Leaching of minerals and heavy metals from muck-amended sandy soil columns

There is a concern over potential leachability of minerals and heavy metals from agricultural soils following muck application. Leaching potential assessment of the minerals and heavy metals in the muck-amended soils is needed for beneficial use of muck in agriculture. A simulation leaching experiment of muck-amended soil columns was conducted to examine effects of muck application on concentrations of minerals and heavy metals in the leachates from two sandy soils. A clayey muck was collected from the St. Lucie Estuary and chemically characterized. Muck was applied to the top of the soil columns prepared from an acidic soil (Wabasso sand) and a calcareous soil (Winder sand) representing typical agricultural soils for commercial citrus production in the Indian River area at five different rates (0, 50, 100, 200, and 300 Mg ha(-1)). Deionized water (250 mL) was applied on a daily basis and repeated for 21 days. A total of 5,250 mL deionized water (equivalent to 1,130 rum rainfall) was leached, and the leachates were collected every 3 days and analyzed for pH, electrical conductivity, and concentrations of minerals and heavy metals. Most heavy metals in the muck sediments predominantly occurred in weakly mobile or nonbioavailable forms. The salts in muck sediments were easily removed from the amended soils by leaching. Electrical conductivity and minerals (Na, K, Ca, Mg) of leachates from the muck-amended soils were higher than soils without muck amendment. The electrical conductivity values and concentrations of the minerals increased with increasing muck rates, indicating that application of high amounts of muck sediments may increase salt concentration in vadose water. The muck application decreased P leaching from the acidic soil but also increased the concentrations of the heavy metals in the leachates. However, the concentrations of most heavy metals in the leachates were below drinking water limits, and percentages of accumulated heavy metals in the total added that was leached from the soils amended with different rates of muck were very low. Field trials may be needed to confirm the results from the column leaching studies because preferential flow may enhance leaching of metals under field conditions.