Effects of Humic Acid Doses and Aplication Times on the Plant Growth, Nutrient and Heavy Metal Contents of Lettuce Grown on Sewage Sludge-Applied Soils

Abstract

Effects of increasing doses of humic acid (HA), applied in different periods, on the plant growth, nutrient and heavy metal contents of lettuce grown on sewage sludge-applied soils were investigated. A constant dose of 20% sewage sludge (SS) was added to experimental soil placed in pots. Various doses of HA (0, 500, 1000, and 2000 mg kg(-1)) were applied to the pots for different time periods (0, 30, 60, and 90 days before seedling transplantation). Increasing doses of HA significantly affected the plant and neck diameter, the plant height, the number of leaves, but not the plant weight. Application times of HA significantly influenced all plant growth parameters mentioned above. Moreover, there were significant effects of the interaction of HA dose and application time on the diameters, heights, and weights of lettuce plants. There were significant effects of increasing doses of HA on the N, P, K, Ca, Mg, Fe, Zn, and Cd contents of lettuce plants. Their Cd, Cr, and Co contents decreased with increasing HA doses. Also, significant differences in N, P, K; Mn, Cd, and Ni contents could be observed for various application times of HA. Moreover, the Mn content of lettuce was affected by the time and dose interaction. As a result, the application of HA had positive impacts on the plant growth and nutrient contents of lettuce, and increasing doses of HA reduced the heavy metal contents, such as Cd, Cr, and Co, in lettuce plants. The early application of HA for lettuce, with a short growing period, had more beneficial effects for the plant growth and nutrient content.