**3. Result and discussion**

#### **3.1. Pure Kala compost**

Pure Kala compost (saturated extract sample) was analyzed for physical, chemical, and biological properties. From **Table 1**, it was found that all measured parameters were within the acceptable level of the international standards and the compost can be applied to improve soil fertility. Actually, Kala compost is a mixture of different municipal wastes such as treated sewage sludge, plant materials, and cow manure. Therefore, it is expected to have low concentration of heavy metals and good values of different nutrients that can support plant growth. It was reported by the Ministry of Agriculture, Forestry, and Fisheries (MAFF) [9] that different compost will behave differently in the soil based on the processes used to generate waste materials.


\*EC: Electrical conductivity; N: nitrogen; OM: organic matter; TOC: total organic carbon; IC: inorganic carbon; TC: total carbon; FC: fecal coliform bacteria.

**Table 1.** Chemical analysis for pure Kala compost.

#### **3.2. Soil samples**

Kala compost was a good source for organic matter and organic carbon that can support soil physical parameters. Organic matter could be the main reason in improving water-holding capacity of the soil amended by Kala compost (**Figure 1**). Moreover, it added more nitrogen to the soil and improved plant chlorophyll content (**Figure 2**). In addition, Kala compost reduced soil-compaction problem by improving soil bulk density where Kala compost gave 1.53 g/cm3 and chemical fertilizer gave 1.72 g/cm3 . The good result for bulk density under Kala compost is supporting Kala application in which organic fertilizer can improve soil aggregate stability, soil structure, and support root growth.

Recent studies indicate that compost of biosolids in combination with woodchips or sawdust is used to grow horticulture crops under field or pots condition. It helps in improving soil Treated Municipal Wastes: Are they Contaminating or Enriching the Soil? http://dx.doi.org/10.5772/64962 47

**Figure 1.** The effect of organic matter (OM) on soil organic carbon and water holding-capacity (WHC) at different treatments.

**Figure 2.** Soil nitrogen and plant chlorophyll values as affected by different treatments.

physical properties such as lowering bulk density, increasing water-holding capacity, increasing total soil porosity, and aggregate stability [10]. According to Wang et al. [11], sludge is shown to be efficient fertilizers as it improves soil physical properties such bulk density, porosity, aggregate stability and water retention and movement. Other properties also can be improved such as pH and contents of organic matter and nutrient contents as the raw sludge is rich in nutrients such as nitrogen, phosphorus, organic matter, and essential trace elements. A study showed clearly that water retention capacity was increased when 0.5% sewage sludge was added to soil. In fact, that increase was higher for raw sludge-amended soil than deposited sludge-amended soil [12].

#### **3.3. Soil salinity**

growth. It was reported by the Ministry of Agriculture, Forestry, and Fisheries (MAFF) [9] that different compost will behave differently in the soil based on the processes used to generate

Kala 1 6.7 23.8 3.15 38.68 22.49 0.011 22.50 0

Kala 2 6.7 24.8 3.20 36.67 21.32 0.89 22.21 0

Kala 3 6.7 24.3 3.18 37.10 21.57 0.84 22.41 0

Kala 1 0.107 <0.001 0.160 0.590 0.068 0.451 2.506 0.127

Kala 2 0.082 <0.001 0.116 0.545 0.063 0.320 1.608 0.105

Kala 3 0.065 <0.001 <0.0004 0.394 0.001 <0.001 0.327 0.098

Kala 1 0.037 15.570 0.053 0.060 0.154 2.991 0.013 0.010

Kala 2 0.039 14.635 0.042 0.068 0.182 3.077 0.009 0.009

Kala 3 0.039 14.185 0.035 0.064 0.128 3.017 <0.001 0.008

**Mn Cd Cu Fe Zn B P Al**

**Ba Ca Cr Co Pb Mg Ni Ti**

\*EC: Electrical conductivity; N: nitrogen; OM: organic matter; TOC: total organic carbon; IC: inorganic carbon; TC: total

Kala compost was a good source for organic matter and organic carbon that can support soil physical parameters. Organic matter could be the main reason in improving water-holding capacity of the soil amended by Kala compost (**Figure 1**). Moreover, it added more nitrogen to the soil and improved plant chlorophyll content (**Figure 2**). In addition, Kala compost reduced soil-compaction problem by improving soil bulk density where Kala compost gave

compost is supporting Kala application in which organic fertilizer can improve soil aggregate

Recent studies indicate that compost of biosolids in combination with woodchips or sawdust is used to grow horticulture crops under field or pots condition. It helps in improving soil

. The good result for bulk density under Kala

**EC (dS/m) N (%) OM (%) TOC (%) IC (%) TC (%) FC (FC media)**

waste materials.

**Samples pH \***

**Samples Elements concentration (mg/l)**

46 Soil Contamination - Current Consequences and Further Solutions

**Samples Elements concentration (mg/l)**

carbon; FC: fecal coliform bacteria.

**3.2. Soil samples**

1.53 g/cm3

**Table 1.** Chemical analysis for pure Kala compost.

and chemical fertilizer gave 1.72 g/cm3

stability, soil structure, and support root growth.

Soil salinity is a good indicator for soil fertility and salt toxicity. Using saturated paste extract method, it can be seen that chemical fertilizer (NPK) gave the highest value (3 dS/m) compared to other treatments (**Figure 3**). Whereas, Kala treatment gave reasonable value that was accepted by many crops. In all cases, salts could be added or diluted or leached down when the land is irrigated by good-quality water. Soil pH for all treatments was around 8. It was slightly affected by compost application.

**Figure 3.** Soil electrical conductivity (ECe) and pH at the beginning of the study.

At the end of the study (**Figure 4**), the salinity value was almost similar to the result found in **Figure 3**. The main difference was that more salts or nutrients were released from Kala fertilizer. All salts found in each treatment were moved up or down the profile depending on air temperature for evaporation or amount of water added as irrigation or rainfall (**Figure 5**). Generally, Kala compost held less salts compared to NPK treatments, but at the same time all those salts were used to support plant growth and released slowly so they can be used as a source of nutrients without any problem of toxicity.

**Figure 4.** Soil electrical conductivity (ECe) and pH at the end of the study.

**Figure 5.** Salt distribution (ECe) along the horizons.

the land is irrigated by good-quality water. Soil pH for all treatments was around 8. It was

At the end of the study (**Figure 4**), the salinity value was almost similar to the result found in **Figure 3**. The main difference was that more salts or nutrients were released from Kala fertilizer. All salts found in each treatment were moved up or down the profile depending on air temperature for evaporation or amount of water added as irrigation or rainfall (**Figure 5**). Generally, Kala compost held less salts compared to NPK treatments, but at the same time all those salts were used to support plant growth and released slowly so they can be used as a

slightly affected by compost application.

48 Soil Contamination - Current Consequences and Further Solutions

**Figure 3.** Soil electrical conductivity (ECe) and pH at the beginning of the study.

source of nutrients without any problem of toxicity.

**Figure 4.** Soil electrical conductivity (ECe) and pH at the end of the study.

Wet sensor is a good device for monitoring soil water content, temperature, and salinity. From **Table 2**, it can be seen that wet sensor confirms what was found in previous figures. Kala compost was maintaining much water that helped in reducing soil temperature with slow release of salts with time.

The application of organic amendment such as sewage sludge compost to agricultural field usually improves soil physiochemical properties through increasing the content of organic matter, the total nitrogen content, and the electrical conductivity, whereas it causes reduction in pH slightly [13]. Electrical conductivity could increase with sewage sludge compost application [14] as a result of acidification in combination with subsequent solubility of metallic elements.


**Table 2.** Wet sensor readings for soil water salinity, moisture content (% vol), and temperature with time.

**Figure 6.** Average yield of cucumber and tomato.

#### **3.4. Plant samples**

From **Figures 6**–**8**, it can be seen that the best productivity of all tested crops was mostly with Kala compost followed by mix treatment and finally by NPK fertilizer. It does not mean that NPK treatment was bad but may be some plants did not get the right amount of fertilizer in the right time. The organic or mix of both organic and inorganic fertilizers usually is the best for consumer and surrounding environment. It seems that Kala compost was creating a good environment for plant by releasing multinutrients, reducing evaporation and keeping much water in the root zone compared to NPK treatment.

Good results were also found in Nielson et al. [15] study when the municipal biosolids were added to cultivate carrots and chard on irrigated soils. A significant increase in yield was found in plants growing biosolid-amended soil as compared to those grown in non-amended soil. In

**Figure 7.** Average yield of lettuce and cabbage.

**Figure 6.** Average yield of cucumber and tomato.

50 Soil Contamination - Current Consequences and Further Solutions

water in the root zone compared to NPK treatment.

From **Figures 6**–**8**, it can be seen that the best productivity of all tested crops was mostly with Kala compost followed by mix treatment and finally by NPK fertilizer. It does not mean that NPK treatment was bad but may be some plants did not get the right amount of fertilizer in the right time. The organic or mix of both organic and inorganic fertilizers usually is the best for consumer and surrounding environment. It seems that Kala compost was creating a good environment for plant by releasing multinutrients, reducing evaporation and keeping much

Good results were also found in Nielson et al. [15] study when the municipal biosolids were added to cultivate carrots and chard on irrigated soils. A significant increase in yield was found in plants growing biosolid-amended soil as compared to those grown in non-amended soil. In

**3.4. Plant samples**

addition, a similar study with cotton (*Gossypium hirsutum*) also showed advancement of flowering and fruiting by 2–3 weeks under sludge-amended soil as compared to fertilizeramended ones [16]. The grain yield of barley increased significantly under repeated sewage sludge application. The leaf protein concentration and dry matter accumulation in the plants grown in sludge-amended soil was higher from the beginning of development to ear emergence [17]. Moreover, it was found that the sludge amendment at the rate of 0.80, 160, and 320 t/ha dry wt. in soil increased the average dry weight of sunflower plants (*Helianthus annus* L.) [18]. Even in saline soil, Verlinden and McDonald [19] showed that compost amendment increased *Limonium sinuatum* and *Celosia argentea* yield. By supplying nutrients, particularly N and P, compost can improve the mineral-nutrient status and growth of plants in saline soils.

Finally, the faster development and greater biomass production in plants grown in sludgeamended soil may be responsible for an early reproductive cycle. Moreover, the complex organic and the inorganic compounds of sewage were broken down into simpler forms, and thus the final treated sludge became useful and beneficial to the seedling growth [20].

**Figure 8.** Average yield of carrot and potato.

#### **3.5. Metal concentrations in soil samples**

#### *3.5.1. At the beginning of the study*

From **Table 3**, it can be seen that all major cations were found in good amount for all treatments, whereas minor cations and heavy metals were detected in low concentrations. This means that all fertilizers had good concentrations of different nutrients in which they positively affected soil fertility. For heavy metals, all measured elements were within the acceptable level of international standards.


\* Mg: magnesium; K: potassium; P: phosphorus; Cd: cadmium; Co: cobalt; Cr: chromium; Cu: copper; Fe: iron; Mn:4 manganese; Ni: nickel; Pb: lead; Ti: titanium; Zn: zinc; B: boron.

**Table 3.** Soil metal concentration (mg/l) in saturation extract at the beginning of the study.

#### *3.5.2. At the end of the study*

Finally, the faster development and greater biomass production in plants grown in sludgeamended soil may be responsible for an early reproductive cycle. Moreover, the complex organic and the inorganic compounds of sewage were broken down into simpler forms, and

From **Table 3**, it can be seen that all major cations were found in good amount for all treatments, whereas minor cations and heavy metals were detected in low concentrations. This means that

thus the final treated sludge became useful and beneficial to the seedling growth [20].

52 Soil Contamination - Current Consequences and Further Solutions

**Figure 8.** Average yield of carrot and potato.

*3.5.1. At the beginning of the study*

**3.5. Metal concentrations in soil samples**

From **Table 4**, it can be seen that elements such as K, P, and Mg were found in good concentrations, which is good for plant growth, whereas microelements and heavy metals were in low concentrations and within the international standards for all treatments. As mentioned before, irrigation water was the main cause of releasing the nutrients to the root zones. However, the similarity in concentrations of most elements in NPK and Kala fertilizers means that original soil was a source for some elements (rock materials) and the added values came from each treatment.


\* Mg: magnesium; K: potassium; P: phosphorus; Cd: cadmium; Co: cobalt; Cr: chromium; Cu: copper; Fe: iron; Mn:4 manganese; Ni: nickel; Pb: lead; Ti: titanium; Zn: zinc; B: boron.

**Table 4.** Soil metal concentration (mg/l) in saturation extract at the end of the study.

Long-term fertilization of biosolids enhances soil condition and shows increase in land production and that increment confirms the potential of substantial revenue expansion [21]. A study was conducted in China by Wang et al. [11] to identify the effects of using sludge in agricultural lands. The study concluded that the biomasses of grass used in the experiment were increased as well as soil organic matter compared to control treatment where no sludge was added. Furthermore, the heavy metals Pb, Cu, and Zn were determined and found not exceeding the standards of acceptable levels of heavy metals. It is wise not to generalize how metals interact in soil and ultimately taken up by plants because many factors influence such interactions and uptake such as the type of metal, physical, and chemical properties of the soil and the type of crop. As it is difficult to take into consideration all such factors, the regulation of sewage sludge application is based on the total metal loading or concentration in soils. Kiekens et al. [22] observed much lower metal solubility in a calcareous clay soil than in sand (pH 6) regardless of whether the metals were added as salt or sludge form.

#### **3.6. Metal concentrations in plant samples**

To evaluate the nitrogen content for the tested crops (**Figure 9**), it can be seen that Kala treatment obtained the highest values which was expected due to the high content of nitrogen in Kala fertilizer compared to NPK. This value was clearly reflected in soil nitrogen and chlorophyll content shown in **Figure 2**. The high value of nitrogen could be one of the reasons for obtaining better productivity with Kala compost compared to NPK.

**Figure 9.** Nitrogen content in tested crops.

For microelement concentration in fruity plants, it can be seen from **Figures 10** and **11** that there were small changes between NPK and Kala treatments. For short-season plants such as cucumber (**Figure 10**), it can be seen that in some cases NPK gave higher values for some elements such as Mn, Pb, and Ni, whereas Kala gave higher values than NPK for others such as Fe, B, and Al.

**Figure 10.** Heavy metal concentrations in cucumber.

Long-term fertilization of biosolids enhances soil condition and shows increase in land production and that increment confirms the potential of substantial revenue expansion [21]. A study was conducted in China by Wang et al. [11] to identify the effects of using sludge in agricultural lands. The study concluded that the biomasses of grass used in the experiment were increased as well as soil organic matter compared to control treatment where no sludge was added. Furthermore, the heavy metals Pb, Cu, and Zn were determined and found not exceeding the standards of acceptable levels of heavy metals. It is wise not to generalize how metals interact in soil and ultimately taken up by plants because many factors influence such interactions and uptake such as the type of metal, physical, and chemical properties of the soil and the type of crop. As it is difficult to take into consideration all such factors, the regulation of sewage sludge application is based on the total metal loading or concentration in soils. Kiekens et al. [22] observed much lower metal solubility in a calcareous clay soil than in sand

To evaluate the nitrogen content for the tested crops (**Figure 9**), it can be seen that Kala treatment obtained the highest values which was expected due to the high content of nitrogen in Kala fertilizer compared to NPK. This value was clearly reflected in soil nitrogen and chlorophyll content shown in **Figure 2**. The high value of nitrogen could be one of the reasons

For microelement concentration in fruity plants, it can be seen from **Figures 10** and **11** that there were small changes between NPK and Kala treatments. For short-season plants such as cucumber (**Figure 10**), it can be seen that in some cases NPK gave higher values for some

(pH 6) regardless of whether the metals were added as salt or sludge form.

for obtaining better productivity with Kala compost compared to NPK.

**3.6. Metal concentrations in plant samples**

54 Soil Contamination - Current Consequences and Further Solutions

**Figure 9.** Nitrogen content in tested crops.

**Figure 11.** Heavy metal concentrations in tomato.

For long-season plants such as tomato (**Figure 11**), it can be seen that NPK was higher in all measured elements than Kala except for Fe.

For leafy plants, it can be seen from **Figures 12** and **13** that similar scenario was repeated and small variations were found between Kala and NPK fertilizers.

**Figure 12.** Heavy metal concentrations in cabbage.

**Figure 13.** Heavy metal concentrations in lettuce.

For very short-season plant such as lettuce (**Figure 13**), it can be seen that all elements were in low concentrations with Kala compared to NPK. Iron (Fe) had the highest concentrations in all crops of both treatments.

For root crops such as carrot and potato (**Figures 14** and **15**), Iron (Fe) was high in both treatments of both crops. However, Kala compost was higher than NPK in some measured values.

**Figure 14.** Heavy metal concentrations in carrot.

**Figure 12.** Heavy metal concentrations in cabbage.

56 Soil Contamination - Current Consequences and Further Solutions

**Figure 13.** Heavy metal concentrations in lettuce.

all crops of both treatments.

values.

For very short-season plant such as lettuce (**Figure 13**), it can be seen that all elements were in low concentrations with Kala compared to NPK. Iron (Fe) had the highest concentrations in

For root crops such as carrot and potato (**Figures 14** and **15**), Iron (Fe) was high in both treatments of both crops. However, Kala compost was higher than NPK in some measured

**Figure 15.** Heavy metal concentrations in potato.

For all treatments of all crops, cadmium (Cd) and copper (Cu) were found in very low concentration of <0.001 mg/l.

Several studies have evaluated the tissue concentrations of nutrients and heavy metals in plants when grown in the sewage sludge-amended soil. The accumulation pattern varied with soil type, plant species, phenology, and chelating effects of other metals [23]. Bonding of potentially toxic elements to sludge solids and soils can limit transfer to roots. Some metals, such as Cr and Pb, have very low solubility in soils and show a particularly strong barrier. Leafy crops tend to have less protection in the uptake of metals in comparison to root crops. Many experiments have shown the metals have lower concentrations in seeds and fruits compared to roots, stems, and leaves. For example, Mo is more concentrated in soybean seeds than in the leaves [24], and Tl concentrations in rapeseed are higher than in the leaves [25]. For slightlymoderately Cd-contaminated soils, the transfer of Cd to the seed of linseed (flax), sunflower, corn, and wheat can be sufficiently high to exceed health standards in some countries [26, 27], whereas Zn uptake by corn (maize) in a multiyear sewage sludge experiment on calcareous soils was within the safe limit [28].

For copper concentration in crops, results for Cu were observed in the long-term field sludge experiments of Hinesly and Hansen [29], Hinesly et al. [30], and Soon et al. [28]. It was observed that Cu concentration increased in maize stover when there was an increase in Cu loading in the soil through sludge application. But interestingly, the increase was not directly proportional to the amount of increased Cu application. Reasons for such behavior are Cu sorption by sludge and soil organic matter and plants' strong physiological barrier to Cu translocation [31].

Because of the complicated nature as to how metals behave in soils especially when they are added through sewage sludge, it is almost impossible to provide generalized guidelines. For any particular situation, various considerations should be given before setting metal application guidelines. Such concentration should include soil physical and chemical properties especially adsorption characteristics, crops to be grown, and usage of grown crops. Contamination of such land by metals should be regarded as irreversible and must be kept to the lowest practicable level [9].

#### **3.7. Biological analysis**

To evaluate microbial contamination, multisamples were sent to the Muscat Municipality laboratory from all crops. Different tests were done such as the total aerobic plate count, Coliform bacteria, *Escherichia coli, Staphylococcus aureus, Salmonella* spp., yeast, and mold. No harmful bacteria were found, and according to that all crops can be eaten safely.

Same finding was reported by Boswell [32], when he noticed that sewage sludge amendment increased the fruit yield significantly compared to the un-amended control and no toxic or detrimental effects on fescue were noted.

#### **3.8. Water productivity**

Water productivity factor can be calculated by comparing water used in this study with plant production (water productivity = total fruit weight, kg/water applied, m3 ). The same amount of water was used to irrigate all crops, and as it was found in **Figures 6**–**8**, Kala compost gave better yield than NPK treatment, which means that water productivity of Kala compost was higher than NPK treatment.

Additions of organic fertilizers enhance soil fertility and improve soil structure. These improvements in soil physical properties increased water-holding capacity by promoting higher water retention in sludge-amended soils [33].
