Residual Time. Time after application of the sludge, the
so-called residual time, may affect the mobility and thus
the metal content in the plants growing on it. When the
organic matter applied with the sludge is decomposed
sorption sites are lost and the accompanying pH decrease
also affects the desorption of heavy metals. It has been
shown that the metal content increased in successively
harvested plants grown on a sewage sludge amended soil.
Thus, the available heavy metals in the amended soil may
change over time. It has also been argued that a fraction
of the organic matter is resistant to degradation and will
provide protection against plant uptake and leachability of
the metals on a long term basis.