Potentially toxic elements in per urban agricultural soils and vegetables with soil to plant transfer pollution assessment and human health risks in Wudil Kano State Nigeria

In the spaces between cities and the wild, where urban sprawl meets the open fields, a quiet exchange takes place. Farmers in these peri-urban zones grow the leafy greens and peppers that feed growing populations, often using water from nearby streams or wells. But as cities expand, they bring with them a hidden cargo: heavy metals and other toxic elements that settle into the soil from car exhaust, industrial runoff, and agricultural chemicals. These elements do not break down like organic waste; they persist. The central question for scientists is whether these invisible contaminants stay locked in the dirt or if they travel up through the roots and into the food we eat. The answer depends on a complex mix of soil chemistry, the specific type of plant, and the nature of the metal itself. Some plants are like sponges, soaking up certain elements, while others act as barriers, keeping them out. Understanding this transfer is vital because it determines whether a meal is a source of nutrition or a slow, cumulative dose of poison.

In the town of Wudil, in the Kano State of Nigeria, a team of researchers set out to investigate exactly this dynamic. They focused on the local agricultural belt, a vital source of fresh produce for the region, to see what was happening in the soil and the vegetables growing there. The team collected samples from five different farms during the dry season, gathering both the topsoil where roots grow and the edible parts of four common crops: spinach, lettuce, cabbage, and pepper. They brought these samples back to a laboratory to measure the precise amounts of five specific elements: cadmium, nickel, lead, manganese, and chromium. Using advanced spectroscopy, they could see exactly how much of each metal was present in the dirt and how much had made its way into the plants.

The first surprise came from the soil itself. When the researchers analyzed the ground, they found that, overall, it was remarkably clean. The levels of toxic metals were low, and the soil did not show signs of the heavy pollution often seen in industrial areas. In fact, the overall pollution score was so low that the land was considered unpolluted. However, one element stood out as an exception. Cadmium, a metal known for its toxicity even in tiny amounts, was found at slightly higher levels than the natural background. While not a crisis, this small enrichment suggested that the soil was not entirely free from human influence. The other metals—nickel, lead, manganese, and chromium—were present but at levels that matched what one would expect in natural, healthy earth.

The story changed, however, when the researchers looked at the vegetables. Even though the soil was mostly clean, the plants had absorbed these elements to varying degrees. The team discovered that the type of vegetable mattered more than the type of soil. Different plants have different ways of taking up nutrients, and this biological trait dictated how much metal ended up in the food. For instance, lettuce was a very efficient absorber of manganese, a nutrient that is essential for plant growth but can be harmful in excess. The lettuce contained significantly more manganese than the other crops. Similarly, spinach showed a strong ability to pull chromium from the ground. In contrast, lead, which is notorious for sticking tightly to soil particles, remained mostly in the dirt and did not travel far into the edible parts of the plants. This finding highlighted a crucial point: you cannot judge the safety of food just by looking at the soil; the plant itself plays a massive role in determining what ends up on the plate.

The most significant concern arose when the team calculated the potential health risks for people who eat these vegetables every day. They focused on how much of each metal a person would consume over a lifetime and compared that to known safety limits. For most of the metals tested, the risk was low. The amounts of nickel, lead, manganese, and chromium that a person would ingest from eating these vegetables were well within safe boundaries. But cadmium told a different story. Because cadmium is so toxic and accumulates in the body over time, even the small amounts found in the vegetables added up to a level that exceeded the safety threshold. The researchers calculated that the risk from cadmium was high enough to pose a potential non-cancer health threat to people who rely on these vegetables as a staple part of their diet. This was a critical finding: the vegetables were generally safe, but the presence of cadmium meant that long-term consumption could lead to health problems, particularly affecting the kidneys and bones.

The study concluded that while the agricultural soils in Wudil are not heavily contaminated, the situation requires careful attention. The fact that the soil looked clean did not mean the food was entirely risk-free. The specific way that plants like spinach and lettuce interact with the soil allowed them to concentrate certain metals, turning a low-level soil issue into a dietary concern. The researchers emphasized that the primary culprit was cadmium, which, despite its low concentration in the ground, managed to reach levels in the vegetables that warrant worry. To protect public health, the team recommended that farmers and local authorities start monitoring these elements more closely, especially cadmium and chromium. They suggested improving how irrigation water is managed and being more careful with the chemicals used in farming. The goal is to ensure that the vegetables feeding the community remain a source of health rather than a slow-acting hazard, securing the future of peri-urban agriculture in the region.