In order to use sewage sludge as an agricultural material without any harmful effects, it is necessary to estimate the accumulation of heavy metals in the soil after sludge application. We developed a successive calculation method to predict the heavy metal content in soil using the results of the long-term application experiments of sewage sludge. This method is based on the assumption that the heavy metals carried by the sewage sludge decreased according to their concentration according to the equation, m_n=(m_<n-1>+M_n)e^<-k'> where m_n is the metal content in soil just before the n-th sludge application, M_n is the metal content increment carried by n-th sludge application, and k' is the decrease rate constant. The constant k' was considered to depend upon the soil pH and soil types. The k' of Zn was significantly affected by the soil pH. It was considered that the decrease rate of Zn content increased rapidly in the soil pH region of 5.0 to 5.5 or below. According to this calculation, the loading limit of sewage sludge to keep the Zn content below 120 mg kg^<-1> over 15 y was estimated at 7 to 13 t ha^<-1>.
