Abstract |
Balances between removed and supplied quantities of trace elements indicate slow depletion of the micro nutrients Zn, Cu and Mn in farming based on cash crops and conventional fertilization. In farming based on livestock there is a slow increase caused by the utilization of manure and additions in feed additives. The soil levels of Cd, Hg and Pb are slowly increasing in both types of farming, mainly due to additions in commercial fertilizers (Cd) and by deposition (Cd, Hg, Pb). The rate of increase is for Cd ca 0.20-025%, for Hg 0.10-0.20% and for Pb 0.04-0.09% annually. Where recommended applications of sewage sludge are utilized, these rates are increased by factors of 2, 4 and 12, respectively. Despite improvements, the major parts of most trace elements in sewage sludge are still of other origins than food and agricultural soils. Probable quantities of trace elements added to agricultural soils during this century have been estimated. Sources considered were commercial fertilizers, lime, feed additives, fungicides and the atmospheric deposition. The estimates indicate that the concentrations of Cd, Hg and Pb have been increased by ca 33, 46 and 14%, respectively, based on the levels 1900. For Zn, Cu, Mn, Ni and Cr estimated increases are 10% (Zn) or below, i.e. rather small and insignificant. Locally the situation may, however, be different. Any increase of the present soil contents of toxic elements has to be considered unfavourable. Therefore, in order to obtain steady state conditions the additions of Cd, Hg and Pb should be reduced to 0.25-0.75, 0.025-0.095 and 0.9-7 gxha(sup -1)xyear(sup -1), respectively. (72 refs., 14 tabs.) (au). |