Biosolid Mixtures Applied in Tropical Soils and Their Effect on Coriandrum sativum and Ocimum basilicum Nutritional Uptake

Abstract

Agricultural soils are degraded worldwide as result of anthropogenic activities; environmental contamination; and excessive use of chemicals, fertilizers, and pesticides. Scientists are concerned about this problem; during the past few years sewage compost and sludge have been used as alternatives to improve the soil’s physical and chemical characteristics. Recycling solid waste residues can be a cost-effective way to reduce landfill material disposal and improve macro and micronutrients’ availability in agricultural soils. In this study, two types of biosolids (compost and sludge) were added to tropical soils (coloso and voladora series) to improve the nutritional content in two edible herbs (basil and coriander). Soil mixtures were made by volume percentage; compost was constant, at 25%, and soil and sludge were incorporated in different proportions to complete the 100% (25 comp/75 soil, 25 comp/62.5 soil/12.5 sludge, 25 comp/50 soil/25 sludge and 25 comp/37.5 soil/37.5 sludge). pH and electrical conductivity were measured in soil mixtures. Basil and coriander were cultivated in pots for 30 days at an open greenhouse (5 replicates per soil mixtures). Germination percentage, stems’ length, total chlorophyll (SPAD 502), acid digestions of plant tissues and an ICP-OES analysis were performed for both plants cultivated in all the soil mixtures. In voladora soil, the pH increased from 4.55 ± 0.10 to 5.64 ± 0.22 and EC from 0.0563 ± 0.0003 dS/m to 1.39 ± 0.01 dS/m in a 25% comp/37.5% soil/37.5% sludge mixture. In the coloso soil, the pH increased from 6.38 ± 0.13 to 6.82 ± 0.07 and electrical conductivity from 0.117 ± 0.001 to 1.310 ± 0.009 dS/m in 25% compost/37.5% soil/37.5% sludge mixture. Significant differences (p < 0.05) were found in the chlorophyll content and stem length for both plants. The highest chlorophyll value was in basil leaves from a 25% compost/50% soil/25% sludge mixture (43.20 ± 074) compared to the coloso soil (26.99 ± 0.43). In the voladora soil, the highest chlorophyll content was in a 25% compost/37.5% soil/37.5% sludge mixture (39.97 ± 0.83). In coriander leaves, the highest chlorophyll content was 33.01 ± 0.84 in the 25% compost/62.5% coloso/12.5% sludge mixture. In the voladora soil, chlorophyll content in leaves did not show a significant difference between treatments. Larger basil stems were found at 25% compost/75% voladora mixture (17.50 ± 2.39 cm) and in a 25% compost/62.5% coloso/12.5% sludge mixture (9.95 ± 0.71 cm) compared to control plants (3.45 ± 0.18 cm). Greater coriander stems were observed in a 25% compost/50% voladora/25% sludge mixture (2.43 ± 0.11 cm) and in a 25% compost/50% coloso/25% sludge mixture (2.17 ± 0.10 cm) compared to control plants (2.05 ± 0.07 cm). Macro and micronutrient content in plants increased with biosolids’ incorporation to soils. Mg content in basil leaves increased from 8.61 ± 0.70 mg/g in the voladora soil to 10.31 ± 0.60 mg/g in a 25% compost/37.5% soil/37.5% sludge mixture. In coriander leaves, Mg increased from 6.91 ± 0.06 mg/g in a 25% compost/75% soil mixture to 9.63 ± 0.02 mg/g in a 25% compost/50% soil/25% sludge mixture. The Mn uptake by basil leaves increased from 0.076 ± 0.005 mg/g in the coloso soil to 0.152 ± 0.019 mg/g in a 25% compost/75% soil mixture. In coriander leaves, Mn increased from 0.357 ± 0.002 mg/g in a 25% compost/75% soil mixture to 0.651 ± 0.006 mg/g in a 25% compost/37.5% soil/37.5% sludge mixture.

1. Introduction

More than 90% of global food are agricultural products, sustained by cultivated land [1]. For the past three decades, the soil degradation has increased in proportion to the anthropogenic activity and environmental pollution. Soil is a non-renewable resource that is essential for living organisms; there is a global interest in improving physical and chemical properties of agricultural soils like pH and nutrient availability. Geologists have classified world soils in twelve orders, based on their physical and chemical characteristics like pH, electrical conductivity, porosity, water hold capacity, and soil density, among others [1]. Puerto Rico (PR), a 100-mile-long and 35-mile-wide tropical island, is located between the Caribbean Sea and the Pacific Ocean. PR has ten out of twelve soil orders where Ultisols and Inceptisols predominate [2]. Some characteristics of Ultisols are acidic pH, low fertility, a horizon with translocated silicate clay, and low water hold capacity; Voladora belong to this order. Coloso belongs to the Inceptisol order, and it is associated with flood regions, minimal horizon development, a dark color due to the presence of organic matter, slightly acid pH, low water hold capacity, and high compaction [3].

In PR, Voladora (clay) and Coloso (silty clay loam) soil series are used for agriculture purposes. However, these types of soils have a nutrient deficiency caused by soil leaching (Ultisols) and high compaction (Inceptisols) [4]. To increase crops’ nutritional content and decrease the use of traditional chemical fertilizers, scientists are looking for better practices to improve the nutrient availability in agricultural soils. Biosolids’ incorporation to arable land has been considered a cost-effective and environmentally friendly practice that is currently applied in several countries around the world [5]. USEPA defines biosolids as the solid residual from water treatment plants which are rich in organic matter and nutrients. They also classify biosolids in Class A and Class B. Class A biosolids are used in agriculture and ornamental plant production because pathogens like coliform (<1000 MPN/g) and salmonella (<3 MNP/4 g) are under regulatory limits. Class B cannot be in contact with humans, domestics, and farm animals because they contain pathogenic species [6]. PRASA (Puerto Rico Aqueduct and Sewer Authority) biosolids (compost and sludge) are classified as Class A biosolids. PRASA compost is elaborated with vegetative matter, wood from construction pallets, and residual silt from water treatment plants. Vegetative materials and silt give compost specific characteristics to balance the C:N ratio of 20 to 40 carbon biodegradable atoms per one nitrogen biodegradable atom to provide an optimum environment for microorganism’s reproduction [7]. PRASA sludge is a solid residual from rivers’ water purification process.

Since 2002, European countries like Spain and United Kingdom recycle more than 50% of the total sludge produced annually. Farmers in year 2000 reported that biosolids reduced their fertilizer bill for 130 euros/ha. Researchers attribute this finding to the amount of phosphate present in sewage sludge. Biosolids’ application in sandy loams improve the soil density as well as the corn and potato productivity. In Europe, the use of sewage sludge as an ecological fertilizer for more than 25 years increased productivity, allowing businesses to supply crops for other farmers [8]. In USA, Environmental Protection Agency (EPA 503 Risk Assessment) regulates the application of biosolids for agricultural purposes, including inorganic and organic pollutant limits, pathogen density, and vector attraction [9]. Some research studies reveal that biosolids are an affordable option to recycle nutrients and improve some soil physical chemical characteristics [10]. De Figueiredo et al. [11] incorporated sewage sludge into Brasil clay oxisol (pH = 4.90) and reported that micronutrients’ concentrations increased as the amount of sludge applied was increased. The authors concluded that the sewage sludge application to acidic soils can improve the nutritional content at a faster rate than chemical fertilizers because of the mineral retention and improvement of soil porosity. Johansen et al. [12] evaluated the incorporation of sewage sludge for two decades; results showed an increment in micronutrients and microbial decomposition activity, because of the organic matter content in sludge. They concluded that the amount of nutrients was below the regulation limits and recommended sewage sludge as an organic soil fertilizer. Ouoba et al. [13] cultivated rice with sewage sludge as a soil amendment in Japan and reported that the sludge amendment improved the soil nutritional content and reduced nutrients leaching into water ecosystems compared to chemical fertilizers. They concluded that nutrients’ retention efficiency in soil amended with sludge is due to an increment in the water hold capacity. This research project aims to recycle PRASA sewage compost and sludge as soil amendments to increase edible crops’ nutritional content. To study the effect of PRASA biosolids on plants’ growth and development, Coriandrum sativum (coriander) and Ocimum basilicum (basil) were cultivated. These edible herbs are used worldwide because of their unique characteristics. Coriander is commonly used as a food seasoning herb, it has medicinal properties (antibacterial and antifungal) and is considered as an antioxidant; coriander is also used in food and pharmaceutical industries (as an oil extract) approved by the FDA (US Food and Drug Administration) [14]. Basil is also commonly used as a culinary herb; its peculiar aroma comes from the phenol content. These compounds also give basil high antioxidant properties [15]. Both herbs grow in tropical areas, tolerate climate changes, and can be harvested in one month; these characteristics qualify basil and coriander to accomplish the study objectives. This project aims to incorporate sludge into the compost/soil system to mitigate the effects caused by high EC, acidic pH, and high nutrient concentrations, thereby providing an optimum environment for plant growth and development. This approach aims to reduce sludge transportation costs to landfills and the use of chemical fertilizers, which also contribute to groundwater contamination.

To perform these research project, soil and biosolid mixtures were made at different proportions of the soil/sewage compost/river sludge. The pH and electrical conductivity were measured for soil mixtures and the control group. Basil and coriander were cultivated in pots in an open greenhouse for 30 days. After this time, the germination percentage, stem length, and total chlorophyll content (SPAD 502) were recorded. Plants were harvested, and the nutritional content was quantified using the ICP-OES technique.

2. Materials and Methods

2.1. Soil, Compost, and Sludge Mixtures

Soil mixtures were prepared using two different types of Puerto Rican soils series; Coloso, Voladora, and two biosolids; PRASA sewage compost and river sludge from a water treatment plant located at Mayaguez, a southwest town of PR near our university campus. Coloso soil is a dark brown soil found in Toa (a group of northeast towns of PR) associated with flood plain regions. The voladora soil is found in Moca (the northwest town of PR). This soil order has a characteristic reddish color that is representative of the high content of iron oxides. The compost is elaborated with vegetative matter, wood from construction pallets, and residual silt from WTP. Vegetative materials and silt have compost-specific characteristics to balance the C: N ratio of 20 to 40 carbon biodegradable atoms per one nitrogen biodegradable atom to provide the optimum environment for microorganism reproduction. Sludge is a solid residual from the river water purification process. In our previous research, compost maturity was evaluated and incorporated into the soil to find the desirable compost/soil proportion in order to cultivate crops. Sludge was combined with compost/soil mixtures at different proportions by volume percentage in pots (

Table 1. Controls and soil mixture proportions in %.

Controls	Soil Mixtures
100% compost	25% compost/75% soil
100% sludge	25% compost/62.5% soil/12.5% sludge
100% Coloso soil	25% compost/50% soil/25% sludge
100% Voladora soil	25% compost/37.5% soil/37.5% sludge

).

2.2. Soil Mixture pH

The pH was measured according to the Sparks soil analysis method [16]. Soil mixtures were transferred to cardboard boxes in the greenhouse for a week to sundry. After the drying process, soil samples were passed through a 2 mm mesh. Soil mixtures were transferred to a 250.0 mL Erlenmeyer flask with distilled water (1:1 proportion) and mixed in a shaker for 30 min. Samples were left standing for 15 min, and suspensions were transferred to beakers. The pH was recorded using an ORION pH meter (MODEL 710A Boston, MA, USA); 3 replicates per mixture were analyzed. Initial pH: Voladora—4.55 ± 0.10, coloso—5.93 ± 0.05, compost—6.5 ± 0.02, and sludge—7.6 ± 0.08.

2.3. Soil Mixtures’ Electrical Conductivity

Electrical conductivity was measured according to the Sparks soil analysis method [16]. A total of 40.0 g of soil mixture was transferred to a 250.0 mL Erlenmeyer flask, mixed with 80.0 mL of deionized water, and placed in the shaker for one hour. Suspensions were left standing for 30 min and filtered with a Whatman #541 filter paper. Electrical conductivity was measured with a HACH CO 150 Conductivity Meter (Loveland, CO, USA). For statistical purposes, 3 replicates per soil sample were measured.

2.4. Plants’ Germination and Harvest

The seed germination was recorded according to The Ecological Effects Test Guidelines [17]. A total of 15 coriander and basil seeds purchased from Ferry Morse Organic Seeds were planted per pot of each soil mixture (pots of 13.0 cm/1.0 kg) in the greenhouse located inside the university campus (with a tropical climate). For statistical purposes, five replicates were placed. Plants were irrigated with fresh water every day. After 30 days, plants were harvested, stem length was measured, and total chlorophyll content was recorded with a SPAD 502 chlorophyll meter (Minolta Ltd., Osaka, Japan). Plants were separated into roots, stems, and leaves. Plant tissues were transferred to 10.0 mL plastic tubes and dried in the oven at 70.0 °C for three days. Plant tissues were pulverized and saved for a further analysis.

2.5. Nutrient Quantification in Plant Tissues

Acid digestions were performed according to the USEPA 3051 method in a microwave oven (CEM Corporation Mathews, NC, USA). A total of 0.20 g samples of dried tissues were mixed with 3.00 mL of 70% plasma pure HNO₃ at 150.0 °C for 30 min. After digestions, solutions were transferred to 50.00 mL centrifuge tubes and diluted with deionized water (n = 3). Macronutrients (K, Mg, and Ca) and micronutrients (Cu, Zn, Fe, and Mn) were quantified by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-OES Perkin-Elmer Optima 4300 DV, Shelton, CT, USA). Certified standard reference materials (NIST-SRF 1570A and 1547, Metuchen, NJ, USA) and certified standards from all elements were used for quality assurance and quality control purposes (QA/QC) every 10 samples.

2.6. Statistical Analysis

The statistical analysis was performed using Minitab software version 21 (Minitab Inc., Pine Hall Road, State College, PA, USA) with Fisher’s exact test with a One-Way ANOVA, assuming a normal distribution and equal variance. Results are reported as mean ± standard error (n = 3) and significant differences with a 95% of confidence.

3. Results and Discussion

3.1. Effect of Biosolid Incorporation (Compost and Sludge) into Voladora and Coloso Soils on pH and Electric Conductivity

Table 2. pH and electrical conductivity values in voladora soil mixtures and biosolids. Values are means ± standard error, n = 3; letters represent significant differences between treatments with a p value < 0.05 according to Fisher’s exact test.

Samples (%)	pH	EC (dS/m)
100 Compost	6.5 ± 0.02 A	8.4 ± 0.04 A
100 Sludge	7.6 ± 0.08 B	0.32 ± 0.001 B
100 Voladora	4.55 ± 0.10 C	0. 0563 ± 0.0003 C
25 Comp/75 Voladora	4.84 ± 0.11 C	1.126 ± 0.004 D
25 Comp/62.5 Voladora/12.5 Sludge	5.15 ± 0.09 D	0.8330 ± 0.0006 E
25 Comp/50 Voladora/25 Sludge	5.57 ± 0.02 E	0.806 ± 0.006 E
25 Comp/37.5 Voladora/37.5 Sludge	5.64 ± 0.22 E	1.39 ± 0.01 F

shows pH and electrical conductivity values in compost, sludge, voladora soil, and soil mixtures. Sludge has the highest pH; when it was incorporated into a soil/compost mixture at 37.5%, the pH increased to 5.64 ± 0.22 compared to soil alone. pH changes can be attributed to the interaction of humic acids from organic matter found in biosolids and soils. Humic acids are approximately 10% of the sludge organic matter and are a bio product of the anaerobic digestions from a microorganism [18]. The incorporation of biosolids can increase or decrease the soil pH depending on the soil acidity and sludge type. For example, Tesfahun et al. [19] incorporated 12 t/ha of brewery sludge to vertisols from Ethiopia, and the pH decreased from 7.4 to 6.5. This was due to the sludge’s nitrification and humic acid formation from organic compounds found in brewery sludge. On the other hand, Urionabarrenetxa et al. [20] reported agricultural soils from Spain that were fertilized with 250 m of sewage sludge in 3.38 Ha, where the pH increased from 6.81 to 7.42. As we mentioned earlier, the sludge type is the main factor to increase or decrease the soil pH as well as soil buffer capacity. Another factor to consider is whether the sludge was previously treated with lime or CaO. In this research, 37.5% of sludge increased the soil pH to an optimal value for plant growth (pH = 5.64). The electrical conductivity was higher in compost with a value of 8.4 ± 0.04 dS/m. According to USDA soil quality indicators, the optimal EC value for plant growth is usually between 0.8 and 1.8 dS/m, and it should not exceed 2.5 (USDA-NRCS, 2011). The EC optimum value in Table 1 was obtained in the treatment of 25% Comp/37.5% Voladora/37.5% sludge (1.39 ± 0.01 dS/m), which is better than the lower EC value reported by Tesfahun et al. [19] (0.005 dS/m) that is caused by salinity from brewery sludge. Authors stated that EC provides insight into the soil salinity levels and cation exchange capacity in the soil.

Table 3. pH and electrical conductivity values in coloso soil mixtures and biosolids. Values are means ± standard error, n = 3; letters represent significant differences between treatments with a p value < 0.05 according to Fisher’s exact test.

Samples (%)	pH	EC (dS/m)
100 Compost	6.5 ± 0.02 a	8.4 ± 0.04 a
100 Sludge	7.6 ± 0.08 b	0.32 ± 0.001 b
100 Coloso	5.93 ± 0.05 c	0.117 ± 0.001 c
25 Comp/75 Coloso	6.38 ± 0.13 c	0.762 ± 0.002 d
25 Comp/62.5 Coloso/12.5 Sludge	6.60 ± 0.03 d	0.699 ± 0.002 e
25 Comp/50 Coloso/25 Sludge	6.69 ± 0.03 d	0.588 ± 0.003 f
25 Comp/37.5 Coloso/37.5 Sludge	6.82 ± 0.07 e	1.310 ± 0.009 g

shows pH and electrical conductivity values from compost, sludge, coloso soil, and mixtures. Sludge has the highest pH value of 7.6 ± 0.08 and coloso the lowest—5.93 ± 0.05. The incorporation of sludge to the compost/soil mixture increases pH from 6.60 ± 0.03 (11%) to 6.82 ± 0.07 (15%), as the sludge amount increases in the media (12.5% and 37.5%, respectively). Arlo et al. [21] incorporated the sewage sludge (pH = 7.5 ± 0.4) in two different soils from Uruguay. For sandy loam, the pH decreases from 5.13 to 5.01 and for silty clay loam, the pH increases from 7.64 to 7.71. Authors attributed pH differences to soil textures. Sandy loam has a lighter texture which increases nitrogen mineralization and acidifies the soil. On the contrary, the soil compaction from silty clay loam lowers oxygenation and reduces mineralization effects, increasing the soil pH. Table 2 shows the EC value in compost (8.4 ± 0.04 dS/m), 0.32 ± 0.001 dS/m in sludge, and 0.117 ± 0.001 dS/m in the coloso soil. The highest EC value in soil mixtures was 1.310 ± 0.009 dS/m in a 25% comp/37.5% coloso/37.5% sludge mixture. The incorporation of biosolids can increase the soil EC due to the number of dissolved ions. Measurements greater than 4.0 dS/m can cause detrimental effects on plant growth due to ions’ saturation and poor nutrient availability [2]. Zoghlami et al. [22] incorporated 120 t/ha of the sewage sludge to sandy loam soil in Tunisia, North Africa, and EC increased from 0.313 to 0.722 dS/m. They reported that the amount of sludge incorporated is directly related to the soil salinity and ion exchange capacity. Authors also concluded that precipitation and soil permeability reduced the impact of soil salinization due to the amount of biosolids’ application—120 t/ha (722 mS/cm and 76 mg/kg, respectively) as compared to the control (313 mS/cm and 48 mg/kg, respectively). EC is also related to soil fertility, because this is an indicator of the nutrient availability given by the charge/mass ratio of each chemical element.

3.2. Effect of Biosolid (Compost and Sludge) Incorporation into Voladora and Coloso Soils in Basil and Coriander Germination Percentages

Figure 1 and Figure 2 show germination percentages of basil and coriander seeds in different compost/soil/sludge proportions (%). All mixtures contain 25% compost and different proportions of soil/sludge. Proportions did not show the symbol % to avoid confusion with the germination percentage. In Figure 1, the germination percentage of basil seeds was 27% in a 25 comp/75 voladora mixture and 38% in a 25 comp/75 coloso mixture (Figure 1). Coriander seed germination percentages were greater than in basil seeds (Figure 2); 44% of the germination was observed in 25 comp/75 voladora and 77% of the germination was observed in a 25 comp/75 coloso mixture. Basil germination percentages in soil with biosolid mixtures did not reach 50% compared to coriander seeds that exceeded 60% in germination. The greater germination percentage for basil seeds was 50% in sludge and, for coriander seeds, 89% in a 25 comp/50 coloso/25 sludge mixture. The difference between basil and coriander germination percentages can be attributed to chemical and structural compositions of seeds. Coriander seeds contain 11% protein, 53% carbohydrates and 36% fat compared to basil seeds, which have 43% protein, 32% carbohydrates and 24% fat [2]. No significant differences were found in germination percentages between soil mixtures. Mixing two different types of biosolids with tropical soils can provide a good environment for seeds’ germination; however, voladora is classified as a sandy soil with a lower pH (4.55 ± 0.10). The sludge improves the voladora water hold capacity, and the organic matter from the compost increases the soil porosity. The voladora soil series is considered moderately well drained and has a slow permeability. The coloso soil series is considered poorly drained and has a moderately low permeability [2]. Some studies reveal that amendments like compost and sludge can improve the soil permeability and moisture [23,24]. Urionabarrenetxae et al. [20] evaluated the effect of sewage sludge (250 m) incorporation in agricultural soil from Spain (3.38 Ha), and the germination percentage of lettuce was 70% in comparison with the control group, at 88%. In contrast, Elkhlifi et al. [25] cultivated grass in alkaline soil from China with sewage sludge (1% application rate by weight), and the germination percentage improved from 10% in the control group to 67% in soil/sludge mixtures. Authors concluded that the germination improvement and plant growth is related with the P and K content in soil mixtures. Germination percentages can provide information about the seed viability and plant sustainability. Seeds need water and oxygen for the primary germination stages in order to remobilized nutrients and provide root development. Differences in seeds’ germination can also be influenced by the soil order and biosolid composition [2].

3.3. Effect of Biosolid (Compost and Sludge) Incorporation into Voladora and Coloso Soils on Basil and Coriander Growth

Basil plants cultivated in the voladora soil (control group) have a low germination percentage (12%) and detrimental growth (Figure 3). Larger basil stems were found in a 25 comp/75 vol (17.50 ± 2.39 cm) mixture and 25 comp/62.5 coloso/12.5 sludge mixture (9.95 ± 0.71 cm) compared to control plants (3.45 ± 0.18 cm) (Figure 3). Figure 4 shows greater coriander stems in a 25 comp/50 voladora/25 sludge mixture (2.43 ± 0.11 cm) and 25 comp/50 coloso/25 sludge mixture (2.17 ± 0.10 cm) compared to control plants (2.05 ± 0.07 cm). Results show that the incorporation of biosolids into voladora and coloso soils can improve stems’ elongation, since plants’ growth can be related to nutrients’ availability, soil pH, and EC. Ouoba et al. [13] cultivated rice in sandy loam soil from Japan with composted sewage sludge (260 kg/ha); their results show an increment in the shoot biomass by 20% and no difference in stem length. Dad et al. [26] cultivated wheat in seven types of biosolids (3.33, 6.66, and 9 tons/ha) with sandy loams from Pakistan. Their results show a direct relationship between the stem length and biosolid amount. Authors conclude that the incorporation of biosolids had a fertilizer effect on plants. Elkhlifi et al. [25] cultivated grass in alkaline soil from China with sewage sludge (1% application rate by weight); their results show an increase in plants’ length from 10.30 ± 0.16 cm (control) to 12.69 ± 0.46 cm (soil/sludge). Bastida et al. [27] cultivated barley in sandy loam soil from southeastern Spain with sewage sludge (84 kg/ha), and the plant height increased from 26.75 ± 3.30 cm (control group) to 32.75 ± 1.71 cm (soil/sludge). Basil and coriander are species with different morphologies. For example, coriander leaves grow from the same node and basil leaves grow across the entire stem, which makes a difference in plant length. Plants cultivated in the voladora soil with biosolid mixtures have longer stems than plants cultivated in the coloso soil with biosolids. Differences in plant growth can be attributed to soil density, since voladora and coloso belong to a different soil order. Coloso as inceptisols have a high compaction, affecting normal root development and stem elongation. The vascular plant system (phloem and xylem) transport nutrients from roots to leaves, meristem, and nodes (where plant metabolism, elongation, and ramification are performed) using the capillarity property of water and osmotic pressure to translocate macro and micronutrients [28]. Biosolids can increase the soil moisture and nutrient availability, since significant differences were found in stem length when measuring the plant growth in soil mixtures. The growth differences between plants cultivated in different soils can be attributed to the nutrients availability in the soil, considering that the coloso soil had a greater compaction than the voladora soil.

3.4. Effect of Biosolid (Compost and Sludge) Incorporation into Voladora and Coloso Soils on Basil and Coriander Total Chlorophyll Content, SPAD 502

The chlorophyll content was higher in plants cultivated in the coloso soil compared to the voladora soil. In coloso, the highest chlorophyll value was in basil leaves grown from 25 comp/50 coloso/25 sludge mixture (43.20 ± 074), compared to the coloso soil (26.99 ± 0.43) (Figure 5). Plants grown in the voladora soil had the highest chlorophyll content in leaves from plants grown in 25 comp/37.5 voladora/37.7 sludge mixture (39.97 ± 0.83). Figure 6 shows the chlorophyll content in coriander leaves from plants grown in both soils. The highest chlorophyll content was found in leaves from the 25 comp/62.5 coloso/12.5 sludge mixture (33.01 ± 0.84). However, the chlorophyll content in leaves from plants grown in the voladora soil did not show a significant difference compared to the chlorophyll content in the voladora soil. Chlorophyll measurements provide information about plants’ photosynthetic activity and nutritional content. The chlorophyll content does not depend on leaves’ physical structure, and the amount of pigment is not related to the shape of leaves; this can vary between species and leaf age. Biosolids’ incorporation of compost and sewage sludge improve the total chlorophyll content in leaves. Our results agree with other findings. Ouoba et al. [13] incorporated the compost sewage sludge (260 kg/ha) to sandy loam soil from Japan to cultivate rice; their results show that the chlorophyll content (SPAD 502) increased by 36.7% compared with the control group. Song et al. [29] cultivated sawtooth oak in pots with soil and sewage sludge from Korea; results show an increment in the chlorophyll content (SPAD 502) from 8.2 ± 1.19 (100% soil) to 37.7 ± 1.03 (50% soil/50% sludge). Chlorophyll captures light energy for the synthesis of glucose, and some macro and micronutrients involved in the photosynthesis process include Mg, Mn, Fe, and Cu [28]. Incorporating biosolids into soils can enhance the nutrient availability, thereby increasing the rate of the chlorophyll synthesis. The differences in chlorophyll between soils can be attributed to the content of organic matter, considering that coloso had a greater amount than voladora.

3.5. Effects of Biosolid (Compost and Sludge) Incorporation into Voladora and Coloso Soils on Macronutrient (Ca, K, and Mg) Content in Basil Leaves

Macronutrients, Ca, K, and Mg are essential for plant growth and development. Differences in the macronutrient uptake by basil plants cultivated in the coloso and voladora soil with compost and sludge mixtures can be explained by chemical interactions and nutrient competition within the soil/plant system (Figure 7 and Figure 8). In the coloso soil (Figure 7), basil leaves show a Ca content of 24.34 ± 1.7 mg/g, which increased to 31.97 ± 5.2 mg/g when the sludge was added to the soil, indicating the contribution of biosolids to the improved Ca uptake. Basil leaves grown in a 25 comp/50 voladora/25 sludge mixture (Figure 8), resulted in the highest Ca content (41.17 ± 4.7 g/kg). This suggests that a balanced mixture of compost, soil, and sludge optimizes the Ca availability in the media (Ca in compost—54.09 ± 0.09 mg/g and in sludge—2.11 ± 0.07 mg/g). Our results agree with those found by Elkhlifi et al. [25], who demonstrated that some biosolids are Ca-rich, improving the Ca level in plants (control—0.51 ± 0.02 cmol/kg and soil/sludge mixture—2.95 ± 0.09 cmol/kg) and optimizing the cell wall structure and plant health. Mixtures with biosolids may affect the soil pH, altering the availability of Ca (the Ca availability increased the pH range from 6.5 to 7.5). In soil mixtures, Ca competes favorably with other cations (Mg and K) for binding sites on the root surface. Its availability also changes with an alkaline environment [2]. The K content in basil leaves grown in the coloso soil was 24.76 ± 1.0 mg/g and increased to 42.66 ± 2.6 mg/g in a 25 comp/75 coloso mixture. The highest K content (52.90 ± 1.3 mg/g) was observed in basil leaves cultivated in 25 comp/50 voladora/25 sludge (Figure 8). This agrees the hypothesis that biosolids as sludge have a positive impact on promoting the K uptake. Tepecik et al. [30] grow cotton plants in sewage sludge-amended soils from Turkey (20 t/ha). Authors emphasized the capacity of biosolids to enhance the K uptake due to the positive interactions between K ions and organic matter present in the sludge. The K concentration increased by 41% in cotton seeds. This supports our findings, in which biosolids contributed significantly to the K improvement in basil leaves. K plays a vital role in enzyme activation and osmoregulation within plant cells by inducing conformational changes in proteins and helping macromolecules’ stabilization. Like Ca, the K uptake by plant roots can be affected by ions competition (Mg and Ca). Soils with higher Mg content might obstruct K uptake leading to lower K levels due to the cation exchange in roots’ ion channels. On the contrary, mixtures with a balanced Ca, Mg, and K ratio can promote the uptake [28]. In the coloso soil (Figure 7), the Mg content in basil leaves was 5.88 ± 0.3 mg/g and increased to 8.61 ± 0.7 mg/g in plants grown in sludge. The highest Mg uptake (10.31 ± 0.5 mg/g) was found in basil leaves cultivated in a 25 comp/37.7 voladora/37.5 sludge mixture (Figure 8). Results show that the compost and sludge incorporation into coloso and voladora soils can improve the Mg content in basil. Antonkiewicz et al. [31] reported the Mg uptake in crops grown in biosolid-amended soils. Their results indicated that the Mg availability increased significantly with biosolids’ incorporation (0.33 ± 0.01 mg/g soil and 2.80 ± 0.06 mg/g with sewage sludge). Mg also has a chemical affinity with organic compounds found in biosolids (PRASA biosolids’ Mg content: compost—10.11 ± 0.01 mg/g and sludge—11.10 ± 0.06 mg/g). The Mg uptake can be depressed by other cations like NH₄, Ca, K, and Mn. Sometimes, an excess of Ca in chloroplasts can inhibit the Mg content affecting the photophosphorylation pathway. Mixtures with an appropriate ion-balance create an environment where the Mg uptake is efficient [28].

3.6. Effect of Biosolid (Compost and Sludge) Incorporation into Voladora and Coloso Soils on Micronutrients (Cu, Fe, Mn, and Zn) Content in Basil Leaves

Figure 9 and Figure 10 show the micronutrient content in basil leaves from different soil/biosolid mixtures. Differences can be attributed to biosolids’ composition and soil type. In this study, the compost-based biosolid significantly impacted the micronutrient content (Cu, Fe, Mn, and Zn) in basil leaves in plants grown in coloso and voladora soils. Santana et al. [32] stated that tropical agricultural soils amended with biosolids increased their nutritional potential. In the coloso soil (Figure 9), a mixture of 25 comp/75 soil increased the Mn and Zn uptake, indicating the positive impact of compost on micronutrients’ availability. In the voladora soil (Figure 10), a 25 comp/75 soil mixture led to a significantly elevated Fe and Cu uptake. The Cu uptake in basil leaves shows a variation across different soil/biosolid mixtures. Leaves from a 25 comp/75 voladora soil mixture in (Figure 10) had a higher Cu uptake (0.033 ± 0.002 mg/g), indicating a relationship between the compost and Cu availability. Cu is an essential micronutrient that participates in various plant physiological processes, including photosynthesis and defense mechanisms against oxidative stress [28]. The Fe uptake in basil leaves varied significantly across the different soil/biosolid mixtures. A mixture of 25 comp/75 voladora in Figure 10 shows an Fe content of 0.67 ± 0.06 mg/g, increasing the Fe absorption due to the soil pH and organic matter content in compost (Fe is more available in soil when the pH is between 5.0 and 5.5). Fe participates in the mechanisms of both photosystem I and II on the electron transfer chain inside the thylakoid membrane for chlorophyll biosynthesis in plants leaves [28]. Biosolid-amended soils exhibit a notable Fe uptake in basil leaves, especially in voladora soil mixtures. Previous research Tepecik et al. [30] reported that increasing the organic matter in biosolids enhances Fe chelation, making it more accessible for plant uptake, as it is incorporated with other organic compounds. Basil cultivated in voladora soil mixtures (Figure 10) contain a greater amount of Mn than plants cultivated in the coloso soil (Figure 9). This micronutrient is a cofactor for several enzymes involved in plant defense mechanisms and photosynthesis because of its oxidative states (II, III, and IV) [28]. A 25 comp/62.5 voladora soil/12.5 sludge mixture (Figure 10) shows a significant increase in Mn content (0.59 ± 0.05 mg/g). However, the Mn uptake in 25 comp/75 coloso soil mixtures (Figure 9) is 0.15 ± 0.02 mg/g. Data reveal different Mn uptake patterns across soil mixtures. De Figuereido et al. [11] demonstrated the influence of sewage sludge on the Mn bioavailability due to its high mineral content. Soils amended with biosolids exhibited different Zn levels for both coloso and voladora soils. The mixture of 25 comp/75 voladora soil (Figure 10) shows a Zn uptake of 0.168 ± 0.003 mg/g. This indicates the potential role of compost for the Zn availability to improve the basil Zn uptake. Barbarick et al. [33] concluded that biosolids can be used as soil fertilizers because they can increase micronutrients’ availability due to the formation of soluble complexes. Zn is considered the second most abundant transitional metal after Fe, and it is present in enzymes such as oxido-reductases, transferases, hydrolases, lyases, isomerases, and ligases, which are all involved in the DNA synthesis.

3.7. Effect of Biosolid (Compost and Sludge) Incorporation into Voladora and Coloso Soils on Coriander Leave Macronutrients (Ca, K, and S) Content

Results show significant differences in Ca, Mg, and S uptake across soil mixtures. In the 25 comp/50 coloso/25 sludge mixture (Figure 11), cilantro leaves had high Ca levels (34.58 ± 0.28 mg/g), showing that the positive effect of this mixture improves the Ca availability. The Mg content was notably high (9.67 ± 0.10 mg/g) in a 25 comp/37.7 coloso/37.5 sludge mixture (Figure 11). A 25 comp/62.5 voladora/12.5 sludge mixture (Figure 12) reveals elevated S levels compared with other mixtures (10.30 ± 0.03 mg/g). Comparing 25 comp/75 coloso and 25 comp/75 voladora mixtures (Figure 11 and Figure 12), different Ca levels were observed (34.05 ± 0.12 mg/g and 19.32 ± 0.11 mg/g, respectively). In a 25 comp/50 coloso/25 sludge mixture (Figure 11), the Ca content was 34.58 ± 0.28 mg/g, showing the sludge potential to improve macronutrients’ availability. These results are like those found by Antonkiewicz et al. [31] where an increase in the Ca content was observed in a grass-legume grown in sewage sludge and a sandy loam soil mixture from Poland. Additionally, findings of Dad et al. [26] demonstrated the Ca availability provided by biosolids across wheat species. The Ca content in plants is influenced by various factors, including the soil pH, organic matter, and cation exchange capacity, because it is a bivalent cation like Mg. Our results show the variation in Ca levels that can be attributed to the interaction between sludge and coloso/voladora soils (Ca in sludge—2.11 ± 0.07 mg/g, coloso—0.11 ± 0.01 mg/g, and voladora—0.033 ± 0.001 mg/g). Sludge, rich in Ca, interacts with the soil matrix and influences the availability of Ca ions. Differences observed in Ca content also can be due to differences in Ca retention and ion exchange between sludge particles and soil components [31]. Mixtures of 25 comp/37.7 coloso/37.5 sludge and 25 comp/50 voladora/25 sludge (Figure 11 and Figure 12) show higher Mg levels (9.67 ± 0.03 mg/g and 9.62 ± 0.02 mg/g), indicating the positive impact of sludge on the Mg uptake in cilantro plants. Results agree those found by Barbarick et al. [33]. The authors investigated the impact of biosolids as a soil conditioner for corn and wheat cultivation in dry land. This study revealed that the Mg availability is strongly correlated with the soil texture and pH. The soil texture influences cations’ competition, thereby affecting the retention and release of ions. The diverse ratios of compost, sludge, and soil components play a crucial role in these processes, resulting in varying levels of Mg in plants [27]. Mixtures of 25 comp/37.7 coloso/37.5 sludge and 25 comp/62.5 voladora/12.5 sludge (Figure 11 and Figure 12) show a greater S content (7.85 ± 0.09 mg/g and 10.30 ± 0.03 mg/g, respectively), showing the positive effect of sludge-enriched soils on the S availability. Results reported by Glab et al. [34] demonstrated the biosolids’ efficiency (compost, sewage sludge, and biochar) in increasing the S content in sandy soils. The S availability in soils is influenced by microbial activity and organic matter decomposition. As sludge and compost are organic-rich, they contribute to microbial activity and enhance the S mineralization. Additionally, biosolids contain S compounds which enrich and condition the soil [35].

3.8. Effect of Biosolid (Compost and Sludge) Incorporation into Voladora and Coloso Soils on Coriander Leaf Micronutrient (Cu, Fe, Mn, and Zn) Content

The Cu, Fe, Mn, and Zn uptake by coriander leaves varied between the two tropical soils, coloso (Figure 13) and voladora (Figure 14). Voladora soil mixtures (Figure 14) showed a greater Mn uptake than the coloso soil (Figure 13) across all mixtures. Differences in the Mn availability between two soils can be related to pH, organic matter, or clay content. The coloso soil has higher Cu levels in coriander leaves. For example, at a 25 comp/75 soil ratio, the Cu content was 0.05084 ± 0.0004 mg/g in coloso (Figure 13) versus 0.037165 ± 0.0002 mg/g in the voladora soil (Figure 14). The coloso soil provided a greater Cu availability for plants. In contrast, the Mn uptake was higher in the voladora soil (Figure 14). The addition of sewage sludge (Figure 13) improved the Mn content in coriander grown in a 25 comp/62.5 voladora/12.5 sludge mixture (1.7812 ± 0.004 mg/g) compared to coriander grown in a 25/comp/37.5 coloso/37.5 sludge (0.6511 ± 0.002 mg/g). Fe ranged from 1.9836 ± 0.03 mg/g (25 comp/75 soil) to 4.731 ± 0.04 mg/g (25 comp/37.5 soil/37.5 sludge) in the coloso soil (Figure 13) and 3.235 ± 0.03 mg/g (25 comp/50 soil/25 sludge) to 5.779 ± 0.12 mg/g (25 comp/75 soil) in the voladora soil (Figure 14). Similarly, Zn varied from 0.204374 ± 0.001 mg/g (25 comp/50 soil/25 sludge) to 0.23813 ± 0.001 mg/g (25 comp/75 soil) in the coloso soil (Figure 13) and 0.23803 ± 0.002 mg/g (25 comp/37.5 soil/37.5 sludge) to 0.2958 ± 0.0002 mg/g (25 comp/62.5 soil/12.5 sludge) in the voladora soil (Figure 14). This suggests that the availability of these nutrients was less affected by soil chemical differences (Fe in the coloso soil—49.12 ± 6.41 mg/g and the voladora soil—54.09 ± 7.75 mg/g). Tepecik et al. [30] reported linear increases in cottonseed Fe, Zn, Mn, and B, with higher sewage sludge application rates up to 30 tons/ha. Similarly, Yang et al. [36] found a significant increase in the Zn and Cu content in wheat and maize grains with sludge amendments’ application over 10 years. The micronutrient uptake is likely attributable to a direct nutrient contribution from compost and sludge, which contain higher trace element concentrations relative to natural soils [37]. Furthermore, the organic matter addition can improve the cation exchange capacity, nutrient availability, and plant root activity. In this study, the voladora soil shows a much higher Mn accumulation and lower Zn uptake compared to the coloso soil.

4. Conclusions

Biosolids’ incorporation as a soil amendment in tropical soils is a cost-effective practice to reduce landfill disposals (approximately, a PRASA annual transportation cost of USD 37,000.00), recycle nutrients, improve the soil nutrient availability, and increase edible herbs’ nutritional content. In this research, sludge reduces the compost acidity effect and improves the soil electrical conductivity. Soil/compost/sludge mixtures improve some plants’ physical characteristics like the germination percentage, stem length, and chlorophyll content as well as increase the basil and coriander nutritional content. A total of 25% comp/37.5% soil/37.5% sludge mixture shows a positive impact on pH and EC for both soils. In contrast, the nutrient uptake for the coloso soil is variable. A greater impact on the nutrient uptake was found in a 25% comp/37.5% soil/37.5% sludge mixture, and the voladora soil was found in a 25% comp/50% soil/25% sludge mixture. Soil physical/chemical properties influence the effectiveness of biosolids’ incorporation.