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Crop performance and soil
fertility improvement using
organic fertilizer produced
from valorization of Carica papaya
fruit peel
S. O. Dahunsi1*, S. Oranusi2, V. E. Efeovbokhan3, A. T. Adesulu‑Dahunsi4 & J. O. Ogunwole5
In recent times, research attention is focusing on harnessing agricultural wastes for the production
of value‑added products. In this study, the valorization of Carica papaya (Pawpaw) fruit peels was
evaluated for the production of quality organic fertilizer via anaerobic digestion (AD) while the eects
of the fertilizer on maize crop were also assessed. Pawpaw peel was rst pretreated by thermo‑
alkaline methods before AD and analyses were carried out using standard methods. The resulting
digestate was rich in nutrients and was dewatered to form solid organic fertilizer rich in microbes
and soil nutrients. When applied to maize plants, organic fertilizer showed a better eect on plant
traits than NPK 15–15–15 fertilizer and without fertilizer application. These were more pronounced at
mid to high organic fertilizer applications (30‑to‑60‑kg nitrogen/hectare (kg N/ha)) rate. Comparison
between the values obtained from the eld experiments reveals that the organic fertilizer showed
better performance in all parameters such as the number of leaves, leaf area, plant height, stem girth,
total shoot, and root biomass, and length of the root. However, the chemical fertilizer outperformed
all the organic fertilizer applied rates in the average highest size of the corn ear by 1.4%. After
harvesting, nutrient elements were found to have bioaccumulated in plant organs (leaves, stem, and
root) with the highest values being 29.7 mg/L for nitrogen in the leaf and this value was reported
from the experiment with 50 kg N/ha. For phosphorus and potassium, the highest concentrations of
7.05 and 8.4 mg/L were recorded in the plant’ stem of the experiment with 50 kg N/ha. All the treated
soils recorded an increase in values of all nutrient elements over the control with the highest values
recorded in the experiment with 60 kg N/ha. In soil with 60 kg N/ha, the nitrogen, phosphorus, and
potassium increased by 28, 40, and 22% respectively over the chemical fertilizer applied experiment
while dierent levels of increases were also recorded for all other macro and microelements in all
the experiments. Thus, agricultural practices by using anaerobic digestates as organic fertilizers is a
sustainable method to overcome the dependence on inorganic fertilizers high rate.
Climate change and its attendant issues have been a major subject of discussion between scientists and researchers
over the last decade. e bulk of the contributions to climate change comes from fossil fuels and their deriva-
tives which now call for a radical approach in protecting the environment for human and animal survival1.
One major way to achieve this is by shiing from a fossil fuel economy to a bio-based one which includes the
identication of suitable materials (Wastes, biomass, etc.) that are useful for renewable energy generation and
their subsequent conversion to such2. Of these renewable energies is biogas which does not depend largely on
the volatile weather conditions unlike others such as wind and solar power3. Biogas is usually produced during
OPEN
Microbiology Programme, College of Agriculture, Engineering and Science, Bowen University, Iwo, Osun State,
Nigeria. Department of Biological Sciences, Covenant University, Ota, Ogun State, Nigeria. Department of
Chemical Engineering, Covenant University, Ota, Ogun State, Nigeria. Food Science and Technology Programme,
College of Agriculture, Engineering and Science, Bowen University, Iwo, Osun State, Nigeria. Agriculture
Programme, College of Agriculture, Engineering and Science, Bowen University, Iwo, Osun State, Nigeria. *email:
olatunde.dahunsi@bowen.edu.ng
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the Anaerobic Digestion (AD) of organic materials such as solid wastes streams, greenery biomass, crop residues
and animal dung, municipal and domestic wastes4,5.
Besides these numerous advantages of the AD process is the production of digestate as a by-product of the
process and they vary in their intrinsic characteristics depending on the nature and composition of the material
used in their production7. ere is, therefore, a need to implement a sustainable method for storage, disposal, and
management of digestate to avoid challenges of handling, environmental contamination, and odor8. e most
promising application of digestate is in the agricultural and horticultural sectors where they are oen applied
as soil conditioners and organic fertilizers due to their richness in nutrients and soil viable microorganisms9,10.
Digestates are so used due to their ability to improve and modify soil structures11, while they also improve
soil nutrient status and boost a load of benecial microorganisms for special functions, especially in marginal
or nutrient-depleted soils when applied as organic fertilizers12. In this regard, digestates have been seen to be
potentially able to partially or wholly replace inorganic chemical fertilizers in agricultural practices especially
in tropical countries most of which are facing depletion in soil nutrients, toxicity to soil microorganisms, inad-
equate soil aeration, soil water pollution, and eutrophication13. Similarly, digestates are potent to replace the
widespread application of peat which has special properties making it important for large-scale application as a
growth media in horticulture14. More importantly, peat is a slowly-renewable and nite resource and thus does
not have sustainable management methods which further make its usage environmentally controversial15,16.
Considering the afore-mentioned, a veritable alternative to both inorganic fertilizer and peat is anaero-
bic digestate which has proven to be a slow-release fertilizer providing essential nutrients such as nitrogen,
phosphorous, and potassium (NPK) besides other essential plant macronutrients required for growth, health,
and wellbeing of crop plants without detrimental eect on the soil17,18. is also goes a long way to boost food
production to cater for the teeming human population thereby attaining the sustainable development goals
(SDGs) one, two, and thirteen which are eradication of poverty, zero hunger, and climate action respectively19,20.
Digestates contains dierent concentrations of nutrients especially NPK21 and have therefore made their usage
of immense benet to agriculturists since these nutrients are naturally scarce especially phosphorus which is
usually obtained from mining with huge costs and high energy expenditure besides the serious health hazards
posed by its mining22. Dependence on these natural and limited sources of nutrients for crop plants, therefore,
makes agriculture vulnerable and less economical in the long run. In most cropping systems, the use of inorganic
fertilizers have been abused and besides the huge cost of procurement, shipping, transportation, and distribution
logistics, they pose serious threats to the environment with their attendant tendencies to reduce the integrity of
soil, cause degradation of the environment, poses risks to biodiversity, high contribution to an algal bloom, and
their huge potentials to make soil heavy metals laden16. e situation can however be redeemed via the adoption
and continuous use of organic preparations e.g., such as digestate organic fertilizer produced from renewable
and locally available bioresources6,23.
Carica papaya (Papaya; Pawpaw in the local name) is a globally important and popular fruit presently with
a total of 11.22 metric tons equivalent of 15.36% of the total production of tropical fruits24. About 60 countries
are known for papaya production globally in which the bulk comes from developing nations in the tropics. Pro-
duction of this fruit has grown considerably over the last two decades from a total of 7.25 metric tons in 2000
to 13.02 metric tons in 201725. e ve-leading papaya-producing nations are India, Brazil, Indonesia, Nigeria,
and Mexico with 38.61, 17.5, 6.89, 6.79, and 6.18% respectively. Being the 4th leading papaya producing nation
in the world, Nigeria witnesses massive production of this fruit annually, and its esh/pulp is either consumed
raw or used in various preparations. However, the huge peels accrued from papaya processing remain an envi-
ronmental nuisance as solid wastes and serving as vehicles for transmission of life-threatening diseases since
there are no sustainable management methods for this massive and year-round bioresource. is has created a
huge knowledge gap in sustainably managing papaya fruit peels by converting them to value-added products
while protecting environmental integrity. ough few previous studies have reported the production of organic
fertilizer from pawpaw fruit peels, such studies were carried out using mixed fruit peels such as banana, pine-
apples, papaya, pomegranate, sweet lime, orange, etc. which resulted in some organic preparations that cannot
be distinctly categorized based on the source of its raw materials26,27. is current study represents a novel and
modern trend in the sole utilization of pawpaw fruit peels as a veritable and abundant resource for organic
agriculture improvement. erefore, this study aimed to evaluate the valorization of pawpaw fruit peels for
the production of quality organic fertilizer. is is important to establish the peel as a profound bioresource in
global organic agricultural development, reduce solid waste accumulation in the environment with its attendant
public health menace as well as documenting a sustainable management method for pawpaw peel in the long
run. e eects of the fertilizer on the performance of maize (Zea mays) as test plants will be assessed while the
possibility of soil fertility improvement as a result of the organic fertilizer application will also be evaluated. It
is hoped that these tests will help validate the potency of the produced fertilizer in the general wellbeing of the
crop plant (Maize) and will engender its further usage on other crop plants besides increasing productivity and
environmental protection in a circular economy.
Materials and methods
Sample collection and pretreatment. Papaya fruit peel (Fig.1) was obtained from Landmark Uni-
versity Farms and sta quarters while the microbial inoculum i.e., cattle rumen content was collected into a
sterile container from an abattoir. Due to its high content of lignin and cellulose, papaya fruit peel needed to
be pretreated to make it easily biodegradable and to avoid the usual rate-limiting occurrence especially during
the hydrolysis stage of digestion. In achieving this, three dierent pretreatment methods i.e. mechanical, ther-
mal, and alkaline (NaOH) were applied as described in previous studies28–36. e peels were initially crushed
into sizes of ≤ 20mm using a hammer mill and this was followed with 80°C thermal treatment in a water bath
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(CLIFTON, 88,579, Nickel-Electro Ltd., England). e temperature was chosen base on the results of previous
works which showed it as the optimal for a suitable lignocellulosic pretreatment37,38. e procedure for alkaline
pretreatment was carried out using 3g NaOH/100g TS at 55° C for 24h39. e choice of NaOH was a result of
its earlier performance as a suitable alkali for lignocellulosic biomass pretreatment40.
Anaerobic digestion. e slurry was prepared from the pretreated biomass and water and it was anaero-
bically treated in controlled batch anaerobic reactors using. e collected bovine rumen uid was activated by
a sieve to remove unwanted biomass and dirt, poured into a clean container, and ushed the headspace with
hydrogen gas before incubation for 5 to 7days. is activated inoculum was used to seed the reactors and they
were turned on for digestion to produce biogas over a retention period of 30days28–34. e batch anaerobic reac-
tor (EDIBON, England) with twin digestion chambers each of 5-L capacity was employed. e reactors were
computer-controlled with internal probes to measure the pH and temperature. Before digestion, a sample of
the prepared slurry was taken for physical, chemical, and microbiological analyses. Also, during the digestion,
samples were taken weekly for the same analyses. e produced biogas was collected via the water displacement
unit which is a component of the automated reactors.
Organic fertilizer development procedures. At the expiration of the AD, the digestates were carefully
removed from the reactors, and samples were taken for analysis before digestates have been poured into sterile
sacks to drain. Curing of the dewatered digestate was carried out for 20days using sterile bags and the resulting
solid preparation was stored in the dry forms41,42 before physicochemical and nutrient analysis as well as applica-
tion in a eld experiment.
Analytical procedures. ere is a need to adequately characterize substrates for AD to determine their
intrinsic components which further enhance their performance during production43. Physical properties (pH
Figure1. Pictorial representation of the organic fertilizer development process.
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and Temperature) of the fermenting materials, inoculum, digestates, and solid organic fertilizers were deter-
mined. Aer this, chemical analyses of the same materials were carried out to quantify their elemental and nutri-
ents compositions. An inductively coupled plasma mass spectrometry was used to determine the total carbon,
nitrogen, phosphorus, potassium, phosphate, sulfate, calcium, magnesium, manganese, iron, zinc, aluminum,
and copper35. To determine the Chemical Oxygen Demand (COD) of samples, the method of the American
Public Health Association (APHA) for analyzing water and wastewaters44 was used. e values of Total solids
(TS), volatile solids (VS), ash, and moisture contents were determined using the Finnish Standard Association
SFS 3008 protocol45.
Microbiological analyses. Characterization and enumeration of aerobic and anaerobic bacteria and fun-
gi. e standard method for total aerobic plate count was adopted in analyzing the aerobic bacteria in the
fermenting material; inoculum, digestate, and solid organic fertilizer in this study employing nutrient agar, eosin
methylene blue (EMB) agar, peptone water, and MacConkey agar. All samples were collected aseptically and in
triplicates while presumptive isolates were characterized by phenotypic methods and the probable ones further
identied using appropriate rapid API kits (BioMerieux, France) as previously reported28–33.
To characterize the anaerobic bacteria, samples were initially cultured on two enriched media (Reinforced
Clostridia medium and blood agar) in anaerobic chambers at 37º C between 5 and 7days. is was meant to
detect members of the Clostridia and other facultative anaerobes in the samples. Aer this, Brain Heart Infusion
agar was employed in fully growing developed colonies followed by counting and recording46. e phenotypic
features of the presumptive isolates were determined aer which appropriate rapid API kits were used for their
nal conrmation47. For fungal evaluation, samples were cultured on Potato dextrose agar and grown for 5–7days
before identication by hypha and spore morphology and those of the fruiting bodies48.
Enumeration of methanogen (archaea). In some previous investigations on the characterization and identica-
tion of methanogens, a mineral-rich basal medium was compounded, utilized, and was found very ecient28–34.
e medium was employed in characterizing the methanogens in the sample of fermenting material, inoculum,
digestate, and solid organic fertilizer in this study using the same protocol.
Phyto‑assessment with Maize (Zea mays) using produced organic fertilizer. e organic ferti-
lizer was applied to maize plants to ascertain their nutrient level especially with regards to their nitrogen content
since this element plays a vital role in plant growth plant and during protein formation. e experiments were
carried out during the 2015 and 2016 cropping seasons at the Teaching and Research Farm of Landmark Uni-
versity, Omu-Aran, Nigeria (Latitude 8.9°N, Longitude 50°61 E). Omu-Aran is a town in the derived savannah
ecological zone of North-Central Nigeria and is characterized by an annual rainfall between 600 and 1500mm
spreading between April and October with a peak usually in May to June and September to October. Whereas,
the town experiences its dry season between November and March. e farm used for this experiment has been
used for continuous cropping since 2010 and the predominant vegetation is composed of weeds such as Rottboe-
llia cochinchinensis, (Lour) Claton) (Itch grass), Eleusine indica (L.) Gaertn) (Goosegrass), Echinochloa colona
(L.) Link) (Sour millet), Euphorbia heterophylla (L) (Milkweed) and Ageratum conyzoides (L.) (Goat weed).
Organic fertilizer application rates determination. A standard method of 10kgN/ha was adopted
in applying the fertilizer49. From this application rate, ve other rates were calculated. e application rate then
followed the following order for six dierent runs subsequently performed:
Total Nitrogen (N) in the papaya fruit peels organic fertilizer is 0.334mg/g.
Using 10kgN/ha as standard for 10kg of soil in pot experiments, the quantity of papaya fruit peels organic
fertilizer needed was:
is gave 166.7g of papaya fruit peels organic fertilizer to 10kg of soil. Where one hectare of land contains
2,000,000kg of soil as standard50, the papaya fruit peel organic fertilizer was converted to grams by multiplying
by 1000. erefore, total papaya fruit peel organic fertilizer needed per ha of crop eld was calculated to be:
is gives approximately 33.340kg/ha of papaya fruit peel organic fertilizer. Table1 shows the quantity of
papaya fruit peel organic fertilizer that was used for six dierent application rates (10, 20, 30, 40, 50, and 60)
kg N/ha.
Soil preparation. Low nutrient (< 5% Nitrogen content) sand-loamy soil (Ultisols) as shown in Table2 was
used in the pot experiments to ascertain the nutrient status of the organic fertilizer. A bulk soil sample was col-
lected during the cropping seasons to determine the physicochemical properties. is was done aer the land
was mechanically prepared with the aid of a tractor-drawn disc plough and harrow. Ploughing was carried out
(1)
Conversion of this value to percentage
0.334 ×100
1000 which gave 0.03%
(2)
100
0.03
×10 ×
10
2, 000, 000
×
1000 g
(3)
166.7
10
×2, 000, 000 g/ha =33, 340, 000 g/ha or 33.340 kg/
h
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once while harrowing was twice and the resulting soil was well pulverized. ereaer, a randomized complete
block experimental design was adopted using experimental pots with 4 replicates for each treatment. e size
for each plot was 2m × 2m) which equals 4m2 and this gave a net plot size of 28m × 4m) which equals 122 m2.
Each experimental pot was lled with 10kg of soil to which was added the measured organic fertilizers and
thoroughly mixed before a two-week incubation for proper mixing and mineralization before planting of maize
seeds. In the design, each organic fertilizer application was repeated every 15-day till fruit maturity and harvest-
ing. e NPK 15–15–15 inorganic fertilizer (Positive control) was manually applied two weeks aer sowing by
side placement at 5 to 8cm distance from the base of the plant while the other experiment was without fertilizer
application and served as a negative control. Physicochemical and microbial analyses were performed on soils
collected before planting and aer crop harvesting. Weeding was manually done by hand-picking of weeds at
15days aer sowing (DaS) and was repeated every other 15days.
Planting and data collection. An extra early maturing maize hybrid (Ife Maizehyb-5) from the Interna-
tional Institute of Tropical Agriculture (IITA) was used in this experiment. Seed viability testing was carried out
by 24h soaking in distilled water 30°C. Viable seeds sat at the bottom of the beaker and these were used giving
90% germination percentage when used for the experiments51. In each pot, maize seeds were sown with a plant-
ing distance of 75 × 25cm, and the Phyto-parameters data was collected at 15-day intervals aer the emergence
of seeds as shown in TableS1 (Supplementary materials). ese measurements of plant parameters were termi-
nated at 50 (DaS); before tasselling when the optimal nutrient uptake had taken place before the commencement
of the generative phase. Nutrient analysis taken aer this period i.e. 60–75 DaS were intended to measure the
leover nutrients aer the photosyntate have been mobilized to tassel, ear, corncob, and grain. Measurement of
the last 4 parameters (shoot and root biomass, root length, and total ear size) were carried out aer crop har-
vesting. e nutrients content accumulated in the plant’s leaf, stem, and root was analyzed by using the method
described in Analytical procedures section.
Table 1. Quantity of Carica papaya fruit peel organic fertilizer applied.
S/N Experiments Fertilizer quantity needed Application rate (kg N/ha)
1 Negative Control No fertilizer 0
2 NPK 15: 15: 15 66.7g 26,680
3 10kgN/ha 83.3g 33,320
4 20kgN/ha 166.6g 66,640
5 30kgN/ha 249.9g 99,960
6 40kgN/ha 333.2g 132,880
7 50kgN/ha 416.5g 166,600
8 60kgN/ha 499.8g 199,920
Table 2. Soil physicochemical properties and microbial composition. TBPC total bacterial plate count, TFC
total fungal count.
Chemical properties Microbial composition
Parameter Soil (mg/L)
Bacteria Fungi
Organism TBPC (cfu/ml) Organism TFC (cfu/ml)
Nitrogen (N) 9.2 Bacillus sp.
4.1 × 105
Aspergillus niger
3.0 × 103
Phosphorus (P) 1.4 Mucor sp.
Potassium (K) 2.6 Clostridium sp.
Calcium (Ca) 43.3
Magnesium (Mg) 21.6
Copper (Cu) 1.25
Zinc (Zn) 10
Iron (Fe) 2.1
Aluminium (Al) 0.05
Nitrate (NO3) 1.09
Ammonium (NH4) 0.21
Phosphate (PO4) 44.4
Manganese (Mn) 0.008
Sulphate (SO4) 41.5
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Statistical analyses. Analyses were carried out using the Analysis of variance (ANOVA) while a comparison of
mean was done with Tukey’s test. All statistical analyses were performed using the Statistical Analysis Soware
(SAS/STAT), Version 8, 6th eds.
Results
Eect of pretreatment and AD on C. papaya fruit peel. Table3 shows the results of physicochemical
characteristics of the papaya fruit peel and inoculum used in the production of the organic fertilizer in this study.
As seen from the table, the pH of the reactor was slightly alkaline throughout the digestion period aer experi-
encing an initial fall during the rst 5days. In the same way, the temperature was kept at the mesophilic range
all through the digestion. It is evident from Table3 that the thermo-alkaline pretreatment applied to the bio-
mass caused the solubilization of important structural components especially the lignin-cellulose-hemicellulose
matrix. is eect was further compounded during AD as the condition of digestion also imparted on further
degradation of the substrate thereby yielding nutrients that was initially locked up in the biomass. As a result,
the increment was recorded for most chemical parameters except for ve of them i.e., total solids, volatile solids,
carbon, calcium, and COD. erefore, the anaerobic digestates obtained were nutrient-rich.
Physicochemical compositions of inorganic fertilizer and organic fertilizer. e papaya fruit-
peel-based organic fertilizer produced by the AD method is shown in Fig.2. Aer dewatering, the organic ferti-
lizer and the inorganic fertilizer were evaluated for major and minor nutrients/elements including pH as shown
in Table4. Calcium has the highest concentration in the organic fertilizer with an average value of 14.00% while
manganese had the lowest value of 0.003%. e composition of the inorganic fertilizer used as control is shown
in TableS2 (Supplementary materials).
Microbial evaluation of anaerobic digestate and organic fertilizer. e microbial composition of
the digestate and the dewatered organic fertilizer are shown in Table5. e microbial diversity and popula-
tions in the digestate far exceed those of the organic fertilizer. e total bacterial count of the digestate and
organic fertilizer was 2.4 × 1012 and 9.0 × 106CFU/ml respectively while the total fungal counts were 4.0 × 104
and 4.0 × 102CFU/ml.
Field assessment result. Table6 shows the results of the eld assessment from the control experiments.
In the no fertilizer application, the number of leaves, leaf area, plant height, and stem girth all increased with
progress between 15 and 75 DaS and the highest values of 17 leaves, 99.7 cm2, 165cm, and 2.8cm were for the
four parameters at the end of the experiments. Aer harvesting, the average total shoot biomass was 221g, while
that of the root was 59.1g. e total root length was 23cm while the average total size of the harvested ear was
316.3g. From the NPK 15–15–15 treated plot, the same trend was recorded for the number of leaves, leaf area,
plant height, and stem girth which all increased with progress in the experiment with the highest values recorded
as 17 leaves, 104.1 cm2, 142cm, and 2.9cm for the four parameters respectively. Aer harvesting, the average
Table 3. Physical and chemical characteristics of Carica papaya fruit peel and cattle rumen content38.
Parameters Rumen content Raw C. papaya peel only
Pretreated C. papaya fruit peel + rumen content
Before digestion Raw digestate before dewatering
pH 7.91 ± 0.02 6.23 ± 1.00 7.70 ± 0.02 7.60 ± 0.03
Total Solids (g/kg TS) 90.52 ± 0.11 94.81 ± 1.21 110.97 ± 0.11 93.94 ± 0.02
Volatile Solids (g/kg TS) 80.44 ± 1.12 83.23 ± 0.22 96.22 ± 3.02 50.01 ± 2.02
Ash Content (%) 5.56 ± 1.02 2.54 ± 1.00 2.78 ± 0.00 5.49 ± 0.03
Moisture Content (%) 90.48 ± 3.02 97.26 ± 0.01 94.03 ± 4.01 96.06 ± 1.02
Chemical Oxygen Demand (mg/
kg TS) 168.21 ± 1.12 165.11 ± 2.20 256.5 ± 4.04 83 ± 2.01
Total Carbon (g/kg TS) 265.21 ± 0.10 202.90 ± 4.03 214.90 ± 5.03 200.10 ± 3.03
Total Nitrogen (g/kg TS) 48.00 ± 2.02 37.51 ± 2.02 40.00 ± 1.01 41.60 ± 0.11
Total Phosphorus (g/kg TS) 6.30 ± 0.02 5.32 ± 1.02 6.12 ± 0.01 7.60 ± 1.11
Potassium (g/kg TS) 7.20 ± 0.11 7.32 ± 2.00 8.00 ± 0.11 10.94 ± 0.03
Phosphate (g/kg TS) 3.00 ± 0.02 1.03 ± 0.11 3.00 ± 0.10 4.51 ± 0.02
Sulfate (g/kg TS) 134 ± 2.00 112.20 ± 3.01 136.00 ± 2.03 159.49 ± 0.03
Calcium (g/kg TS) 80.00 ± 0.10 220.81 ± 4.41 226.00 ± 4.09 89.06 ± 2.00
Magnesium (g/kg TS) 96.00 ± 0.10 89.32 ± 1.02 100.00 ± 0.03 200.10 ± 5.05
Manganese (g/kg TS) 1.18 ± 0.22 0.021 ± 1.00 0.028 ± 0.00 0.060 ± 0.01
Iron (g/kg TS) 1.18 ± 0.11 1.06 ± 0.11 1.16 ± 0.21 4.60 ± 1.00
Zinc (g/kg TS) 38.00 ± 0.02 32.32 ± 0.01 36.00 ± 0.03 40.94 ± 1.22
Aluminium (g/kg TS) 0.80 ± 0.11 0.52 ± 1.02 0.76 ± 0.02 0.91 ± 0.03
Copper (g/kg TS) 4.80 ± 0.10 3.87 ± 0.03 4.70 ± 0.03 5.49 ± 0.03
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total shoot biomass was 255g, while that of the root was 63g. e total root length was 26cm while the average
total size of the harvested ear was 325.7g.
Table6 further shows the result of the eld assessments with papaya fruit peel organic fertilizer. From these
experiments, there was an increase in values of all parameters corresponding to the progress of the experiment.
e highest number of leaves (19) was recorded in the maize treated with 30, 40, 50, and 60kgN/ha organic
fertilizer application and recorded at 60 and 75days aer the emergence of seed. e highest leaf area (150.3 cm2)
was recorded in the 30kgN/ha experiment, while those of plant height (173cm) and stem girth (3.5cm) were
Figure2. e organic fertilizer produced and used in this study.
Table 4. Mineral composition of Carica papaya (Pawpaw) fruit peels organic fertilizer. Values shown in table
are means of triplicate analyses.
S/N Parameter Composition (%)
1 pH 7.30 ± 0.01
2Copper 0.65 ± 0.02
3 Calcium 14.00 ± 0.02
4Iron 0.14 ± 0.01
5 Magnesium 4.80 ± 0.02
6Manganese 0.003 ± 0.01
7Phosphate 0.30 ± 0.01
8 Sulfate 1.94 ± 0.05
9Potassium 1.22 ± 0.02
10 Nitrogen 9.18 ± 0.03
11 Phosphorus 0.62 ± 0.01
12 Zinc 3.46 ± 0.01
13 Aluminium 0.08 ± 0.01
Table 5. Microbial composition of Carica papaya fruit peel organic fertilizer. Values shown in table are means
of triplicate analyses; TBPC total bacterial plate count, TFC total fungal count.
Before dewatering Aer dewatering Before dewatering Aer dewatering
Bacteria TBPC (cfu/ml) Bacteria TBPC (cfu/ml) Fungi TFC (cfu/ml) Fungi TFC (cfu/ml)
Bacillus sp.
Enterococcus sp. Pseu-
domonas aeruginosa
Proteus sp.
Fusobacterium s p. Bacte-
roides fragilis Clostridium
sp. Gemella sp.
Methanococcus sp.
Methanosaeta sp.
Methanobacteriales sp.
2.4 × 1012
Bacillus sp.
Enterococcus sp. Pseu-
domonas aeruginosa
Proteus sp.
Fusobacterium s p.
Bacteroides sp. Clostridium
sp. Gemella sp.
9.0 × 106Aspergillus niger
Mucor sp. Rhizopus sp.
Penicillum sp. 4.0 × 104Aspergillus niger
Mucor sp. Rhizopus sp.
Penicillum sp. 4.0 × 102
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Table 6. Results of Phyto-Assessment in control, NPK 15–15–15 and Carica papaya fruit peel organic
fertilized experiments using Maize (Zea mays) as test plant. Values shown in table are means of triplicate
analyses; DAE = Day aer Emergence; italics values are the highest obtained for each phyto-parameter;
superscripts with same letters are statistically the same for a particular parameter across the dierent fertilizer
treatments (Organic fertilizer and control experiments) by the Tukey’s test at 5%.
DAE Leaf numb er Leaf area
(cm2)Plant height
(cm) Stem girth
(cm)
Biomass
above soil
level (g) Root biomass
(g) Root length
(cm) Total ear
size (g)
No Fertilizer Application
15 5 ± 0.01a22.9 ± 0.02a22 ± 1.01a0.7 ± 0.01a– – – –
30 7 ± 0.01a45.1 ± 0.01a42 ± 0.02a0.9 ± 0.01a– – – –
45 10 ± 0.01a62.4 ± 0.01a85 ± 1.01a1.4 ± 0.01a– – – –
60 15 ± 0.01a87.4 ± 0.01a148 ± 1.01a2.1 ± 0.01a– – – –
75 17 ± 0.01a99.7 ± 0.01a165 ± 0.01a2.8 ± 0.01a221 ± 0.01a59.1 ± 0.01a23 ± 2.01a316.3 ± 4.01a
NPK 15–15–15 Fertilizer Application (30kgN/ha)
15 5 ± 0.01a23.5 ± 0.01a18 ± 0.01b0.7 ± 0.01a– – – –
30 6 ± 0.01b46.2 ± 0.01b35 ± 0.03b1.3 ± 0.01b– – – –
45 10 ± 0.01a74.5 ± 0.01b74 ± 0.02b1.9 ± 0.01b– – – –
60 16 ± 0.01b94.9 ± 0.01b121 ± 0.03b2.4 ± 0.01b– – – –
75 17 ± 0.01a104.1 ± 0.01b142 ± 0.02b2.9 ± 0.01a255 ± 0.05b63 ± 0.02b25 ± 1.01a325.7 ± 5.01b
10kgN/ha Application
15 4 ± 0.01b20.1 ± 0.01b15 ± 0.01c0.6 ± 0.01a– – –
30 8 ± 0.01c39.2 ± 0.01c31 ± 2.01c1.2 ± 0.01b– – –
45 11 ± 0.01b70.5 ± 0.02c76 ± 2.01b1.6 ± 0.01c– – –
60 18 ± 0.01c90.5 ± 0.01c91 ± 2.01c1.9 ± 0.01c– – –
75 18 ± 0.01b107 ± 0.02b130 ± 2.01c2.7 ± 0.01a225 ± 4.01a60 ± 3.01b21 ± 1.01a253 ± 5.05c
20kgN/ha Application
15 6 ± 0.01c30.1 ± 1.01c18 ± 2.01b1.1 ± 0.01b– – –
15 9 ± 0.01d50.1 ± 2.01d36 ± 2.01b1.3 ± 0.01b– – –
45 12 ± 0.01c86.1 ± 2.01d86 ± 2.01a1.8 ± 0.01b– – –
60 17 ± 0.01d101 ± 2.01d103 ± 2.01d2.1 ± 0.01a– – –
75 18 ± 0.01b127 ± 3.01c147 ± 4.01d2.9 ± 1.01a219 ± 5.02a65.5 ± 2.01b22 ± 0.05a259 ± 6.03c
30kgN/ha Application
15 7 ± 0.01d30.8 ± 1.01c16 ± 0.03c1.4 ± 0.01c– – –
30 9 ± 0.01d50.8 ± 2.01d63 ± 1.04d1.9 ± 0.01c– – –
45 12 ± 0.01c90.8 ± 2.01e83 ± 2.04a2.3 ± 0.01d– – –
60 19 ± 0.01e120.0 ± 3.01e134 ± 3.04e2.8 ± 0.01d– – –
75 19 ± 0.01c150.3 ± 0.01d155 ± 4.05e3.1 ± 0.01b424 ± 5.05c146 ± 4.05c25.5 ± 2.01a321 ± 6.03b
40kgN/ha Application
15 7 ± 0.01d30.7 ± 1.01c17 ± 2.01b1.6 ± 0.01d– – –
30 9 ± 1.01d50.2 ± 1.01d60 ± 2.01e2.4 ± 0.01d– – –
45 13 ± 0.01d80.2 ± 2.01f. 85 ± 2.01a2.5 ± 0.01e– – –
60 19 ± 0.01e104.5 ± 1.01d132 ± 3.04e2.8 ± 0.01d– – –
75 19 ± 1.01c147.2 ± 2.01d159 ± 3.01f. 3.4 ± 0.01c393.5 ± 3.01d139.5 ± 4.01c23 ± 2.00a312 ± 2.05d
50kgN/ha Application
15 7 ± 0.01d30.8 ± 0.01c16 ± 2.01c1.6 ± 0.01d– – –
30 9 ± 0.02 d51.0 ± 1.02d62 ± 1.04d2.6 ± 0.01e– – –
45 13 ± 0.02d83.2 ± 2.02f. 87 ± 1.04c2.9 ± 0.01f. – – –
60 19 ± 0.01e101.5 ± 2.01d152 ± 2.04f. 3.2 ± 0.01e– – –
75 19 ± 0.01c140.6 ± 4.01e157 ± 2.01f. 3.5 ± 0.01c418 ± 3.01c144 ± 2.01c25.1 ± 0.05a316 ± 5.03d
60kgN/ha Application
15 7 ± 0.01d40.0 ± 0.01d17 ± 0.01b1.5 ± 0.01d– – –
30 9 ± 0.03d50.4 ± 1.02d66 ± 2.01f. 2.8 ± 0.01f. – – –
45 13 ± 0.03d90.1 ± 2.02e96 ± 2.01d2.9 ± 0.01f. – – –
60 19 ± 0.01e114.5 ± 2.01f. 158 ± 2.03g 3.3 ± 0.01e– – –
75 19 ± 0.01c144.7 ± 3.01e173 ± 3.01g 3.5 ± 0.01c414.5 ± 3.01c117.5 ± 2.0d24.5 ± 2.05a309 ± 4.05e
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both recorded in the 60kgN/ha experiment at the end of the experimental period. e highest weight of shoot
and root biomass were 424 146g respectively both recorded in the 30kgN/ha experiment aer crop harvesting.
e highest root length (25cm) was recorded in the 30kgN/ha experiment while the average size of the harvested
ear was 321g also from the experiment with 30kgN/ha organic fertilizer application. In general, the organic
fertilizer showed better performance in all parameters such as the number of leaves, leaf area, plant height, stem
girth, total shoot, and root biomass, and length of the root. However, the chemical fertilizer outperformed all the
organic fertilizer applied rates in the average highest size of the corn ear by 1.4% i.e. 325.7 and 321g for chemical
fertilizer and organic fertilizer (30kgN/ha) respectively. Comparison between the values obtained from all the
eld experiments involving papaya fruit peel organic fertilizer and the controls reveals statistical dierence at 5%
condence interval for all eld parameters except the root length with values that are all statistically the same.
Figure3 shows the root images from the dierent treatments i.e., Fig.3A is the root system of a representative
from the control (No fertilizer application) experiment, Fig.3B represents the root system of an NPK fertilized
maize plant while Fig.3C represents the root system of a 30kgN/ha organically fertilized experiment. On the
other hand, Figs.S1 and S2 (Supplementary materials) show the complete shoot system of representative plants
from the controls i.e. No fertilizer application and NPK 15–15–15 fertilized experiments and the 30kgN/ha
organically fertilized experiment.
Nutrient bioavailability and uptake. Table7 shows the results of the accumulation of three major nutri-
ent elements (N, P, K) in the leaves, stems, and roots of the maize plants from the control experiments. In the
negative control experiments, nitrogen, phosphorus, and potassium had their highest concentrations of 20.5,
2.92, and 3.5mg/L respectively in the plant roots while in the positive control experiment, all the 3 major ele-
ments (NPK) had their highest concentrations of 16.8, 1.9 and 3.3mg/L respectively in the plant stem.
e results of the accumulation of nutrient elements in the leaves, stems, and roots of the maize plants from
the organic fertilized experiments are also presented in Table7. Nitrogen recorded its highest concentrations
of 29.7mg/L in the leaf and this value was reported from the experiment with 50kgN/ha. For phosphorus and
potassium, the highest concentrations of 7.05 and 8.4mg/L were recorded in the plant’ stem of the experiment
with 50kgN/ha. Overall, the stem displayed the highest ability to store nutrient elements, and the 50kgN/
ha experiment showed the highest level of nutrient bioavailability. As shown in Table7, statistical comparison
between the values obtained from all the nutrient bioavailability experiments involving papaya fruit-peel-based
organic fertilizer and the controls reveals statistical dierence at a 5% condence interval for all parameters
across all treatments.
Soil fertility improvement assessment. As shown in Table2, the chemical and microbial composi-
tions of the experimental soil show that it is of low nutrient status and also low in microbial composition when
compared to fertile soils as seen from the table. e total bacterial count was 4.1 × 105CFU/ml while the fungal
count was 3.0 × 103CFU/ml. As shown in Table8, the application of papaya fruit-peel-based organic fertilizer
improved soil fertility in the long run. e lowest recorded fertility enhancement was found in the negative
control and highest in the experiment with 60kgN/ha. Aer plant harvesting, nitrogen, phosphorus, and potas-
sium increased in experimental soils by 28, 40, and 22% in the experiment with 60kgN/ha over the chemical
fertilizer applied experiment, and the same trend was recorded for all other parameters as shown in the table. All
Figure3. Complete root system from (A) the control (No fertilizer application) experiment (B) the NPK
fertilized experiment (C) organic fertilized experiments (30kgN/ha).
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the values recorded across the dierent treatments (Papaya fruit peel organic fertilizer and the controls) reveal
statistical dierences at a 5% condence interval.
Discussion
Eects of pretreatment on Papaya fruit peel. e papaya fruit peel used in this study was solubilized
with evidence of a breakdown in the lignin-cellulose-hemicellulose complex and other important structural
materials aer pretreatment with heat and chemical application. Heating at 80°C ensured the stabilization of
the resulting substrate as against the use of higher temperatures which has been reported to either cause excess
solubilization or the production of complex proteins that in turn hinders AD and ultimately aect the quality of
produced digestate28–32,52,53. ese pretreatments have also been applied to several lignocelluloses and the result
was a structural component breakdown and high product yield54,55.
Physical parameters of biomass. Dierent raw materials with suitable physical and rheological proper-
ties e.g. lignocellulosic biomass are cheap sources of digestate organic fertilizers, making them preferable to
inorganic fertilizers due to production and supply costs56,57. is factor has over the last decades promoted the
use of anaerobic digestates as organic fertilizers with its attendant reduction in chemical usage and its build-up
in the food chain58. During the AD in this study, pH values are within the alkaline range which is very similar
to earlier reports that alkaline pH range is best for microbial activities during digestion25,28–30 while lower pH
ranges can negatively aect microbial functions and could disrupt the entire process43,59. In most studies, micro-
Table 7. Nutrient bioavailability and accessibility to plant organs in control, NPK 15–15–15 and Carica
papaya fruit peel organic fertilized experiments using Maize (Zea mays) as test plant (Measured at 75 DAE).
Values shown in table are means of triplicate analyses; superscripts with same letters across each plant organ
for the dierent fertilizer treatment treatments (Organic fertilizer and control experiments) are statistically the
same by Tukey’s test at 5%; Value in underline indicates highest concentration of Nitrogen in the leaf, while
those in italics and bold indicates highest levels of phosphorus and potassium respectively in the root.
Nutrient (mg/L) Leaves Stem Roots
No Fertilizer application
Nitrogen (N) 17 ± 1.02a18.1 ± 2.02a20.5 ± 2.01a
Phosphorus (P) 2.11 ± 0.01a2.22 ± 0.03a2.92 ± 0.02a
Potassium (K) 3.0 ± 0.01a3.2 ± 0.02b3.5 ± 0.01a
NPK 15–15–15 fertilizer application
Nitrogen (N) 15.5 ± 0.11a16.8 ± 0.11b15.8 ± 1.01b
Phosphorus (P) 1.58 ± 0.10a1.9 ± 0.02a1.82 ± 0.01b
Potassium (K) 2.9 ± 0.01a3.3 ± 0.02a3.2 ± 0.01a
10kgN/ha application
Nitrogen (N) 27.0 ± 5.02b26.1 ± 4.02c26.2 ± 3.05c
Phosphorus (P) 6.76 ± 1.01b6.51 ± 0.01b6.00 ± 1.01c
Potassium (K) 7.1 ± 2.01b7.1 ± 0.01b7.1 ± 2.01b
20kgN/ha application
Nitrogen (N) 28.0 ± 3.03b27.1 ± 2.02c27.4 ± 3.01c
Phosphorus (P) 4.77 ± 0.12c4.52 ± 0.11c5.05 ± 1.00d
Potassium (K) 7.2 ± 1.01b7.3 ± 1.01b7.5 ± 0.01c
30kgN/ha application
Nitrogen (N) 25.9 ± 3.01c29.0 ± 5.01d27.5 ± 3.04c
Phosphorus (P) 5.62 ± 0.02d5.36 ± 0.01d4.89 ± 1.01d
Potassium (K) 7.0 ± 1.01b7.5 ± 1.01c7.4 ± 0.02c
40kgN/ha application
Nitrogen (N) 29.0 ± 4.01b27.1 ± 4.01c27.4 ± 3.02c
Phosphorus (P) 4.79 ± 1.01c4.53 ± 0.03c5.15 ± 1.01e
Potassium (K) 7.5 ± 1.01c7.3 ± 2.00b7.5 ± 1.01c
50kgN/ha application
Nitrogen (N) 29.7 ± 2.01d27.6 ± 4.01c28.0 ± 1.01c
Phosphorus (P) 4.77 ± 1.01c7.05 ± 1.01c4.51 ± 1.01c
Potassium (K) 7.3 ± 1.02c8.4 ± 2.01b7.1 ± 2.01b
60kgN/ha application
Nitrogen (N) 26.8 ± 3.03c29.0 ± 4.01d27.1 ± 3.03c
Phosphorus (P) 4.01 ± 1.01e5.19 ± 2.01e5.01 ± 1.01d
Potassium (K) 6.9 ± 2.01b7.6 ± 0.03c7.4 ± 2.01c
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bial abundance and activities have been closely linked to the alkaline pH range in the digester60–65. us, main-
taining suitable pH during digestion is a vital condition in ecient bioconversion of substrate, digestion stability,
and product yield enhancement66,67. Due to the enormous biochemical reactions during AD, the temperature is
also very important because most bacteria and methanogens implicated in the process usually thrive at either
mesophilic or thermophilic ranges68,69. e mesophilic temperature in this study contributed to digestion stabil-
ity besides the provision of support for bacteria proliferation and activities70,71. A retention time (RT) of 30 ± 2
was employed in this study to provide proper ambiance for anaerobic microbes causing ecient digestion of the
substrate. A similar result was earlier reported by Mao etal.71.
Chemical parameters of biomass. Results show that the papaya fruit peel is composed of nutrients
required by microbes for their growth in a fermentation medium. Besides, the rumen content used as inoculum
is equally rich in nutrients and microbes72,73. e high nutrients and elemental content of papaya fruit peel are
due to its nutrient storage ability especially in its epicarp coupled with variations in the season which most
times determines the availability of nutrients to the plant. ese qualities make papaya fruit peel better than
other lignocellulosic biomass such as the fruit rind of uted pumpkin (Telfairia occidentalis), Pineapple (Ananas
comosus), Peanut (Arachis hypogaea) pod, Cocoa (eobroma cacao) pod husk, Siam weed (Chromolaena odo-
rata) and wild Mexican Sunower (Tithonia diversifolia) shoots28–31,33–36 while if slightly dier from kitchen
wastes and other animal-based wastes such as cow and piggery dungs. e use of nutrient-rich substrates such
as papaya fruit peel in the AD process had been advocated74. e Nitrogen contents of the papaya fruit peel are
suitable for an ideal AD substrate74.
Organic fertilizer quality. e papaya peel-based anaerobic digestate obtained was richer in nutrients
than the raw biomass. is trend was equally inuenced by the application of pretreatments before AD. How-
ever, total solids, volatile solids, carbon, and calcium concentrations were reduced in the digestate as against
their values in the raw pawpaw fruit peel and this can be attributed to the vital roles they play during microbial
metabolism and for the synthesis of the microbial cell wall. e application of pretreatment coupled with high
microbial population and activities made the pretreated biomass easily biodegraded with evidence of organic
matter breakdown which led to COD reduction in the produced digestate. Application of pretreatments to ligno-
cellulosic biomass before AD has been recommended to enhance ease of digestion and improvement in product
quality75–79.
e digestate used in this study can therefore be adjudged to be nutrient-rich and is potent enough to improve
the nutrient and microbial status of soil80. Besides soil improvement, this digestate when applied as organic fer-
tilizer will have a great impact on the growth and health of plants especially in regions facing erosion of topsoil
and depletion of nutrients. is corroborates the submission of previous studies on the use of AD digestates to
supplement or sustainably replace chemical fertilizers due to the many environmental challenges posed by the
latter in dierent cropping systems the world over58,81–84. Moreover, the microbial composition of the digestate
produced in this study makes it potent to increase the population and diversity of benecial microorganisms
and suitable inoculants in the soil.
Table 8. Soil fertility improvement by Carica papaya fruit peel organic fertilizer. Values shown in table are
means of triplicate analyses; superscripts with same letters are statistically the same by the Tukey’s test at 5%;
Values in italics indicates highest concentrations of each parameter measured.
Parameters
(mg/L) Control NPK 10kgN/ha 20kgN/ha 30kgN/ha 40kgN/ha 50kgN/ha 60kgN/ha
Nitrogen (N) 10.0 ± 0.02a27.6 ± 2.02b30.2 ± 0.01c36.2 ± 0.01d36.7 ± 0.12d31.9 ± 0.02b37.6 ± 1.03d38.2 ± 0.02d
Phosphorus
(P) 1.1 ± 0.01a3.8 ± 0.02b5.5 ± 0.01c5.6 ± 0.02c5.8 ± 0.02c6.9 ± 0.02d6.2 ± 0.02c6.3 ± 0.02c
Potassium (K) 1.6 ± 0.01a5.7 ± 0.01b6.5 ± 0.02b6.6 ± 0.02b6.7 ± 0.02b6.9 ± 0.02b7.1 ± 0.02b7.3 ± 0.03b
Calcium (Ca) 36.2 ± 2.02a74.4 ± 2.02b86.5 ± 2.01c87.5 ± 0.02c89.5 ± 0.05c98.3 ± 0.02d93.6 ± 0.03d106.2 ± 0.02e
Magnesium
(Mg) 21.2 ± 1.01a46.3 ± 1.02b55.5 ± 0.03c56.5 ± 0.03c56.6 ± 0.01c58.4 ± 0.02c61.5 ± 0.02c63.3 ± 0.03c
Copper (Cu) 1.01 ± 0.02a2.0 ± 0.02b1.95 ± 0.02b2.8 ± 0.02c3.1 ± 0.02c3.1 ± 0.03c4.5 ± 0.02d4.7 ± 0.02d
Zinc (Zn) 6.9 ± 1.02a16 ± 1.02b14.5 ± 0.02b16 ± 0.01b21.5 ± 0.01c21.7 ± 0.01c23 ± 1.02c23.5 ± 1.02c
Iron (Fe) 1.6 ± 0.01a3.5 ± 0.02b3.5 ± 0.02b4.1 ± 0.02b4.5 ± 0.02c4.7 ± 0.02c4.9 ± 0.02c6.2 ± 0.02d
Aluminium
(Al) 0.11 ± 0.02a0.43 ± 0.02b0.41 ± 0.02b0.52 ± 0.02c0.54 ± 0.02c0.56 ± 0.02c0.65 ± 0.02d0.68 ± 0.02d
Nitrate (NO3)0.4 ± 0.02a1.6 ± 0.02b1.4 ± 0.01b1.45 ± 0.01b1.55 ± 0.00c1.52 ± 0.01c1.7 ± 0.02d2.0 ± 0.02d
Ammonium
(NH4)0.11 ± 0.01a0.36 ± 0.02b0.35 ± 0.02b0.35 ± 0.02b0.36 ± 0.01b0.37 ± 0.12b0.35 ± 0.05b0.39 ± 0.02c
Phosphate
(PO4)43.2 ± 1.02a75.6 ± 4.02b75.4 ± 1.02b76.1 ± 1.02b77.5 ± 1.02b78.3 ± 2.01b83.2 ± 2.01b84.5 ± 1.00b
Manganese
(Mn) 0.006 ± 0.01a0.013 ± 0.02a0.012 ± 0.01b0.012 ± 0.01b0.013 ± 0.01b0.014 ± 0.01b0.014 ± 0.01b0.018 ± 0.01c
Sulfate (SO4)34.4 ± 1.02a62.5 ± 2.02b61.2 ± 2.03b61.1 ± 2.02b62.5 ± 2.01b62.5 ± 0.02b64.2 ± 1.02b67.5 ± 2.03c
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Digestate microbial composition (before dewatering) and functions. During the evaluation
of the digestate, dierent bacteria, fungi and methanogens were implicated most of which have earlier been
reported to play important function during each stage of AD68 whose source is rumen content used as inocu-
lum in this study. e dominant microbial group in digestate is the Clostridia which are common dwellers of
the bovine rumen and are prominent amino-acid utilizers to produce volatile fatty acids (VFAs) with ammonia
given o85. e population of facultative anaerobes was rather high in the digestate and this could be attributed
to the alkaline nature of the digester which encouraged their proliferation and activities33,34,71,86,87. Besides the
Clostridia, other prominent anaerobes Fusobacterium mortiferum, Bacteroides fragilis, and Gemella morbillorum
all of which are regular inhabitants of anaerobic milieu such as AD. Similarly, the methanogens reported in the
digestate are well known in the AD process87. Earlier, Dahunsi etal.29 had reported a rich microbial population
is a major condition needed for improved degradation of substrates leading to the production of nutrient and
microbe-rich digestates. Besides anaerobes, anaerobic digestates usually contain suitable aerobic microbes such
as Pseudomonas, Klebsiella, Aspergillus, and Bacillus among others which are capable of quickening microbial
processes in applied soils thereby increasing nutrients bioavailability to crop plants88.
Microbial composition of organic fertilizer (dewatered digestate). e higher composition of
microbes in the digestate than the dewatered organic fertilizer is a result of pronounced water reduction in
the organic fertilizer because the growth and activities of microbes are water-dependent. However, the organic
fertilizer is very rich in microorganisms which are oen employed as microbial inoculants for soil nutrients88.
Clostridium and Klebsiella species are notable nitrogen-xing bacteria in anaerobic conditions while Bacillus
species are ecient at solubilizing phosphate72 and have also been implicated in nitrogen-xing. ese microbial
processes oen lead to nutrient availability in the organic fertilizer than the undigested or partially digested
components and the overall eect of this is more ecient crops fertilization89–91. Besides, organic fertilizers
provide ecological advantages such as improvement of food quality which makes them benecial over chemical
fertilizers17,18.
One of the most important components of organic fertilizers is the microbial biomass held together in a
partially-degraded matrix between soil particles, beside the inorganic component, and this makes them suit-
able candidates for soil conditioning92. Globally, the over-dependence on fertilizers from chemical origin has
maximally reduced the quality of soil, increased toxicity to soil benecial microbes, and promoted freshwater
pollution by heavy metals and other chemicals13. erefore, organic fertilizers are vital in providing numerous
sustainable benets which include the enhancement of the quality of soil and the resulting products besides the
provision of human and animal well-being17,18.
Field assessments. From the eld experiments conducted, experiments involving papaya fruit peel organic
fertilizer performed better than the controls as shown in the performance of the maize plants measured through-
out the experimental period i.e., 15 to 75 DaS especially at mid to high rates of organic fertilizer application i.e.,
30 to 60kgN/ha. Among all parameters evaluated, the inorganic fertilizer only recorded better results in the
values obtained for the length of the root while the organic fertilizers outperformed the inorganic fertilizer in
every other parameter. is implies that the organic fertilizer is composed of more nutrients that were readily
made available for the plants for performance improvement. Also, the results further show that the nutrients in
the organic fertilizer were slowly released to the plant’ roots thereby causing a gradual and steady plant growth
as witnessed in the increased values recorded for all the parameters from the time measurement commenced i.e.,
15 DaS through to the end of the experiments. Even though, growth was slow at the earlier stage (15 to 30 DaS)
and was more conspicuous later on, a trend of steady plant growth was established in this study which further
validates the presence and release of plant’ benecial nutrients beside continuous microbial interactions. is
assertion supports an earlier study which reported that the nutrient and elemental composition of digestates is
usually high since the nutrients originally present in the raw materials used for their production usually remains
in them at elevated levels even aer digestion thus explaining their huge potentials for replacing fertilizers from
a chemical origin in agricultural practices the world over92. Babalola93 and Suarez etal.94 have also armed
that activities such as plant growth stimulation facilitated by the xing of atmospheric nitrogen, solubilization,
and mobilization of phosphorus, iron sequestration by the actions of siderophores, and production of phyto-
hormones makes organic fertilizers more ecient than inorganic fertilizers. e use of inorganic fertilizers has
several negative impacts on the environment besides that they release nutrients to the soil in a non-sustainable
manner55. All these factors give organic fertilizers an advantage over inorganic fertilizers58.
Improvement of soil quality. One of the major functions of organic fertilizers is soil physical property
modication, soil aggregation and hydraulic conductivity improvement, and mechanical resistance reduction58,95.
Usage as organic fertilizer is a veritable way to eciently manage anaerobic digestates. is method allows for
the maximum recovery of nutrients especially nitrogen and phosphorus besides controlling organic matter loss
from soils83,96. In this study, soils treated with six organic fertilizer doses were richer in essential nutrients than
the controls at the end of the experiments. is is because the papaya fruit peel organic fertilizer is rich in nitro-
gen and other elements which were slowly released to the soils as a result of lots of microbial interactions espe-
cially the actions of siderophores in the rhizosphere and in such a manner as to enrich the soil in the long run.
e application of organic fertilizer is currently a popular practice designed for sustainable soil management to
improve productivity in agricultural practices97. erefore, the application of digestates organic fertilizers has
popularized the utilization of fertilizers in agriculture for the promotion of organic farming and reduced chemi-
cal usage globally58.
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Conclusion
e application of fertilizers is a common soil management practice that enhances soil fertility and ultimately
improves the productivity of agricultural practices. is study has demonstrated that papaya fruit peel is a suit-
able material for organic fertilizer production via the AD route. e resulting digestate organic fertilizer is rich
in both microbes and soil nutrients. When applied to maize plants, the eld assessment results were better than
those from the NPK 15–15–15 inorganic fertilizer and the control especially at medium to high organic ferti-
lizer application (30 to 60kgN/ha) rate. e organic fertilizer also enhanced the growth of the crop as well as
improved soil fertility. Comparison between the values obtained from the eld experiments involving the organic
fertilizer and the controls reveals that the organic fertilizer showed better performance in all parameters such
as the number of leaves, leaf area, plant height, stem girth, total shoot, and root biomass, and length of the root.
However, the chemical fertilizer outperformed all the organic fertilizer applied rates in the average highest size
of the corn ear by 1.4%. Aer harvesting, all the experimental soils recorded an increase in values of all nutrient
elements over the control with the highest values recorded in the experiment with 60kgN/ha. In the 60kgN/ha
soil, nitrogen, phosphorus, and potassium increased by 28, 40, and 22% respectively over the inorganic fertilizer
applied experiment while dierent levels of increases were also recorded for other elements in all experiments.
us, organic fertilizer produced from papaya fruit peel is a rich source of crop plant’ nutrient and soil bene-
cial microbes which are needed to maintain soil balance, enhance plant’ growth and wellbeing, increase food
production and ultimately ensure food security. erefore, the use of organic fertilizer is a medium for promot-
ing organic agriculture and a veritable way to overcome the challenges posed by inorganic fertilizers’ high rate.
Received: 29 July 2020; Accepted: 12 February 2021
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Acknowledgements
e author appreciates the eorts of the following persons: Professor G.O. Agbaje of Adekunle Ajasin University,
Akungba-Akoko, Ondo State, Nigeria for his immense contribution during the research design and execution;
Professor C.O. Aremu, Mr. Owolabi Akinyomade, and Miss Adewumi Adejoke of Landmark University, Omu-
Aran Nigeria for their contributions during the compilation of the results.
Author contributions
Author S.O.D. conceived the research idea, carried out the research, and wrote the manuscript; authors S.O. and
V.E.E. supervised the entire work; author A.T.A.-D. carried out part of the microbiology analyses and wrote part
of the methodology; author J.O.O. corrected the manuscript at the nal stage.
Competing interests
e authors declare no competing interests.
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