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

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.