INTRODUCTION
The
soil is the most important natural resource, because on it, the entire
vegetation is dependent and by extension man. Man directly depends on
vegetation for food, hence it’s necessary to preserve the top soil and help to
improve its organic matter content in order to increase its nutrient providing
ability (Defoer, 2002). Fertilizers, according to the IFA (2008), are materials
that contain 5% or more of the three essential plant nutrients. They are soil
amendments that guarantee the minimum percentages of Nitrogen, Phosphate and
Potassium (Adrian et. al. 2014). Generally, Fertilizer among other advantages,
improves soil nutrient, results in faster growth of crops, increases crop yield
and improves the quality of fruits/crops, by making available the essential
plant nutrients in readily available forms (EPA, 2013). The disadvantages of
inorganic/synthetic fertilizer particularly in uncontrolled/continuous use, can
results in problems of serious soil degradation, negative impact on the
environment like contamination of water bodies close to the site of
application, enhancing the growth of weeds, and also reducing the oxygen
content in water making the water unsuitable for consumption, results in the
death of aquatic animals (Wilfred, 2002), soil acidity and changes in soil
microbial diversity (Carroll and Salt, 2004).
The organic matter content of soils is one of the major key to soil
productivity. The advantage of organic fertilizer over inorganic fertilizer in
soil fertility management is its impact on soil fertility, moisture holding
capacity and structural characteristic. It is helpful in binding the soil
particles together to form aggregates.
In most sandy soils, it improves the moisture – holding capacity,
enhances soil permeability to water, increases the cation exchange capacity,
buffers the soil against excessive or sudden pH change when soil amendments are
added, enhances the formation of metal-organic matter complexes (Fe, Mn, Cu,
Zn). It is a good source of both micro and macro nutrient (Udoh et al., 2005; FAO, 2006). Globally, the
use of organic fertilizer is being advocated for because of its soil
conservation property and its eco-friendliness over the inorganic fertilizer.
MATERIALS AND METHODS
Experimental
Site
The study was
conducted in 2018 and 2019 cropping seasons at the Research farm of Rubber
Research Institute of Nigeria (RRIN), Iyanomo near Benin City, Edo State, which
lies within the Rain Forest zone of Nigeria. The study area falls between
latitude 6000 and 7000′N
and longitude 5000′ and 6000′E. The rainfall
pattern is bimodal with the peaks in the month of July and September but the
highest in July and a short dry spell in August. The soils of this humid forest
belt are mainly ultisols and the site is classified locally as kulfo series with
pH range between 4.0 and 5.5 (Vine,1945; RRIN 1998).
Experimental design and field layout
The treatments involved a combination of sole
rubber and snake tomato and their intercropped combination with NPK (applied at
60kgNha-1) and rubber effluent application rates (0, 50, 60 and
70kgNha-1) laid out in a randomized complete block design in three
replications. For rubber component in the intercrop, the treatments were:
RE1RS-Rubber Effluent at application rate of
50 Kg N ha-1 cropped with rubber and snake tomato (Intercrop)
RE1SR- Rubber Effluent at application rate of
50 Kg N ha-1 cropped with sole rubber
RE2RS- Rubber Effluent at application rate of
60 Kg N ha-1 cropped with
rubber and Snake tomato (Intercrop)
RE2SR- Rubber Effluent at application rate of
60 Kg N ha-1 cropped with sole rubber
RE3RS- Rubber Effluent at application rate of
70 Kg N ha-1 cropped with rubber and snake tomato (Intercrop)
RE3SR- Rubber Effluent at application rate of
70 Kg N ha-1 cropped with sole rubber
RSC- Rubber and snake tomato intercrop
control
RSNPK- 60 Kg NPK applied to rubber and snake
tomato intercrop
SRC- Sole Rubber Control
SRNPK- 60 Kg NPK applied to sole rubber
Cultural
practices, data collection and Analysis
The snake tomato seeds were raised into
seedlings in a polybag nursery filled with a mixture of top soil and poultry
manure in ratio 3:1 for two weeks.
An experimental field
measuring 26 by 60 m was cleared of the existing vegetation manually with the
aid of cutlasses and hoes, the debris were packed out of the site, thereafter
the field was marked out into plots measuring 3 by 7m with a metre pathway. The
rubber effluent was applied immediately to the designated plots as per
treatment two weeks prior to transplanting of rubber saplings, The pulled
budded stump (young rubber) was placed in the hole in such a way that the
budded patch is just above the ground level at a spacing of 3 by 7 m. The snake
tomato seedlings were transplanted to designated plots at a spacing of 0.5 by
0.5 m, a week after the planting out of the rubber saplings. The NPK fertilizer
was applied to the designated plots as per treatment two weeks after
transplanting of snake tomato seedlings.
Standardization
of rubber effluent
Rubber Effluent sourced from three Rubber
Factories in Edo State (Odia, Okomu, and Osse Rubber Estate) and analyzed to
check for possible variation in nutrient composition from the different
sources, which can also give an idea of possible variation due to the type of
clone. This was done using the standard laboratory standard (results of
effluent analysis is shown in table 2). The Effluent was applied two weeks
prior to transplanting of rubber sapling in order to decompose and equilibrate
in the soil while NPK was applied two weeks after transplanting (WAT).
Soil Analysis
Prior to cropping
with rubber and snake tomato, soil samples were randomly collected from the
experimental site at a depth of 0 - 30 cm depth using auger and bulked together
to form a composite sample. The composite soil sample was air-dried and sieved
through a 2 mm mesh and analyzed for its physical and chemical properties using
standard laboratory procedures. After harvest, soil samples were randomly
collected from each plot separately and analyzed for its post-harvest chemical
properties.
Particle size analysis was determined by
hygrometer method (IITA, 1979), The
soil pH was determined in 1:2 soils to water ratio using glass electrode
digital pH meter, Available Phosphorus was extracted using Bray-1
solution and the phosphate in the extract was assayed calorimetrically by the
molybdenum blue colour method and was determined by a spectrometer as described
by IITA (1979). Exchangeable bases were extracted using 1N neutral
ammonium acetate solution. Calcium and magnesium content of the solution were
determined volumetrically by EDTA titration procedure by Houba et al. (1988). The level of calcium,
potassium, and sodium was determined by flame photometer, the total nitrogen of
the soil was determined by Micro kjeldahl procedure described by IITA
(1979).The exchangeable acidity was determined by the KCL extraction and
titration method of Houba et al. (1988).
Data Analysis
Data collected were analyzed with GENSTAT
programme, using analysis of variance and significant differences among
treatments means were separated using the LSD procedure at 0.05 level of
probability
RESULTS
The soils were strongly acidic and low in
organic C, total N, available P and exchangeable Ca (Table 1). The Ca/Mg ratios
were moderate. The soils were texturally sandy loam. The chemical analysis of
the rubber effluent used for the study showed that it was moderately acidic
with total dissolved solids, chemical oxygen demand and biochemical oxygen
demand (Table 2). It contained total N, available P, organic C, K, Mg, Na and
Ca in appreciable amount. However, the composition of the effluent varies with
sources
Post-harvest
soil chemical properties
The results of the
postharvest soil chemical properties as influenced by NPK and rubber effluent
and intercrop in a rubber-snake tomato intercrop in an existing rubber
plantation are presented in Tables 3a and 3b. The pH values were lowest in
RE2ST, RSC and RE2ST which were identical with RE1RS, RSNPK, STC and STNPK
while RE1ST had the highest pH value in the 3rd year experiment but
identical with RE3RS and RE3ST. In the 4th year experiment, the
highest pH values were recorded in STNPK and RE3RS which were identical with
RE3RS. The lowest pH was recorded in RSC and STC. There were no significant
differences among treatments in respond to pH in the combined analysis.
The highest and
lowest organic C content values were recorded in RSC and RE3RS, respectively in
the 3rd year experiment. Plot with RSNPK had the highest organic C
content while RE1ST had the lowest organic C which was similar with RE1RS, RSC
and STC plots in the 4th year experiment. In the combined analysis,
RSNPK had the highest organic C while RE3RS which was similar with RE3ST and
RE2RS had the lowest organic C content. Organic C was higher in the 3rd
year experiment than in the 4th year experiment.
In the 3rd
year experiment, a significant difference existed among treatments for total N
and ranged from 0.35 g kg-1 for RSC and 0.78 g kg-1 for
plot with RSNPK. RSNPK was identical with all other treatments except RE1RS and
RSC. In the 4th year experiment and in the combined analysis, the
lowest total N content was recorded in RSC plot while the highest content was
recorded in RSNPK which was identical with STNPK. Total N was higher in the residual
4th year experimental plots than in the residual 3rd year
experimental plots.
The available P
was highest in plot with RE3ST both
experiments and in the combined analysis. However, available P recorded in the
plot with RE3ST was comparable with STNPK, RE3RS, RE2ST and RE2RS plots in the
4th year experiment and RE1RS, RE2RS, RE2ST, RE3RS, RSNPK, STC and
STNPK plots in the combined analysis. The lowest available P content in the 3rd
year experiment and in the combined analysis was recorded in RSC plot but was
comparable with RE1ST in the combined analysis. The lowest available P was
recorded RE1ST in the 4th year experiment.
Exchangeable Ca was
highest in RE3ST in both experiments and in the combined analysis. The lowest
exchangeable Ca was recorded in RE2ST, RSC and STC in the 3rd year
experiment. In the 4th year experiment and combined analysis, the
lowest exchangeable Ca was recorded inn RSC and STC. Exchangeable Ca was higher
in the 3rd year experiment than in the 4th year.
The highest
exchangeable Mg was recorded in RE3RS and RE3ST in both experiments and in the
combined analysis. However, in the 3rd year experiment, the
exchangeable Mg recorded in RE3RS and RE3ST was comparable with RE1RS, RE1ST,
RSC, STC and STNPK had the lowest concentration. The lowest exchangeable Mg
concentration in the 4th year experiment and in the combined
analysis was recorded in RSC. However, RSC was identical with STNPK, STC,
RSNPK, RE1ST and RE1RS in the 4th year experiment and in the
combined analysis, RSC and STC were comparable with RE1RS and STNPK. The plots
with RE2RS and RE2ST recorded the lowest exchangeable concentration in the 3rd
in the 3rd year experiment.
In the 3rd
year experiment, the highest exchangeable K content was recorded in RE1ST which
was not significantly different from RE3ST, RSNPK and STNPK. The plot with
RE1RS had the highest exchangeable K which was identical with RSNPK and STNPK
in the 4th year experiment. In the combined analysis, the lowest
exchangeable K was recorded in RE2ST which was not significantly different from
STC and RSC.
The lowest
exchangeable Na content was observed in RE1RS which was comparable with RSC,
RSNPK and STNPK while the highest exchangeable Na content was observed in
RE2ST, RE3RS and RE3ST in the 3rd year experiment. In the 4thh year
experiment, the lowest exchangeable Na was recorded in RE1RS, RSC, RSNPK, STC
and STNPK. The highest exchangeable Na content was recorded in RE3ST and RE3RS
in the 4th year experiment. In the combined analysis, the plot RE3RS
had the highest exchangeable Na which was comparable with RE2ST and RE3ST
plots. The lowest exchangeable Na content increased with increasing rate of
rubber effluent application.
Exchangeable H+
ranged from 0.08 and 0.15 cmol kg-1 for STNPK and RSC, respectively
in the 3rd year experiment. However, RSC was comparable with RE1RS
while, STNPK was identical with RE1ST. In the 4thyear experiment,
the highest exchangeable H+ was recorded in STNPK and RSNPK and was
comparable with RE1RS, RE3RSand RE3ST while the lowest concentration was
recorded in RE1ST. RE1ST plot also had the lowest exchangeable H+
concentration in the combined analysis while the highest exchangeable H+
concentration was recorded in RE1RS which was identical with RE3RS, RSC and
RSNPK. Exchangeable H+ concentration was higher in the 4th year
experiment than in the 3rd year experiment. In the 3rd
year experiment the highest exchangeable Al3+ was recorded in RE1RS
plot which was comparable with the plots with RE2ST, RE3ST and RSC. Exchangeable Al3+was lowest in the
plots with RE2RS and STNPK and was comparable with RE1ST, RE3ST, RSNPK and STC
plots. Exchangeable Al3+ was identical in values in all the
experimental plots except the plot with STC in the 4th year
experiment. All treatments had identical
exchangeable Al3+ values in the combined analysis experiment.
DISCUSSION
Contrary
to expectation that sole crop will exhaust the soil less than the rubber/snake
tomato intercrop but this was not to be. The study indicated that intercropping
rubber and snake tomato had similar effect on soil fertility as solely cropped
snake tomato. This was evidenced as plots where rubber was intercropped with
snake tomato and plots where snake tomato was grown alone had similar soil
nutrient contents.
The soils of the experimental site were
strongly acidic with values lower than critical level for some essential
nutrients (table 1). This implied that the soil has low fertility status.
Law-Ogbomo and Osaigbovo (2008) reported that most Nigerian soils are of low in
native fertility owing to the highly weathered soils coupled with leaching and
continuous cropping. Soil fertility is a very important factor in soil
productivity in relation to nutrient and yield (Erhabor, 2005). Plants need
supply of appropriate proportionate essential nutrients from the soil for
optimum growth, development and yield. Low soil fertility status without
adequate soil nutrient amendment will result in growth and yield depression due
to nutrient deficiencies (Law-Ogbomo et
al., 2020).
The analysis of the rubber effluent showed
variability depending on location. They were moderately acidic and contain N,
P, K and Ca in appreciable quantity. The effluent has high concentration of
organic carbon, COD and BOD at safe level. This finding is in agreement with
Orhue et al. (2007) who reported
highly significant amount of total suspended and dissolved solids, phosphate
and total N in rubber effluent. Orhue and Osaigbovo, (2013) reported that
rubber effluent had great potential as organic fertilizer and could be beneficial
to arable crops without additional cost as effluent are waste product of rubber
processing factories and its disposal has been a major concern to factory
owners. This is an indication that rubber effluent which ought to be waste and
pollutant to the environment can be made to be an avenue for wealth creation
through its conversion to organic fertilizer.
The post-harvest fertility status of the soil
was improved. Law-Ogbomo et al.
(2014) reported an increase in fertility status after fertilizer application
which is a reflection of the availability of essential plant nutrients in NPK
and rubber effluent. The increase in soil pH, N, Ca, Mg, K, and Na and decrease
in exchangeable acidity in plots fertilized with NPK and rubber effluent is
attributed to the amending effects of the fertilizer. This finding implies that
rubber effluent and NPK reduced soil acidity as the reaction changed from
strongly acidic to moderately acidic.
The decrease in exchangeable acidity might
have led to higher soil pH. The increase in soil pH could have led to higher
availability of exchangeable cations. The decrease in organic carbon in both
the fertilized and unfertilized plots is not in conformity with the observation
of Odedina et al. (2003), who
reported that organic fertilizer increased soil organic matter.
The increase in N content in soil of rubber
intercropped with snake tomato treated with NPK (RSNPK) compared to the sole
snake tomato soils treated with NPK (STNPK) is a demonstration of N cycling as
reported by Mbow et al. (2014). The
decrease in available P compared to the initial concentrations could have
resulted from decrease in soil organic carbon. The mineralization of available
P due to microbial actions resulted in the production of organic acid, which
make soil P available (Law-Ogbomo et al.,
2016). The higher exchangeable Ca observed in RE3RS and RE2ST plots implies
higher rate of mineralization of Mg as the fertilized plots contained more
nutrient reserve than the unfertilized plots. The increase in exchangeable cation
implies increase in the soil effective cation exchange capacity (ECEC) brought
about through fertilizer application.
CONCLUSION
·
The
soils of the experimental sites were of low fertility status.
·
Rubber
effluent was variable in chemical composition but contained appreciable amount
of plant essential nutrients.
·
The
soil analysis after the harvest of snake tomato showed that the soils benefited
from the amendment with fertilizers (NPK and rubber effluent).
·
The
soil analysis after harvest showed that soil was improved with the application
of NPK and rubber effluent
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