inorganic NPK or soil amendments
like Green grow have depressive
effects on the seedlings.
Competing interests
The
authors have declared that no competing interests exist.
Authors’ contributions
This work was carried out in collaboration
among all authors. Author Alfred Arthur handled the data compilation, performed
the statistical analysis, and wrote the final manuscript. Author Solomon Acquaye designed the experiment and wrote the first draft
of the manuscript. Author Jerome Agbesi Dogbatse managed the literature search and read proof the
final manuscript. Authors Alfred Arthur and Jerome Agbesi
Dogbatse read and approved the final manuscript.
Acknowledgements
The authors are grateful to the Technical Staff at the Soil Science
Division of Cocoa Research Institute of Ghana, for the data collection and soil
sample analyses. This paper is published with the kind permission of the
Executive Director, CRIG.
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