Different organic manure sources and NPK fertilizer on soil chemical properties, growth, yield and quality of okra

Abstract

Use of organic manures to meet the nutrient requirement of crop would be an inevitable practice in the years to come for sustainable agriculture since organic manures generally improve the soil physical, chemical and biological properties. Hence, field experiments were carried out in 2017 and 2018 to compare the impact of different organic manures and NPK fertilizer on soil properties, growth, yield, proximate and mineral contents of okra (Abelmoschus esculentus L.). The treatments each year 2017 and 2018 consisted of: rabbit manure, cow dung, poultry manure, green manure [Mexican sunflower (Tithonia diversifolia Asteraceae)], pig manure, NPK 15-15-15 fertilizer applied at 120 kg N ha −1 and a control (no manure/inorganic fertilizer). The seven treatments were laid out in a randomized complete block design with three replication. Organic manures and NPK fertilizer increased the soil organic matter (OM), N, P, K, Ca and Mg (NPK fertilizer did not increase OM, Ca and Mg significantly), growth, yield, minerals, protein, ash, carbohydrate and mucilage contents of okra fruit as compared with control. Organic manures improved okra yield compared with NPK fertilizer. Okra growth and yield parameters were significantly higher in 2018 compared with 2017. Control, rabbit manure, cow dung, poultry manure, green manure, pig manure and NPK fertilizer in 2018 increased the pod yield of okra by 9.7%, 35.3%, 57.9%, 36.2%, 39.2%, 45.5% and 3.2%, respectively compare with the same treatment in 2017. Amongst various organic manures, poultry manure produced significantly higher plant growth, yield, mineral and proximate composition of okra because of its high soil chemical properties which could be related to its lowest C: N ratio, lignin and lignin: N ratio. Results also showed that okra grown during high intensity rainfall has higher yield but with reduced quality except its mucilage content. Therefore, planting of okra with poultry manure under moderate rainfall will enhance the health benefit from the fruit, however, those that desire its mucilage content planting during high rainfall is recommended.

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Dierent organic manure
sources and NPK fertilizer on soil
chemical properties, growth, yield
and quality of okra
Aruna Olasekan Adekiya1*, Wutem Sunny Ejue1, Adeniyi Olayanju2, Oluwagbenga Dunsin1,
Christopher Muyiwa Aboyeji1, Charity Aremu1, Kehinde Adegbite1 & Olanike Akinpelu1
Use of organic manures to meet the nutrient requirement of crop would be an inevitable practice in the
years to come for sustainable agriculture since organic manures generally improve the soil physical,
chemical and biological properties. Hence, eld experiments were carried out in 2017 and 2018 to
compare the impact of dierent organic manures and NPK fertilizer on soil properties, growth, yield,
proximate and mineral contents of okra (Abelmoschus esculentus L.). The treatments each year 2017
and 2018 consisted of: rabbit manure, cow dung, poultry manure, green manure [Mexican sunower
(Tithonia diversifolia Asteraceae)], pig manure, NPK 15-15-15 fertilizer applied at 120 kg N ha−1
and a control (no manure/inorganic fertilizer). The seven treatments were laid out in a randomized
complete block design with three replication. Organic manures and NPK fertilizer increased the soil
organic matter (OM), N, P, K, Ca and Mg (NPK fertilizer did not increase OM, Ca and Mg signicantly),
growth, yield, minerals, protein, ash, carbohydrate and mucilage contents of okra fruit as compared
with control. Organic manures improved okra yield compared with NPK fertilizer. Okra growth and
yield parameters were signicantly higher in 2018 compared with 2017. Control, rabbit manure, cow
dung, poultry manure, green manure, pig manure and NPK fertilizer in 2018 increased the pod yield
of okra by 9.7%, 35.3%, 57.9%, 36.2%, 39.2%, 45.5% and 3.2%, respectively compare with the same
treatment in 2017. Amongst various organic manures, poultry manure produced signicantly higher
plant growth, yield, mineral and proximate composition of okra because of its high soil chemical
properties which could be related to its lowest C: N ratio, lignin and lignin: N ratio. Results also
showed that okra grown during high intensity rainfall has higher yield but with reduced quality except
its mucilage content. Therefore, planting of okra with poultry manure under moderate rainfall will
enhance the health benet from the fruit, however, those that desire its mucilage content planting
during high rainfall is recommended.
Okra (Abelmoschus esculentus L.) is an important tropical and subtropical vegetable crop grown for its fresh
leaves, buds, owers, pods, stems and seeds. e fresh pods can be eaten as vegetables in form of salads, in soups
and stews or boiled. Okra fruits contain mucilage upon cooking. e mucilage has medical importance as it is
used as a plasma replacement or blood volume expander and binds cholesterol and bile acid carrying toxins
dumped into it by the liver1. Okra fruits contain ber, vitamin C, folate and antioxidants2. e seeds contain oil
that is edible by man as well as in soap industry3.
Despite the enormous potentials of okra fruits production, its yield per hectare and quality in Nigeria had
been greatly hampered by the low fertility status and organic matter contents of the soils which translate into low
productivity and consequently reducing income for the farmer4. According to Adekiya etal.4. the yield of okra
in Nigeria is currently very low about 2.7 t ha−1 owing to low native soil fertility status among other factors. Lack
of sucient amounts of nutrients result in poor performance of the crop with growth been aected resulting to
low yield. It has been reported that the maintenance of soil organic matter (OM) is the basis of sustainable crop
open
Department
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production in Nigeria and other tropical countries5. Hence, there is need to improve the fertility of the soil for
continuous and increased crop production.
In years to come, utilization of organic manure to meet crop nutrient requirement will be an unavoidable
practice to enhance sustainable agriculture, this is because, the physical, chemical and biological properties of
soil is generally improved by the addition of organic manures which in turn enhances crop productivity and
maintains the quality of crop produce6. Although, in comparison to inorganic fertilizers, organic manures con-
tain smaller quantities of plant nutrients. e use of inorganic fertilizer to increase yield has been found to be
eective as a short-term solution but demands consistent use on a long-term basis. e high cost of inorganic
fertilizers makes it uneconomical and out of reach to poor farmers and it is also undesirable due to its hazardous
environmental eects7. erefore, it is essential to investigate the use of locally sourced organic materials which
are environment friendly, cheap and probably an eective way of improving and sustaining the productivity of
soils and arable crops such as okra.
Okra yield responses to organic and inorganic fertilizers have been reported by several workers2,4,8–10. How-
ever, since organic and in organic fertilizers contain dierent chemical composition and quality they therefore
may react dierently when applied to the soil with respect to soil chemical properties, crop yield and quality. is
aspect needs investigation especially in Nigeria where such data on the eects of dierent organic and inorganic
amendments on the mineral and proximate contents are lacking. Available work dealt with organic amendment
on the yield of okra without putting into consideration the quality of okra fruits. In Sokoto northern Nigeria10,
investigated dierent sources of organic manure (cow, sheep and poultry manure) on growth and yield of okra.
eir results revealed that poultry manure promotes higher growth and yield of okra compared with cow and
sheep manure.
Fagwalawa and Yahaya11 investigated the eects of Sheep, cow and poultry manures and their combinations
on the growth and yield of okra, their result also revealed that poultry manure has the highest yield. Also in
Malaysia9, six dierent treatments (no fertilizer, NPK fertilizer, poultry manure, rat manure, goat manure and
rabbit manure) were investigated on the growth and yield of okra, According to the study, application of poultry
manure signicantly increased the growth and yields performances on okra compared to other types of organic
fertilizers. To have a holistic approach on the response of organic manures on soil chemical properties and okra
performance, data on the quality of such okra fruit is imperative. erefore, the study is to compare the impact
of dierent organic manures and NPK fertilizer on soil properties, growth, yield, proximate and mineral contents
of okra grown in derived savanna zone of Nigeria.
Results
Pre-planting chemical and physical analysis of the experimental eld and chemical composi-
tion of organic manures used for the experiment. e result of the pre-planting chemical and physi-
cal analysis of the experimental eld (Table1), indicated in both years that the soil was sandy loam and slightly
acidic with a pH of 6.10 and 6.20 respectively in 2017 and 2018. e soil organic matter, total N available P and
exchangeable Ca and K were low with both a little below the critical levels of 3.0%, 0.20%, 10.0mgkg−1, 2.0
cmol kg−1 and 0.15 cmol kg−1 respectively in both cropping seasons of 2017 and 201812. Exchangeable Mg was
adequate in 2017 but low in 2018. Analysis of the soil amendments used for this experiment is shown in Table2
with poultry manure having the highest percentage of N, P, K, Ca, Mg and the lowest lignin, lignin: N ratio, C:
N ratio and organic C.
Response of soil chemical properties and bulk density to dierent organic manure sources and
NPK fertilizer. Table3 shows the result of the eect of dierent organic amendments and NPK fertilizer
Table 1. Pre-planting physico-chemical characteristics of the soil (0–15cm) at the experimental sites in 2017
and 2018.
2017 2018
Soil properties
pH (H2O) 6.10 6.20
OM (%) 2.56 2.44
Total N (%) 0.10 0.15
Available P (mg/kg) 9.50 8.90
Exchageable cations (cmol kg−1)
K 0.15 0.14
Ca 1.80 1.90
Mg 0.41 0.38
Particle size distribution (%)
Sand 76 74
Silt 13 15
Clay 11 11
Textural class Sandy loam Sandy loam
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on soil chemical properties. In both years, all organic sources of soil amendment increased soil OM, N, P, K,
Ca and Mg signicantly (p < 0.05) with respect to the control. NPK fertilizer did not increase OM, Ca and Mg
contents of the soil relative to the control but increased N, P and K. Among organic manure sources, poultry
manure has the highest values of all soil nutrients (except SOM) which was closely followed by green manure
treatment with rabbit manure having the least values. e order of SOM among organic manures was: Rabbit
manure > cow dung > Pig manure > green manure > Poultry manure. All organic manure sources increased soil
N, P and K compared with NPK fertilizer. In both years, the control has the highest value of pH while NPK fer-
tilizer has the least value, there were no signicant dierences between the pH values of control, rabbit manure,
cow dung, poultry manure, green manure and pig manure. Also there were no signicant dierences in the pH
values between organic manures and NPK fertilizer, but there was a signicant dierencs between the pH values
of the control and NPK fertilizer. Soil chemical properties in year 2018 were increased compared with year 2017
except soil pH. Soil bulk density in both years was signicantly reduced in organic manure soils compared with
the control and NPK fertilizer which has similar values. All organic manures have statistically similar values
of bulk density. e interactions between year and amendment (Y × M) were signicant for all soil properties
except soil pH and bulk density.
Response of okra growth and yield to dierent organic manure sources and NPK fertilizer. e
results of the response of okra growth and yield to dierent organic manure sources and NPK fertilizer are pre-
sented in Table4. In 2017 and 2018, all amendments (both organic and inorganic) increased the growth (plant
height and leaf area) and yield (okra pod weight and number of pods per plant) of okra compared with the
control. Also in both years, poultry manure has the highest values followed by green manure. e order of okra
pod yield was: poultry manure > green manure > pig manure > cow dung > NPK fertilizer = rabbit manure > con-
trol. Okra growth and yield parameters were signicantly higher in 2018 compared with 2017. Control, rabbit
manure, cow dung, poultry manure, green manure, pig manure and NPK fertilizer in 2018 increased the pod
yield of okra by 9.7%, 35.3%, 57.9%, 36.2%, 39.2%, 45.5% and 3.2%, respectively compare with the same treat-
Table 2. Analysis of organic manures. Values followed by similar letters under the same column are not
signicantly dierent at p = 0.05 according to Duncan’s multiple range test.
Manure Organic C (%) N (%) C: N Lignin (%) Lignin: N P (%) K (%) Ca (%) Mg (%)
Rabbit manure 30.1a 1.01e 29.8a 14.8a 14.7a 0.54b 1.95d 1.15d 0.40c
Cow dung 26.5b 1.86d 14.24b 14.6a 7.8b 0.82a 2.11c 1.01d 0.51b
Poultry manure 17.8e 2.91a 6.12e 6.1c 2.1e 0.84a 3.79a 3.34a 0.64a
Green manure 23.6c 2.51b 9.40d 8.1b 3.2d 0.52b 3.04b 3.01b 0.10d
Pig manure 20.1d 2.16c 9.77c 7.9b 3.7c 0.80a 2.16c 1.45c 0.54b
Table 3. Post-Harvest soil chemical properties. Values followed by similar letters under the same column
are not signicantly dierent at p = 0.05 according to Duncan’s multiple range test. *Signicant dierence at
p = 0.05; ns, not signicant at 0.05.
Year Amendments pH (water) OM (%) N (%) P (mg kg−1) K (cmol kg−1) Ca (cmol kg−1) Mg (cmol kg−1)Bulk density
(g cm−3)
2017
Control 6.02a 2.47f 0.09f 7.9f 0.14g 2.81e 0.38g 1.55a
Rabbit 5.84ab 3.87a 0.14d 9.6e 0.19e 3.28d 0.40ef 1.20b
Cow dung 5.81ab 3.57bc 0.19c 10.3d 0.20de 4.80c 0.46d 1.19b
Poultry 5.85ab 3.08e 0.25a 27.4a 0.70a 5.84a 0.81a 1.19b
Green manure 5.87ab 3.23d 0.24a 15.9b 0.50b 4.96b 0.56b 1.18b
Pig manure 5.82ab 3.47c 0.21b 12.3c 0.26c 4.84c 0.49cd 1.18b
NPK 5.51b 2.50f 0.13e 9.5e 0.16f 2.91e 0.39fg 1.54a
2018
Control 6.01a 2.39e 0.08f 7.2g 0.12f 2.69f 0.36e 1.55a
Rabbit 5.81ab 3.41a 0.17d 10.6e 0.22d 3.66d 0.45d 1.19b
Cow dung 5.81ab 3.11b 0.21c 13.4d 0.22d 5.15c 0.51c 1.19b
Poultry 5.79ab 2.95d 0.27a 29.1a 0.75a 6.96a 0.88a 1.19b
Green manure 5.77ab 3.07c 0.27a 19.4b 0.55b 5.48b 0.61b 1.19b
Pig manure 5.0ab 3.09c 0.24b 15.4c 0.31c 5.10c 0.53c 1.18b
NPK 5.51b 2.41e 0.13e 9.3f 0.15e 2.72e 0.37e 1.54a
Year (Y) Ns * * * * * * Ns
Manure (M) Ns * * * * * * *
Y × M Ns * * * * * * Ns
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ment in 2018. e interactions between year and amendment (Y × M) were signicant for all growth and yield
parameters.
Response of okra quality to dierent organic manure sources and NPK fertilizer. e eect
of dierent organic manure sources on proximate and mineral content of okra in 2017 and 2018 are shown in
Table5. In 2017 and 2018 dierent organic manure sources and NPK fertilizer increased protein, ash, carbohy-
drates, mucilage N, P, K Ca and Mg contents of okra fruits compared with the control. Fat contents of okra fruits
were reduced with dierent organic sources and NPK fertilizer compared with the control. Among dierent
amendment applied, poultry manure had the highest values of protein, carbohydrate, mucilage, N, P, K, Ca and
Mg contents. Green manure had statistically similar value with poultry manure in protein, ash, fat, carbohydrate,
mucilage, N, P and Mg contents in 2017 and in 2018, ash, fat, carbohydrate, mucilage, N and P were statisti-
Table 4. Growth and yield parameters. Values followed by similar letters under the same column are not
signicantly dierent at p = 0.05 according to Duncan’s multiple range test. *Signicant dierence at p = 0.05.
Year Amendments Yield (t ha−1) No. of pods/plant Plant height (m) Leaf area (m2)
2017
Control 3.1f 10g 0.29f 0.46e
Rabbit 3.4e 12f 0.37e 0.54d
Cow dung 3.8d 14de 0.41d 0.57d
Poultry 5.8a 23a 0.56a 0.81a
Green manure 5.1b 21b 0.51b 0.71b
Pig manure 4.4c 17c 0.47c 0.64c
NPK 3.5e 13ef 0.38e 0.55d
2018
Control 3.4g 12h 0.30f 0.50f
Rabbit 4.6e 15e 0.41de 0.60e
Cow dung 5.2d 16de 0.44d 0.65de
Poultry 7.9a 26a 0.62a 0.94a
Green manure 7.1b 24b 0.55b 0.80b
Pig manure 6.4c 18c 0.50c 0.71c
NPK 3.6f 14fg 0.39e 0.60e
Year (Y) * * * *
Manure (M) * * * *
Y × M * * * *
Table 5. Proximate and mineral contents of okra. Values followed by similar letters under the same column
are not signicantly dierent at p = 0.05 according to Duncan’s multiple range test. *Signicant dierence at
p = 0.05.
Year Amendment Protein (%) Ash (%) Fat (%) Carbohydrate (%) Mucilage (%) N (%) P (%) K (%) C a (%) Mg
(%)
2017
Control 9.91g 5.6f 12.9a 22.0d 6.35g 0.82e 10.6d 0.88e 2.41e 2.10e
Rabbit 19.1e 6.2e 13.1ab 26.8c 7.34de 2.87d 12.3b 1.10c 3.22c 3.22c
Cow dung 19.3de 6.4de 13.9ab 27.0c 7.58c 2.96d 12.6b 1.06c 3.10c 3.32c
Poultry manure 23.5a 7.1a 10.4c 32.0a 9.88a 4.41a 13.4a 1.87a 4.43a 4.31a
Green manure 22.1b 6.9ab 10.9c 30.1ab 9.57ab 4.21ab 12.9a 1.44b 3.61b 4.01b
Pig manure 19.6c 6.1e 10.2c 29.0b 7.03e 3.11c 11.6c 1.74ab 3.55b 3.10c
NPK 17.5f 6.7cd 11.9b 27.1c 6.89f 3.21c 11.4c 0.94d 2.96d 2.22d
2018
Control 9.69c 5.1f 12.10a 19.9e 6.84d 0.75d 9.31c 0.81c 2.51e 2.00c
Rabbit 18.6b 5.6e 11.9ab 23.3d 7.58bc 2.65c 10.0b 0.96b 2.76c 2.80b
Cow dung 18.91b 5.8de 11.6ab 24.2cd 7.99b 2.65c 10.9b 0.90b 2.75c 2.88b
Poultry manure 20.88a 6.6a 9.6c 28.0a 10.7a 3.24a 11.9a 1.16a 3.26a 3.68a
Green manure 20.55a 6.5ab 9.8c 27.6ab 10.4a 3.04a 11.8a 0.99b 2.91b 3.61a
Pig manure 18.51ab 5.9d 9.2c 26.2b 7.8b 2.85b 10.6b 1.10ab 2.96b 2.86b
NPK 17.20b 6.0cd 11.0b 23.0d 7.1c 2.86b 10.1b 0.99b 2.61de 2.21c
Year (Y) * * * * * * * * * *
Manure (M) * * * * * * * * * *
Y × M * * * * * * * * * *
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cally similar. Poultry manure and green manure increased protein, carbohydrate, mucilage, N, P, K, Ca, Mg and
reduced fat compared with NPK fertilizer. 2018 cropping of okra reduced protein, ash, fat, carbohydrate, N, P,
K, Ca, Mg and increased mucilage contents in the fruits compared with 2017. e interaction (Y × M) was also
signicant for proximate and mineral contents of the okra fruit.
Discussion
e dierent organic manures increased the soil organic matter (SOM), N, P, K, Ca and Mg contents of the soil
compared with control. is result is consistent with the analysis recorded for the amendments in the present
study that they contain these nutrients and also attested to the fact that the soil was decient in these nutrients
(Table1). ese nutrients were released into the soil as the organic manures were decomposed. Studies by13,14
have shown that animal manures and green manure increased soil OM, N, P and CEC and this was attributed to
the availability and adequate supply of organic matter. e slightly lower pH of organic amended soil compared
with the control could be due to the fact that during microbial decomposition of the incorporated manures,
organic acids may have been released, which neutralized the alkalinity of the manures, thereby lowering the pH
of the soil below their initial values. Adekiya etal.4 observed a similar trend in their work on organic amend-
ments of soils. NPK fertilizer has the lowest pH as a result of leaching of bases from soil surface. Poultry manure
signicantly produced the highest soil chemical properties, this could be related to its lowest C:N ratio, lignin
and lignin: N ratio (Table2) which favours quick mineralization and release of nutrients to the soil compared
with other soil amendments. Consequently, the quality of an organic amendment is dened in term of the relative
content of nutrients (especially nitrogen), lignin, lignin: N and the C:N ratio14. Rabbit manure increased SOM
compared with other organic manures. e increase in SOM of rabbit manure could be related to its high C:N
and lignin content. Plant constituents, such as lignin retard decomposition. Organic materials with high C:N
and lignin generally would favor nutrient immobilization, organic matter accumulation and humus formation14.
In both years, poultry manure increased N, P, K, Ca and Mg compared with other manures. is was due to
the fact that poultry manure was low in C:N ratio, lignin and lignin/N and high in N, P, K, Ca and Mg compared
with other manures. Due to high quality of this amendment, it decomposes quickly and release nutrients to the
soil. e signicant increase in soil nutrients especially OM, N, P and K in organic amendments compared with
NPK fertilizer was due to leaching in NPK fertilizer treated plots. e reduction in Ca and Mg in NPK fertilizer
plots compared with organically amended plot was due to the fact that NPK fertilizer did not contain Ca and Mg.
e reduction of soil bulk density observed in both years with organic manures compared with control and NPK
fertilizer could be attributed to increase in soil organic matter resulted from the degraded organic residues by
soil microorganisms. Organic matter is known to improve soil structure, aeration and reduce soil bulk density15.
e decreasing order of okra yield were poultry manure > green manure > pig manure > cow dung > NPK
fertilizer = rabbit manure > control. e positive eect of organic manure on growth and okra yield could be due
to the contribution made by amendments to fertility status of the soils as the soils were low in organic carbon
content. Manure when decomposed increases both macro and micro nutrients as well as enhances the physio-
chemical properties of the soil for the betterment of okra growth.
Okra grown on poultry manure performed better in terms of growth and yield compared with other sources
of organic soil amendment and NPK fertilizer. is could also be related to low C:N ratio, lignin and lignin/N
values ese attributes of poultry manure will lead to fast mineralization and early release of nutrients to a short
gestation crop like okra, hence there was a boost in the morphological growth of the plant which translate to
greater yield compared with other amendments. Wolf and Snyder16 reported that C:N ratio of organic materials
markedly inuences the decomposition rate and the mineralization of N because N determines the growth and
turnover of the microorganisms that mineralize organic carbon.
e reduced growth and yield of okra in plots treated with other manures in comparison with poultry manure
could be as a result of immobilization of soil nutrients which occurs when soils are treated with animal manure
of high lignin contents which results from the feed eaten by the animal17.
Higher yields were observed in poultry manure plots compared with inorganic fertilizer because, the nitrogen
content of poultry manure is released to the soil gradually and steadily over longer time for the growth of the
plant compared with nitrogen from NPK fertilizer which is prone to losses by run-o, volatilization, leaching
and/or denitrication. Poultry manure has been said to be a better soil amendment compared with chemical
fertilizers because of the greater capability of poultry manure to preserve its N18. e superior N supply by poultry
manure during okra cropping in this experiment may be the reason for better growth and yield of okra in plots
with poultry manure18. e obtained results corroborated the nding of19 that poultry manure increased the
height of okra relative to other amendments.
e better performance of okra under NPK fertilizer plots compared with the control was due to release of
nutrients (N, P and K) from the fertilizer which are absorbed by the okra plants. Okra growth and yield in second
year (2018) was better than that of rst year (2017). is was due to the dierences in the amount of rainfall
between the two years (Table6). Year 2017 had 1238mm of rainfall while it was 1428mm in 2018. ere was
higher moisture in the rst few weeks (month of May) of incorporation of the manures in 2018 compared with
2017 which may have led to better and quicker decomposition of the organic materials in 2018. Soil biological
activities which causes degradation of organic materials is severely limited during limited moisture, but with the
onset of the rains (2018), there is a ush in microbial activity20.
e fact that organic manures and NPK fertilizer increased okra mineral contents compared with the control
was attributed to increased availability of the nutrients in soil as a result of the mineralization of the manures
leading to increased uptake by okra plants. In both years, the correlation coecient between soil and okra fruit
N, soil and okra fruit P, soil and okra fruit K, and okra fruit Ca, soil and okra fruit Mg were all signicant with
R values of 0.83, 0.71, 0.66 0.81 and 0.88, respectively at P < 0.05. Poultry manure had the highest values of N, P,
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K, Ca and Mg in the okra fruit compared with other amendment and NPK fertilizer. is result is also consist-
ent with the soil chemical properties and growth and yield for this treatment. e poultry manure may have
improved the availability of nutrients to the crop by enhancing the mineralization and supply of readily available
nutrients to the soil21.
e increased nitrogen to the soil by the incorporation of organic manures increased the nitrogen uptake by
the okra fruit thereby increasing protein. is explains the reasons for increase in crude protein values between
organic amendment and control and NPK fertilizer treatment. Poultry manure has the highest value of crude
protein, this is also consistent with its high soil N level (Table3). Nitrogen is a major constituent of chlorophyll,
protein, amino acids, various enzymes, nucleic acids and many other compounds in the cell of plants22. e crude
protein contents in all the organically amended soils were above the critical 13–17%23,24. erefore the organic
materials sustained good nutritive quality of okra as opposed to NPK and control. e ash content in poultry
manure treatment was signicantly higher possibly because of the balanced nutrient in the manure, unlike NPK
with inferior contents of N, P, and K and the control with lower concentration of nutrients. In this study, poultry
manure gave the highest N, P and K concentrations of 3.04%, 11.9% and 1.16% in 2017 and 4.41% 13.4% and
1.87% in 2018 (Table6) respectively. e fat content of poultry manure was lowered compared with others, this
was due to its high protein content of poultry manure. It has also been reported25 that there is a negative cor-
relation between fat and protein content. High nitrogen application reduced fat and increased protein content.
Organic amendments and NPK fertilizer increased the mucilage contents of okra compared with the control.
is could be as a result of increase in NPK from these amendments. is could be attributed to the increase in
-galactose, -rhamnose and -galacturonic acid contents in okra fruits by the application of nutrients through
organic and inorganic sources which might have resulted in increase of mucilage content26. e mucilaginous
polysaccharide in the okra is rich in uronic acid (65%) and consists of rhamnose, galactose, glucose, galacturonic
acid and glucuronic acid in addition to 3.7% acetyl groups27. Ahmad etal.27 also reported that compost and NPK
fertilizer application increased the mucilage of borage plant. e highest value of mucilage in poultry manure
could be related to increased soil nutrients compared with other treatments. Okra produced under organic
amendments has better qualities (N, P, K, Ca, Mg, protein, ash and mucilage) compared with NPK fertilizer and
control. is is because organic manures not only increase soil nutrients but also improves the physical by pre-
vention of erosion and leaching of nutrients (in this experiment reduce bulk density) and biological properties28.
Organic manures also contains both micro and macro nutrients unlike NPK fertilizer that contains only N, P
and K. Lumpkin29 also, was of the opinion that organically produced vegetables were of higher qualities than
those produced using conventional methods. Cropping of okra in 2018 reduced protein, ash, fat, carbohydrate,
N, P, K, Ca and Mg contents of okra fruit compared with 2017 cropping. is could be adduced to high rainfall
in 2018 compared with 2017. High rainfall has been reported to reduce the nutrient and proximate content of
vegetables30. Climatic conditions have a great eect on the concentration of mineral in plants. Variation in tem-
perature and rainfall have been reported to inuence the chemical composition in plants30,31.
Conclusion
Results of this experiment showed that organic manures and NPK fertilizer increased the soil chemical properties
(NPK fertilizer did not increase OM, Ca and Mg signicantly), growth, yield, minerals, protein, ash, carbohydrate
and mucilage contents of okra fruit as compared with control. Organic manures improved okra yield compared
with NPK fertilizer. Amongst various organic manures, poultry manure produced signicantly higher plant
growth, yield, mineral and proximate composition of okra because of its high soil chemical properties which
could be related to its lowest C:N ratio, lignin and lignin: N ratio. Results also showed that okra grown during
high intensity rainfall has higher yield but with reduced quality except its mucilage content. erefore, planting
of okra with poultry manure under moderate rainfall will enhance the health benet from the fruit, however,
those that desire its mucilage content planting during high rainfall is recommended.
Material and methods
Site description, treatments and experimental layout. During 2017 and 2018 cropping seasons,
okra was grown in a eld experiment at the Landmark University Teaching and Research Farm, Omu-Aran,
Kwara state (8° 9′ N, 5° 61′ E), with altitude of 562m above sea level. e rainfall pattern was bimodal with peaks
Table 6. Meteorological data of the study area. Source Meteorological unit, Teaching and Research Farm,
Landmark University, Omu-Aran, Kwara State, Nigeria.
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
2017
Rainfall (mm) 0 0 97.28 92.46 158.24 174.49 71.11 179.33 231.39 119.88 0 114.04
Relative humidity (%) 54.1 44.9 69.7 81.0 85.3 89.9 91.9 93.4 92.3 85.2 71.3 56.9
Mean temp (°C) 29.1 31.2 31.9 29.9 28.9 27.9 26.9 26.3 26.5 27.8 29.9 28.4
2018
Rainfall (mm) 0 12.19 157.73 96.26 214.88 228.6 160.02 95.25 248.66 195.07 19.55 0
Relative humidity (%) 34.3 63.7 78.8 85.2 87.5 90.2 92.7 92.7 91.3 88.4 76.4 47.1
Mean Temp (°C) 26.6 29.6 29.3 29.2 28.1 27.7 26.4 26.2 26.4 27.5 28.8 28.9
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in August (Table6). e total annual rainfall in the area was about 1238mm in 2017 with mean air temperature
of 28.7°C and mean relative humidity of 83.9%. In 2018, the total annual rainfall in the area was about 1428mm
with mean air temperature of 30.0°C and mean relative humidity of 77.4%. e soil at the site of the experiment
is an Alsol classied as Oxic Hap-lustalf or Luvisol2. e experimental site falls under the derived savanna agro-
ecological zone of North-Central Nigeria. Weeds in the experimental soil before cultivation included Mexican
sunower (Tithonia diversifolia Asteraceae) and Guinea grass (Panicum maximum Jacq). Dierent sites were
used for the experiment in 2017 and 2018, but the two sites were very close to each other.
e treatments each year 2017 and 2018 consisted of: rabbit manure, cow dung, poultry manure, green
manure [Mexican sunower (Tithonia diversifolia Asteraceae)], pig manure, NPK 15-15-15 fertilizer and a con-
trol (no manure/ inorganic fertilizer). Both the organic manures and NPK fertilizer were applied at the rate of
120kgNha−132. is was equivalent to 800kgha−1 for NPK 15-15-15, 11.9 t ha−1 for rabbit manure, 5.5 t ha−1 for
cow dung, 4.1 t ha−1 for poultry, 4.8 t ha−1 for green manure and 5.6 t ha−1 for pig manure. e seven treatments
were laid out in a randomized complete block design (RCBD) with three replications. Each block consisted of 7
plots measuring 3 × 2m with 1m block and 0.5m between plots.
Land preparation, incorporation of manures, sowing of okra seeds and application of NPK
fertilizer. e experimental eld was cleared manually using cutlass and thrashed removed from the site
before mechanical (ploughing and harrowing) land preparation aer which plots were marked out to the require
plot size of 3 × 2m. A hand-held hoe was used to incorporate the manures into the soil to a depth of approxi-
mately 20cm. All animal manures were obtained from the livestock section of Landmark University Teaching
and Research Farm. Fresh top Mexican sunower was collected from a nearby farm and hedge containing green
tender stems and the leaves. e plots were allowed for 4week before sowing of okra seeds. Sowing of okra vari-
ety NHAe-47-4 was done late April for both 2017 and 2018 croppings. Two seeds of okra were sown per hole at
inter-row spacing of 0.6m and 0.6m intra-row spacing manually giving a plant population of 27,778 plants ha−1.
At 2weeks aer sowing, thinning to one plant per stand was done and this was followed by manual weeding
using hand hoe before treatment application. Subsequent weeding was done as needed. NPK 15-15-15 fertilizer
was applied by side placement at about 8–10cm away from the sown seeds two weeks aer sowing. Insect pests
were controlled by spraying cypermethrin weekly at the rate of 30ml per 10 l of water from 2weeks aer sowing
till 4weeks aer sowing.
Determination of soil properties. Before the start of the experiment, soil samples from topsoil (0–15cm)
were taken from random spots in the study area and were bulked together, air-dried and sieved using a 2-mm
sieve and their physical and chemical characteristics were determined2. Textural class of the soils were deter-
mined by the method of33. Soil organic carbon (OC) was determined by the procedure of Walkley and Black
using the dichromate wet oxidation method34. Total N was determined by the micro-Kjeldahl digestion method35.
Available P was determined by Bray-1 extraction followed by molybdenum blue colorimetry36. Exchangeable K,
Ca, and Mg were extracted using 1M ammonium acetate37. ereaer, concentration of K was determined on a
ame photometer, and Ca and Mg were determined by EDTA titration method. Soil pH was determined using
a soil–water medium at a ratio of 1:2 with a digital electronic pH meter. At the termination of the experiment in
2017 and 2018, soil samples were also collected (on plot basis) and similarly analysed for soil chemical proper-
ties as described above. Also at the end of the experiment each year, soil bulk density were determined all plots.
Five undisturbed samples (0.04m diameter, 0.15m high) were collected at 0–0.15m depth from the centre of
each plot at random and 0.15m away from each okra plant using core steel sampler4. e samples were used to
evaluate bulk density aer oven-drying at 100°C for 24 h4.
Determination of growth and yield parameters. Collection of data for growth (plant height and leaf
area) was done at mid-owering of okra plant (about 47days aer sowing). Plant height was determined by the
use of meter rule while leaf area was determined by using the model {LA = 0.34(LW)1.12} developed by38, where
LA = leaf area, L = leaf length and W = leaf width. Edible pods were harvested at four day intervals, counted and
weighed. Pod weight was evaluated based on the cumulative harvests per plot. For 2017 cropping of okra, fruits
were harvested until November 2017 while for 2018 okra cropping, fruits were harvested until July 2018.
Chemical analysis of okra fruits and organic manures. At harvest in 2017 and 2018, eight okra fruits
of uniform sizes were randomly collected from each plot and analyzed for mineral contents according to meth-
ods recommended by the Association of Ocial Analytical Chemists39. One gram of each sample was digested
using 12cm−3 of the mix of HNO3, H2SO4, and HCLO4 (7:2:1 v/v/v). Contents of N, P, K, Ca and Mg were deter-
mined by atomic absorption spectrophotometry. Lignin content of the organic amendments was determined
from acid-free detergent ber using the method described by40.
Samples of okra fruits from each plot was taken for proximate analysis. e ash, crude protein, crude fat and
carbohydrate contents of the okra fruits were determined using standard chemical methods described by Asso-
ciation of Analytical chemists39. Samples of okra fruits were sliced with a knife and blended. Aer blending, it
was diluted with ten times its weight with water (1:10). e viscous solution was separated from the debris using
ne cloth41. Mucilage of the extracted viscous liquid was measured using viscometer.
About 2g of each organic manures used was collected and analysed for N, P, K, Ca, and Mg as described
by42. N was determined by the micro-Kjeldahl digestion method. Samples were digested with nitricperchloric-
sulphuric acid mixture for the determination of P, K, Ca, and Mg. Phosphorus was determined colorimetrically
using the vanadomolybdate method, K was determined using a ame photometer and Ca and Mg were deter-
mined by the EDTA titration method43.
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Statistical analysis. e data collected were subjected to statistical analysis of variance (ANOVA) using the
Genstat statistical package44 and treatment means were separated using Duncan Multiple Range Test (DMRT)
at 5% probability level.
Received: 20 May 2020; Accepted: 27 August 2020
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Acknowledgements
I would like to thank Landmark University who provided enabling environment in carrying out the experiment
and also willing to pay the article processing charges of this paper
Author contributions
A.O.A.—design, statistical analysis & write up; W.S.E.—typing of manuscript; A.O.—proof reading of manuscript;
C.M.A.—design of experiment; O.D.—data collection; O.A.—eld work; K.A.A.—proof reading of manuscript;
C.A.—statistical analysis of data.
Competing interests
e authors declare no competing interests.
Additional information
Correspondence and requests for materials should be addressed to A.O.A.
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