Cadmium (Cd) is a toxic metal with no known benefits to plant or soil health. Although Cd is typically found in minute concentrations, certain fertilizers and soil amendments can be contaminated with Cd, leading to soil toxification. Crops can take up excess Cd from the soil, posing a risk to plant and human health. Cadmium enters the soil primarily through anthropogenic inputs. Industrial contaminants, mining wastes, sewage sludge, and even manure may contain significant amounts of Cd. In addition, specific sources of soft rock phosphate fertilizers have been shown to contain 50-100 ppm Cd. The adverse health effects of Cd in humans are well documented and severe, and these effects extend to many species. Soil organisms are negatively affected by excess Cd, inhibiting soil productivity. In plants, Cd diminishes yield and reduces plant quality.
Effects of Cadmium on Plant and Soil Health
Basic Information on Cadmium
Outcomes of Cadmium Toxicity
The toxicity of Cd is determined by soil characteristics such as pH, organic matter content, clay content, and cation exchange capacity (CEC). pH is considered to be the most important of these factors. Cd uptake is facilitated by lower pH values, whereas high pH values can buffer Cd toxicity by favoring adsorption of Cd to soil colloids. Organic matter in the soil increases the CEC, providing more sites for Cd adsorption. Adding organic materials to the soil may decrease Cd availability. In a study of wheat, the authors found that a 5.0% amendment of biochar decreased Cd uptake and significantly reduced the accumulation of Cd in shoots by 47%. When Cd accumulates in the plant, it causes widespread damage. Cd toxicity is associated with inhibited root elongation and disrupted photosynthesis–visible symptoms include leaf chlorosis, leaf rolling, necrosis, and growth inhibition. Cd toxicity causes oxidative stress and can potentially cause damage to DNA strands. Cd also disrupts the plant’s ability to take up other nutrients, exacerbating the stress.
Decreasing Cadmium Uptake
Nitrogen, phosphorus, and potassium fertilization can help plants tolerate Cd stress by promoting photosynthesis and biomass production, allowing the plant to sequester Cd in vegetative tissues. Calcium decreases Cd uptake by competition at the calcium transport channels and increases soil pH, decreasing Cd availability. Proper levels of micronutrients such as magnesium, zinc, iron, manganese, selenium, and silicon are essential for the plant’s antioxidant defense system and enzymatic activity. Loading the soil with biochar increases the CEC of soil, which protects nutrients from leaching away and presents them in plant-available form. Pacific Gro has a biochar product that is a liquid suspension, loaded with nutrients from ocean fish, crab, and shrimp, stabilized with acid and vortexed together to solubilize the carbon in a colloidal suspension. Pacific Gro Oceanic Hydrolysate facilitates humic formation to build up soil organic matter and improve soil tilth, particularly by the growth of fungal hyphae.
Source: Nazar et al., (2012)
Figure 1. Plants take up cadmium from the soil through their roots and transport it to their leaves, where it accumulates. Cadmium is taken up by plants through their xylem, which transports water and nutrients from the roots to all parts of the plant. The uptake of cadmium by plants is important because it competes with other elements for transport through this system.
Source: Riaz et al., 2021
Figure 2. Cadmium can cause metabolic, physiological, and biochemical alterations within the plant. These alterations are caused by cadmium’s ability to inhibit photosynthesis and disrupt calcium metabolism.
Further Reading
Abbas, T., Rizwan, M., Ali, S., Zia-ur-Rehman, M., Qayyum, M. F., Abbas, F., Hannan, F., Rinklebe, J., and Ok, Y. S. (2017). Effect of biochar on cadmium bioavailability and uptake in wheat (Triticum aestivum L.) grown in a soil with aged contamination. Ecotoxicology and Environmental Safety.
Grant, C. A. (2011). Influence of phosphate fertilizer on cadmium in agricultural soils and crops. Pedologist.
Liu, K., He, W., Zhang, H., Cao, Y., and Dai, Y. (2015). Major factors influencing cadmium uptake from the soil into wheat plants. Ecotoxicology and Environmental Safety.
Nazar, R., Iqbal, N., Masood, A., Khan, M. I. R., Syeed, S., and Khan, N. A. (2012). Cadmium toxicity in plants and role of mineral nutrients in its alleviation. American Journal of Plant Sciences.
