The effect of fertilizer additions on the solubility and plant-availability of Cd, Ni and Zn in soil

We conducted a pot experiment to investigate the effect of fertilizer additions on the solubility of Cd, Ni and Zn in soil solution and their uptake by plants. Radish (Raphanus sativus cv. Crystal Ball), oat (Avena sativa cv. Thule) and water spinach (Ipomoea aquatica cv. Kangkon) were grown in a naturally metal-rich soil. From day 7 after planting, fertilizers were added daily to each pot. Additions of fertilizer nutrients affected the pH of soil and soil solution, soluble and tissue concentrations of Ca, Mg, K, Cd, Ni and Zn differently in three plant species. The trend of soil and solution pH was in the order: water spinach < radish < oats, that resulted highest soluble and plant tissue concentrations of Cd and Zn in the water spinach followed by radish and then oats. However, Ni concentration in the soil solution increased in all pots and was not affected by pH changes. Soil solution pH increased by more than 1unit in the pots with radish and oats, indicating that mechanisms other than acidification, such as ion exchange and root exudation, may be responsible for the increased heavy metal uptake in these two plant species. Paired t-test showed significantly higher uptake of Cd and Zn in the radish plants resulting in lower concentrations of these elements in the solution. The contribution of mass flow to the supply of major cations and heavy metals varied among elements and plant species. Cadmium, Zn and K were taken up rapidly by all plant species in response to the amount supplied by mass flow. In contrast, the supply of Ni was in excess of its uptake by radish and water spinach. The uptake of all elements was positively correlated (p<0.0001) with mass flow and the transpiration rate in individual plant species. The study suggests that fertilizer cations increased the uptake of metals by improving growth conditions, but the magnitude of increase depended on plant species.