Application of N-ZEO-SR Plus Fertilizer on the Dynamics of Lead (Pb) Heavy Metal in Soybean Plants (Glycine max L.)
Abstract
Lead (Pb) contamination in agricultural soil poses serious risks to crop productivity and food safety. This study investigated the effects of N-ZEO-SR Plus fertilizer dosage and Pb concentration on the growth and yield of soybean (Glycine max L.), as well as on Pb content in soil and seeds, and their interactions. A randomized block design (RBD) with two factors was employed: N-ZEO-SR Plus fertilizer dose (0, 50, 100, and 150 kg ha⁻¹) and Pb concentration (0, 50, and 100 ppm), with three replications. N-ZEO-SR Plus fertilizer significantly increased the number of leaves at 6 weeks after planting (WAP) (R² = 0.937, linear), chlorophyll content (optimum at 111.33 kg ha⁻¹, quadratic), and seed weight per plant, while significantly reducing soil Pb content (R² = 0.906, linear). Pb concentration significantly increased soil Pb content (R² = 0.957) and the percentage of Pb reduction (R² = 0.891), but did not affect growth or yield variables. Significant interactions between N-ZEO-SR Plus and Pb were observed for the number of filled pods, chlorophyll content, soil Pb content, and Pb reduction percentage. The interaction revealed competitive ion exchange at the highest combination (150 kg ha⁻¹ N-ZEO-SR Plus + 100 ppm Pb), where NH₄⁺ displaced adsorbed Pb²⁺ from zeolite exchange sites. All soybean seed samples exceeded the 0.5 ppm maximum permissible Pb limit (Permentan No. 53/2018), indicating that N-ZEO-SR Plus alone is insufficient to reduce seed Pb to safe levels. Moderate N-ZEO-SR Plus application (100 kg ha⁻¹) provided the best balance between growth enhancement and Pb immobilization.
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Introduction
Soybean (Glycine max L.) is one of the most important leguminous crops in Indonesia, serving as a primary source of plant-based protein processed into tofu, tempeh, soy sauce, and soy milk [25]. However, soybean cultivation in Pb-contaminated soil raises concerns about food safety, as Pb can accumulate in plant tissues and enter the food chain [1, 16].
Lead (Pb) is a non-biodegradable toxic heavy metal that accumulates in soil and is readily absorbed by plant roots. Sources of Pb contamination include fertilizers, pesticides, industrial wastewater, mining waste, and vehicular emissions [4]. Pb inhibits plant growth by disrupting physiological processes such as photosynthesis, respiration, and nutrient absorption, ultimately leading to chlorosis, necrosis, reduced seed germination, and plant death [4]. Plants absorb Pb through roots and translocate it to stems, leaves, and seeds through three principal mechanisms: (1) root absorption, (2) translocation from roots to aerial parts, and (3) cellular localization [5].
Remediation of Pb-contaminated agricultural soil is essential for food security. Among various approaches, the use of zeolite-based slow-release fertilizers has shown promise. N-ZEO-SR Plus is a nitrogen fertilizer combining urea with nano-zeolite, nano-montmorillonite, nano-silicate, and humic substances [7]. Zeolite, a microporous aluminosilicate mineral with high cation exchange capacity (CEC) and large specific surface area, can adsorb Pb²⁺ ions through ion exchange and surface complexation [6]. In a slow-release fertilizer formulation, zeolite serves a dual function: regulating N release to improve nitrogen use efficiency, and immobilizing heavy metals through adsorption [2, 22].
Conclusion
1) N-ZEO-SR Plus fertilizer significantly increased the number of leaves at 6 WAP (linear, R² = 0.937), chlorophyll content (quadratic, optimum at 111.33 kg ha⁻¹), and seed weight per plant, while significantly reducing soil Pb content (linear, R² = 0.906).
2) Pb concentration (0–100 ppm) significantly increased soil Pb content (R² = 0.957) and Pb reduction percentage (R² = 0.891), but did not affect growth or yield, indicating that Deja-1 soybean can tolerate moderate Pb levels without visible toxicity.
3) Significant D×K interactions on the number of filled pods, chlorophyll content, soil Pb content, and Pb reduction percentage revealed that competitive ion exchange between NH₄⁺ and Pb²⁺ on zeolite sites can reduce chlorophyll and increase soil Pb at the highest combination (D3K2).
4) All soybean seed samples exceeded the 0.5 ppm maximum permissible Pb limit (Permentan No. 53/2018), indicating that N-ZEO-SR Plus alone is insufficient to reduce seed Pb to safe levels. Complementary remediation strategies such as phytoremediation, biochar amendment, or soil washing should be considered.
5) Moderate N-ZEO-SR Plus application (100 kg ha⁻¹) provides the best balance between enhancing soybean growth and immobilizing soil Pb, while avoiding negative interactions at higher Pb concentrations.
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