Introduction
Food insecurity is partially explained by declining soil fertility, higher
chemical fertilizer prices on the global market, and water scarcity
especially in the Sahelian region. Although chemical fertilizers are widely
used worldwide to increase fertility of agricultural soils, their use is
still very low in Sub-Saharan Africa (SSA) with an application rate of
8 kg ha-1 year-1 (Morris et al., 2007). These researchers
reported that low fertilizer use is one of the major factors explaining
lagging growth in agricultural productivity in Africa, specifically SSA,
relative to other regions.
In addition, the shortage of freshwater resources affects agricultural
production in SSA countries. To mitigate this food crisis, closed-loop
sanitation systems provide a way to reduce health risks while also recovering
useful nutrients for sustainable agriculture (Esrey et al., 2001). Thus,
nutrients obtained from urine and faeces recycled as fertilizers can help
relieve poverty by improving household food security and saving money used to
buy chemical fertilizers, which can be put to other productive uses according
to the WHO (2006). Human urine is a liquid fertilizer containing nitrogen
(mostly as urea), phosphates and potassium in dissolved form that is
available for plant use (Kirchmann and Pettersson, 1995). Because of this,
the application of human urine has been gaining popularity as a fertilizer
for agricultural practices (Heinonen-Tanski and van Wijk-Sijbesma, 2005;
Germer et al., 2011; Boh et al., 2013). Nutrients and organic matter found in
human faeces are already widely recognized to improve soil fertility and crop
productivity when recycled as fertilizers (Mnkeni and Austin, 2009; Useni et
al., 2013). Several studies have shown that reused wastewater is a good
management option for increasing water supplies for agriculture
(Brzezińska et al., 2011). Akponikpè et al. (2011) reported that
irrigation with treated wastewater results in 40 % more eggplant yield
compared to irrigation with dam water. Furthermore, van Leeuwen et al. (2015)
confirmed that organic farming can enhance soil biomass.
Despite the growing interest, negative environmental and health risks are
sometimes associated with excreta and greywater reuse in agriculture. Several
studies have focused on the health risks of human urine and composted faeces
with regards to pathogens, pharmaceutical residues, and trace elements in
agricultural (Höglund et al., 2002; Winker et al., 2010; Fidjeland et
al., 2013). Additionally, Mara et al. (2007) showed that pathogens in
greywater may cause diseases through direct contact as well as through the
consumption of contaminated plants. However, health risks associated with the
reuse of these sanitary products will not be investigated in this present
study.
Soil salinization is one of the major causes of declining agricultural
productivity in many arid and semi-arid regions of the world (Qadir et al.,
2001). Qadir et al. (2008) estimated that about 34 million ha in Iran,
including 4.1 million ha of the irrigated land, is salt-affected and annual
economic losses due to salinisation in this country are more than
USD 1 billion. In Australia, salinity, sodicity and acidity are three major
soil constraints that limit crop and pasture yields and costless removal of
these constraints would increase annual profits by AUD 187 million,
AUD 1034.6 million, and AUD 1584.5 million respectively (Hajkowicz and
Young, 2005). This phenomenon is particularly important in arid and semi-arid
regions characterized by low rainfall, high evaporation, and low salt
leaching from the topsoil (Muyen et al., 2011). Thus, the salinity issue is
significant in the SSA region and to the reuse of sanitary products,
particularly urine (Boh et al., 2013) and greywater (Travis et al., 2010),
which are potential salt sources. Generally, fertilizer application increases
the content of soluble salts in the soil, and WHO (2006) has cautioned
against the use of urine fertilizer in saline soils. Concentrations of sodium
(Na+) and chloride (Cl-) salts in urine are often too high, placing
an additional risk if used in salt affected soils. Thus, over-applying of
human urine may cause agricultural soil sodicity (Sene et al., 2013; Boh and
Sauerborn, 2014) and increase soil electrical conductivity (Mnkeni et al.,
2008; Hijikata et al., 2013), which inhibit plant growth. Moreover, greywater
is often a source of elevated levels of compounds such as surfactants and
salt, which can alter soil properties (Travis et al., 2010). Hijikata et
al. (2013) showed that Na+ and major cations in pilot gardens mainly
derived from greywater following combined application with human urine.
Several studies have demonstrated the effects of salt accumulation in soil
and impacts on vegetables growth and yields. Dasgan et al. (2002) observed
that plant growth is affected by the osmotic and ion specific effects and by
ionic imbalance when soil accumulates Na+. Moreover, Na+ inhibits
plant growth by disrupting the water uptake in the roots, dispersing soil
particles, restricting root growth, and reducing nutrient availability by
competition with major ions (i.e. Na+ and Cl-) in the substrate
(Franzen, 2007). Boh et al. (2013) concluded that a reduction of dry-matter
in the urine compared to ammonium nitrate-fertilized plants under saline
conditions could be associated with toxicities in plants caused by high
concentrations of Na+.
To alleviate this problem, Ganjegunte et al. (2008) demonstrated that
successful wastewater reuse requires selection of salt tolerant crops,
appropriate irrigation systems, soil suitability, and salinity management
strategies. Additionally, to reduce salt accumulation in soil, compost
supplemented with human urine is recommended by Shrestha et al. (2013). Oo et
al. (2013) reported that compost and vermicompost are effective in
alleviating soil salinity and improving maize productivity in Thailand. The
researchers explained that the soil salinity was reduced because the compost
and vermicompost increased exchangeable potassium (K+), calcium
(Ca2+), and magnesium (Mg2+) while decreasing exchangeable Na+
This suggests that Ca2+ was exchanged for Na+, and exchangeable
Na+ thus leached out of the soil.
To date, there is very limited information on soil salinity and sodicity from
greywater in conjunction with human urine/compost reuse on agriculture under
Sahelian conditions. Hence, the objective of this study is to evaluate the
effects of sanitary products on vegetable productivity and soil chemical
quality. Specific objectives are: (1) assessment of urine, toilet composts
and greywater combined effect on okra productivity and (2) their effect on
soil salinity during the harvest period.
Material and methods
Research site
The experiment was implemented at the International Institute for Water and
Environmental Engineering (2iE) at Kamboinsin (12∘27′40.6′′ N and
01∘32′56.0′′ W), Ouagadougou, Burkina Faso. The climate is
semi-arid and characterized by a 25–30 ∘C mean monthly temperature,
an annual average rainfall of 773 mm, and a reference evapotranspiration is
1900 mm year-1 (Ouédraogo et al., 2007). The study was performed
during the dry season (March to June 2014).
Soils are of a tropical ferruginous type and the surface layer (0–15 cm) is
sandy loam (58 % sand, 23 % silt and 19 % clay) while the
sub-layer (15–30 cm) is sandy loam clay (48 % sand, 29 % silt and
23 % clay).
Experimental setup
The experiment was arranged in a completely randomized complete block design
with six treatments replicated three times. The treatments were based on the
nitrogen (N) requirement of okra plant which is 100 kg ha-1 (Grubben
et al., 2004). Sene et al. (2013) suggested that the application of human
urine volume based on the plant N requirement is a good method for urine
reuse. The treatments were as follows : (1) fresh dam water (FDW) as a
negative control, (2) FDW plus chemical fertilizer (i.e. NPK) (FDW + NPK)
as a positive control, (3) treated greywater (TGW), (4) FDW plus Urine/Toilet
Compost (UTC) (FDW + UTC), (5) TGW + UTC, (6) TGW + NPK. Thus,
there were 18 plots of 1.50 m2 each with an interval of 0.5 m.
Chemical fertilizer, NPK 14:23:14, was applied at the rate of 23 grams per
plot (g plot-1); equivalent to 0.15 tons per hectare (t ha-1)
through ring method before seeding. Proportions of 75 % nitrogen from the
urine and 25 % nitrogen from the human faece-based toilet compost were
applied according to Sangare et al. (2014).
Urine was applied two weeks after sowing at the rate of 1.5 liter per plot
(L plot-1) which was equivalent to 9615 liter per hectare
(L ha-1). A second application was supplied three weeks after the first
application and a third three weeks after the second application. Toilet
compost was supplied once before sowing with 70 g plot-1, which is
equivalent to 4.45 t ha-1. Seven days after the seedlings emerged,
they were reduced to two plants per hole which is equivalent to twelve plants
per plot. During the experiment, the daily irrigation quantity was modified
during the cultivation period. The total volume of 2.5 L plot-1 during
the 15 days after sowing (DAS) was increased to 10 L plot-1 until
harvest. The total irrigation water volume and the total applied amount of
Total Nitrogen (TN), sodium (Na+) and chloride (Cl-) derived from
the irrigation water and human excreta during the experiment are shown in
Table 1. The plants were watered twice per day following the watering-can
practices of local gardeners. Plots were manually weeded three (3) times
during the growing period and periodically treated with insecticides
(K-Lambda and Pacha) to fight insects, nematodes, fungi, and pests.
Vegetable growth and yield
The experiments were carried out using okra (Abelmoschus esculentus
(L.) Moench). Okra is a well-known tropical vegetable in the sub-Saharan area
whose fruits can be harvested and dried for off-season consumption (Nana et
al., 2009). The vegetative growth and yields were recorded by plant height,
fresh fruit production, and aboveground biomass. The leaves, stems, and
fruits were weighed for the wet aboveground biomass and were oven dried for
24 h at 105 ∘C to obtain the total dry biomass.
Irrigation water collection and sample analyses
Treated greywater was collected from High Rate Algal Ponds (HRAPs) at 2iE
(treatment capacity: 21 m3 day-1). The HRAPs were discussed
previously at the lab scale by Derabe (2014). Raw greywater came from the
student residence hall of 2iE and was treated with primary purification. The
raw greywater was maintained in rotation (88 turns min-1) by an
electromechanical mixer during seven and a half (7.5) days inside the algal
pond. To remove the excess sludge a secondary clarifier received the effluent
of HRAPs. The effluent from this clarifier was then stored in a pond before
being pumped to a small tower. Finally, treated greywater was collected to
supply an intermediate tank from which the plants were irrigated.
Fresh dam water (FDW) was collected from the small dam located in Kamboinsin,
18 km from Ouagadougou, which is used for gardening by the population during
the dry season. For this study, FDW was collected from the tank installed in
the field experimental site.
FDW and TGW samples were collected every three weeks and analyzed for the
following parameters: pH and electrical conductivity (EC) by the
Electrometric method, calcium (Ca2+) and magnesium (Mg2+) by the
EDTA titrimetric method, sodium (Na+) and potassium (K+) by the
Flame photometric method. The sodium adsorption ratio (SAR) describes the
amount of excess Na+ relative to Ca2+ and Mg2+ cations. SAR is
a measure which indicates the sodicity of water and predicts possible adverse
effects of Na+ in soils. It has been estimated using the following
Eq. (1):
SAR=[Na+][Mg2+]×[Ca2+]2
Na+, Ca2+ and Mg2+ are expressed in milli-equivalents per
liter (meq L-1).
Total nitrogen (TN) was measured with the direct Hach method. Total
phosphorus (TP) was determined using the ascorbic acid method with
persulfate digestion. Anionic surfactants were measured by methylene blue
method.
Microbiological analysis of wastewater irrigation samples targeted relevant
hygiene indicator bacteria, such as Escherichia coli (E. coli) and faecal coliforms. Spread plate method was used after an
appropriate dilution of the samples in accordance with the procedure in
Standard Methods for the Examination of Water and Wastewater (APHA, 1998).
The samples were diluted with sterile Ringer. After dilution, 0.1 mL of the
diluted sample was spread out over the media (Chromo cult Agar), contained in
Petri dishes which were placed in the drying oven for incubation at
44.5 ∘C for 24 h. E coli were identified by their blue or
purple color and faecal coliforms were identified by their red-pink color.
The concentration was expressed by forming colony unit (CFU) reported
relative 100 mL of sample. The bacteria load was expressed by Eq. (2)
according to Rodier (2009):
N=nV×d×Vs,
where N = Bacteria load (CFU/100 mL); n = Number of colonies in
Petri dishes; Vs = Reference volume (100 mL);
V = Volume of test (1 mL); d = dilution factor.
Human excreta collection and sample analyses
Human urine and toilet compost were collected in a urine diverting toilet
implemented with a pilot family in the peri-urban area of Ouagadougou,
Burkina Faso. The compost was collected from the composting toilet designed
to avoid accumulation of moisture in the composting reactor. The composting
toilet was a continuous feed system with constant reaction conditions in
terms of temperature and moisture contents of the matrix. Unlike traditional
composting systems, biodegradation rates of organic matter are very important
because faeces were daily added into the composting reactor of the bio-toilet
and thus accelerated decomposition (Lopez et al., 2004). The reactor can be
removed and carried to the appropriate area, such as farmland, and just be
turned with opening top hatch. In this study, toilet compost was taken from
pilot families reactors after 3 months and stocked at the research site where
daily temperatures ranged from 35–48 ∘C during the study period.
Composting was used to balance the relation of carbon and nitrogen and if
there was excess nitrogen the amount of compost was reduced (Heinonen-Tanski
et al., 2005). Before reuse, the toilet compost was spread in the sun for a
week to completely inactivate bacteria and pathogens that were not destroyed
during composting.
Human urine was also collected from the same composting toilet, stored in a
plastic drum, and exposed to sunlight in bottles (1.5 L) before reuse as
fertilizer.
Undiluted urine sample was used for pH and EC measurements by WTW 350i
multi-parameters before reuse. For compost, pH and EC were determined from a
ratio 1 : 10 (compost: deionized water) using WTW 350i multi-parameters.
Cation and anion determination was performed with an urine sample diluted at
ratio 1 : 250 (urine: deionized water) and with a compost sample diluted at
ratio 1 : 10 (compost: deionized water). The cation and anion
concentrations were determined following the same procedure used for the
irrigation water sample.
Regarding microbiological analysis, feacal coliforms in urine were determined
following the same procedure used for the irrigation water sample. Compost
samples of 25 g were homogenized in 225 mL of buffer (phosphate) and a 10
fold dilution series was performed in maximum recovery diluents (Ringer
solution). Feacal coliforms were cultured following the method 9215 A in
APHA (1998). Relevant dilutions were spread on plates in duplicate on the
media chromo cult coliform agar ES (Difco, France) and were incubated at
44.5 ∘C for 24 h. The bacteria load was expressed according to
Eq. (3):
N=log10nPV×Vt×d×DW,
where N = Bacterial load in compost; n = Number of colonies in
Petri dishes; P = Weight of compost or soil samples (25 g);
V = Volume of Buffer phosphate; Vt = Volume of test
(1 mL); d = factor of dilution; DW = Dry Weight.
Soil physico-chemical measurements
The samples were taken at depths of 0–15 and 15–30 cm, air dried, and
ground and sifted through a 2 mm mesh sieve in the laboratory. Soil pH and
EC were determined in a 1 : 2.5 and 1 : 5 soil/water ratio respectively.
Particle size analysis was performed by sieving and the sedimentometry
method. Total organic carbon (TOC) in the soil samples was determined
according to the modified Walkley and Black (1934) method.
For cation elements, soil was saturated with pure water (1 : 10) and the
soil suspension was centrifuged at 3000 rpm for 10 min and the supernatant
filtered with 0.45 mm glass microfiber filters (Whatman). Ionic
concentrations (Na+, Mg2+ and Ca2+) were determined by the
same methods described above. The results were expressed in meq kg-1
and were used in Eq. (1) to determine the SAR value. These parameters were
tested to determine the effects they may have on plant growth.
Statistical analyses
The data collected for different parameters were analyzed for variance. The
means of the parameters for all treatments were calculated. The significance
of the difference between all treatment means was evaluated by the Tukey
multiple comparisons of means test at 95 % confidence intervals
(p < 0.05). Statistical analysis of these results was carried out using
R for windows version 2.15.1.
Results and discussion
Physical-chemical and microbiological characteristics of<?xmltex \hack{\newline}?> the
irrigation water
The average physical-chemical and microbiological parameters measured during
this study for both sources of irrigation water are shown in Table 3. Overall
values of the analyzed parameters were higher for TGW than for FDW. The
physical characteristics (pH and EC) of both water sources were in agreement
with the recommendations of the FAO (Pescod, 1992). In terms of nutrient
contents, high nitrogen, phosphorus, and potassium values were observed in
TGW compared to FDW. These two major nutrients in TGW irrigation can promote
plant growth (Travis et al., 2010). Additionally, the sedimentable HRAPs
treatment can reduce the anionic surfactant (Derabe, 2014). Anionic
surfactant in FDW was not detected.
The microbiological quality of TGW showed higher levels of coliforms than
that of FDW. The faecal coliforms count in TGW and FDW averaged
4.05 × 103 and 2.05 × 103 CFU 100 mL-1
respectively, and did not meet current standards which are < 103 CFU
100 mL-1 for unrestricted irrigation, according to WHO (2006).
E. coli was not detected in any of the irrigation water samples.
Toscano et al. (2013) who reported that the UV rays significantly improved
the quality of wastewater in terms of inactivation of pathogens such as total
coliforms, E. coli, salmonella and enterococci.
Physical-chemical and microbiological characteristics of human
excreta
The physical and chemical properties of human urine and toilet compost are
summarized in Table 2. The urine sample had the highest electrical
conductivity (EC), 2.5 times more than that of toilet compost. Mnkeni et
al. (2008) showed that excessive urine application inhibited plant growth due
to increasing soil EC. Human urine characterized by a higher SAR value (119),
could be sources of soil salinity and/or sodicity. Total N in the urine
sample was within the range of previous studies such as Kirchmann and
Pettersson (1995).
The bacteria load results showed that, urine and compost can be used for
cooking vegetables like Okra, cereal crops, industrial crops, fodder crops,
pasture and trees.
Plant height
The results showing the effect of bio fertilizers and both irrigation waters
on okra growth are given in Fig. 1. Overall, the greatest height enhancement
was obtained in plot irrigated with TGW compared to plots irrigated with FDW
during all the cultivation days.
The plant height measurement for all treatments was not statistically
different among them until after 56 cultivation days. This study showed that
plants irrigated with TGW (31.63 ± 2.41 cm) were not significantly
higher compared to plants treated with TGW + UTC (30.33 ± 8.78 cm)
during the cultivation days. However, the plant heights in plots treated with
TGW + UTC (30.33 ± 8.78 cm) were significantly higher than those
treated with FDW + NPK (19.02 ± 2.25 cm) at 69 cultivation days.
These results corroborate other studies which demonstrated that the use of
wastewater and/or treated greywater can encourage good plant growth, mainly
because of the significant amount of nutrients (Singh et al., 2012). The
total N amount applied from TGW was 90 % more than those from FDW during
the cultivation growth.
Compared to plots irrigated with TGW, the lowest plant heights were reported
for plots irrigated with FDW. Plant growth in these plots was not
significantly affected when supplied with human urine and toilet compost.
Vegetable yields
The different components of okra yield obtained with treatments are
summarized in Table 4. Similar to plant height, the number and yield of okra
fresh fruit was significantly (p < 0.05) increased in plots irrigated
by TGW compared to plots irrigated by FDW. Indeed, irrigation with TGW alone
improved significantly (p < 0.05) the number of okra fruit
(9.50 ± 2.71) compared to irrigation with FDW alone
(4.90 ± 2.25). Additionally, a significantly higher number of fruits
were obtained with plants treated with TGW + UTC (11.33 ± 2.57)
compared to FDW + NPK (4.25 ± 2.86). However, no significant
differences were observed between fruit number obtained from plant irrigated
with TGW (9.50 ± 2.71) and TGW + UTC (11.33 ± 2.57). This
result showed that TN derived from TGW and TGW + UTC is sufficient for okra
plant growth.
Regarding okra fruit yields, the effect of TGW
(0.71 ± 0.33 t ha-1) was significantly higher (p < 0.05)
compared to FDW (0.23 ± 0.16 t ha-1), which is in accordance
with previous studies (Akponikpè et al., 2011). This improved yield could
be ascribed to water and nutrient supplies, but it most likely occurs because
nutrients are provided and released continuously by TGW.
In addition, the fruit yields using TGW + UTC
(0.67 ± 0.32 t ha-1) were significantly higher (p < 0.05)
than those obtained using FDW + NPK (0.22 ± 0.17 t ha-1). A
possible reason for this result might be that the urine and toilet compost
(UTC) were richer in nutrients which are required for plant production. The
nutrients and the variety of inorganic and organic compounds in human urine
promote crop growth (Kirchmann and Pettersson, 1995). At the same time, the
addition of organic amendments (i.e. compost) improves soil structure,
aggregate stability, and moisture retention capacity (Bhattacharyya et al.,
2008). Higher fruit production in the plots with UTC could be explained by
enhanced organic carbon content and microbial activity in the soil (Nakhro
and Dkhar, 2010). Pradhan et al. (2009) showed also that compost or ash
reduced emissions from nitrogen fertilizer and maintained soil potential.
This agrees with the observations of Shrestha et al. (2013), who showed an
improved yield, for sweet pepper when UTC was used. In general, the main
advantages of toilet compost are that it releases nutrients gradually, raises
soil organic matter contents, and minimizes N evaporation.
However, the lower fruit yields of plants treated with FDW + NPK may be
associated with larger gaseous nitrogen losses from NPK and poor nutrients
contained in FDW. Indeed, Mermoud et al. (2005) reported the high evaporation
which leads to high volatilization of nitrogen in Kamboinsin area. Besides,
chemical fertilizer can remain undissolved lying in the upper layer of dry
soil during the dry season, if there is insufficient irrigation. Furthermore,
commonly used chemical fertilizers include fertilizers containing a single
nutrient which is usually nitrogen (N), phosphorus (P), and potassium (K) and
no other nutrients such as Ca, Mg, Zn, Fe, B important for vegetable growth.
In regards to only fertilizer sources, okra yields of the plots treated with
UTC were not significantly higher (p < 0.05) than those fertilized with
NPK with the same irrigation water sources. In fact, okra fruit production
obtained with TGW + UTC (0.67 ± 0.32 t ha-1) was not
significantly higher compared to those with TGW + NPK
(0.61 ± 0.42 t ha-1). For FDW, okra fruit yield was not
significantly different (p < 0.05) between FDW + UTC
(0.47 ± 0.18 t ha-1) and FDW + NPK
(0.22 ± 0.17 t ha-1). These results confirmed that the nutrients
supplied, especially TN from TGW are more important than FDW for plant
production, as shown in Table 1. Unfortunately, excess N derived from TGW and
especially urine leads to over-fertilization of plants. This excess N
application affects nitrogen concentration in plant shoots and roots (Mnkeni
et al., 2008; Sene et al., 2013).
As opposed to fruit productions, the dry aboveground biomass yields were not
significantly different (p < 0.05) between all the treatments applied
during this study.
Physical characteristics of soils
The soil pH values at different depths (0–15 and 15–30 cm) before and at
the end of the cultivation periods are indicated in Fig. 2. Overall the soil
pH in all treatments increased at the end of cultivation compared to that
before cultivation at both depths (0–15 and 15–30 cm). However, there was
no significant difference (p < 0.05) between pH values of the soil
(5.42 ± 0.23) before and after the cultivation period apart from pH of
soil treated with TGW (7.07 ± 0.80), TGW + UTC (7.74 ± 0.11)
and FDW + UTC (7.04 ± 0.88) at the depth of 0–15 cm. This study
thus shows that the application of TGW and UTC could lead to an increase in
soil alkalinity over time. Similar observations were found by Mnkeni and
Austin (2009) who showed that pH of human manure is alkaline and will have a
liming effect on acidic soil (i.e. increasing soil pH). Indeed, soil pH
increase may be due to the mineralization of carbon and the subsequent
production of OH- ions from compost application (Mkhabela and Warman,
2005).
At the depth of 15–30 cm, no significant difference (p < 0.05) was
observed between soil pH values before and after the cultivation period
(Fig. 2).
Salinity and sodium hazards of cultivated soil
The soil salinity, measured through EC values in all treatments after
irrigation compared to this initial soil at both depths (0–15 and
15–30 cm), is presented at Fig. 3a. The general results showed that soil EC
values at the depth of 0–15 cm were higher compared to those at the depth
of 15–30 cm. This result coincides with the study of Ben-Hur (2005) who has
shown that the salts are located mainly at the depth of 0–20 cm in sandy
soil.
The highest significant EC value (p < 0.05) was obtained with
TGW + UTC (447.2 ± 278.4 µS cm-1) compared to FDW
(211.5 ± 21.0 µS cm-1), FDW + NPK
(222.3 ± 53.5 µS cm-1), and initial soil
(96.1 ± 3.5 µS cm-1) at the depth of 0–15 cm. These
results agree with Hijikata et al. (2013) who indicated that the combined
application of urine and greywater significantly elevates soil EC values.
Additionally, greywater irrigation also increases soil EC values
(Al-Hamaiedeh and Bino, 2010; Faisal Anwar, 2011). In fact, Faisal
Anwar (2011) reported that the salinity level for different plots irrigated
with greywater is between 707–789 µS cm-1. Similar to the
0–15 cm depth, at the depth of 15–30 cm, soil EC values were
significantly higher in plots treated with TGW + UTC
(350.1 ± 92.5 µS cm-1) compared to those plots
fertilized with FDW + NPK (174.9 ± 9.2 µS cm-1) and
compared to the soil before cultivation
(92.4 ± 7.0 µS cm-1) (Fig. 3a). However, no significant
difference was observed between plots treated with TGW
(147.8 ± 18.8 µS cm-1) and FDW
(138.2 ± 11.2 µS cm-1).
The sodium in the soil was indicated as sodium adsorption ratios (SAR) at the
depths of 0–15 and 15–30 cm before and after cultivation days, as
presented in Fig. 3b. Overall SAR values of soil in all treatments increased
after cultivation compared to the soil before cultivation at both depths
(0–15 and 15–30 cm). One possible reason might be a fast concentration the
mineral constituents brought by the irrigation water (Sebastian et al., 2009)
and by human urine occurs as a result of a high evapotranspiration rate in
the study area (Mermoud et al., 2005).
At the depth of 0–15 cm, the SAR value of soils treated with TGW + UTC
(10.55 ± 1.85) was significantly higher than those treated with
FDW + NPK (2.71 ± 0.67) and soil before cultivation
(0.95 ± 0.21). Thus, this study shows that all plots irrigated with TGW
resulted in higher SAR values compared with those irrigated with FDW excepted
the plot fertilized by FDW + UTC. A possible reason could be the high Na
applied through TGW and the urine volume application in our experimental
conditions. Apart from nitrogen, urine contains dissolved salts of Na+
and Cl- which explain the increase in substrate salinity at a higher
application dosage (Boh et al., 2013). The sodium effects derived from
greywater have been shown by several studies (Al-Hamaiedeh and Bino, 2010;
Travis et al., 2010). Travis et al. (2010) reported the highest SAR in sandy
soil irrigated with raw greywater. On the other hand, Hulugalle et al. (2006)
reported that irrigation with treated sewage effluent increases the
exchangeable Na, and the exchangeable Ca and K in the clay-textured surface.
This could be due to high quantity of sodium salts typically found in laundry
detergents included in greywater (Christova-Boal et al., 1996). Additionally,
plots amended with UTC resulted in higher SAR values which are likely due to
the largely Na+ content in the urine. Sene et al. (2013) showed that
continuous urine application increases the SAR value of the fertilized soil.
Problems with salt accumulation in soil can be worse in dry climates, where
increased water needs in combination with high evapotranspiration rates are
common. An accumulation of salts can result in a decrease in the soil's
capability to absorb and hold water (Mungai, 2008).
The lowest SAR values in plots treated with FDW (5.61 ± 1.45) and
FDW + NPK (2.71 ± 0.67) are likely explained by the low Na+
concentrations in FDW.
At the depth of 15–30 cm, SAR values were significantly higher in plots
treated with TGW and fertilized with UTC compared those treated with FDW and
FDW + NPK (Fig. 3b).
Moreover, SAR values of soil at 0–15 cm depths were significantly higher
(p < 0.05) compared to those at 15–30 cm depths. Generally, surface
horizons are often more susceptible to increased Na+ concentrations
(Johnston et al., 2013). Qureshi and Qadir (1992) reported that Na+
replaced from cation exchange sites stays suspended or moves little to the
lower depths. To mitigate this problem, Ahmad et al. (2013) reported that
crop rotation improves leaching of sodium (Na+) and other ions.
Additionally, Johnston et al. (2013) showed that soil salinity decreases
following the addition of gypsum plus sulfur to saline-sodic (i.e.
SAR > 40) irrigation water. Nevertheless, these researchers advised that
the application of the required dose of sulfuric acid with pre-sowing
irrigations is better for the reclamation of saline-sodic soils to avoid
plant phytotoxicity.
Total organic carbon in cultivated soil
Figure 4 shows the effect of different treatments on total organic carbon
(TOC) in soil before and after the cultivation periods at the depth of
0–15 cm. There were no significant differences (p < 0.05) in the TOC
values between treatments receiving TGW and UTC. However, TOC of plots
treated with TGW + UTC (6.09 ± 0.99 g kg-1) was
significantly higher compared to those of FDW + NPK
(4.46 ± 0.22 g kg-1) and soil before cultivation
(4.00 ± 0.20 g kg-1) at the depth of 0–15 cm. On the other
hand, no significant difference was observed in soils treated with TGW +
UTC (6.09 ± 0.99 g kg-1) and TGW
(5.79 ± 0.66 g kg-1). Generally, the application of organic
amendments increases carbon pools which are significantly correlated with
soil organic carbon (Srinivasarao et al., 2014). As such, the application of
compost can increase TOC in soils as shown by Rivero et al. (2004). That
study reported the positive influence of compost on soil organic matter
quality under tropical conditions. Furthermore, Jaiarree et al. (2014) showed
that soil carbon storage increased significantly after compost application to
tropical sandy soil. Similarly, Oo et al. (2013) showed that compost and
vermicompost amendments improved cation exchange capacity, soil organic
carbon, total nitrogen, and extractable phosphorus in saline soil.