Rather than dumping sewage sludge into the North Sea, it has been suggested that it could be used as fertiliser on agricultural land. However, sewage sludge often contains waste from industry which is contaminated with heavy metals, some of which (e.g. zinc and cadmium) can be absorbed by plants. Zinc is generally present in the sewage in greater amounts than other metals and is a problem for crop production because it will kill plants at concentrations which are too low to cause toxicity to animals. However, concentrations of cadmium can become high enough to make plants dangerous as food without the cadmium having any obvious effect on the growth of plants.