Abstract
We estimated the zinc (Zn) and copper (Cu) balance in Japanese farmland soil in 1997. Zinc and Cu uptake by crops amounted to 563 Mg and 108 Mg, respectively, while Zn and Cu doses from chemical fertilizer were 297 Mg and 63 Mg, respectively. Zinc and Cu contents of livestock excreta were estimated to be 2,863 Mg and 713 Mg, and 1,551 Mg and 337 Mg of this was applied to farmland soil. As a result, there was an excess of 1,285 Mg of Zn and 292 Mg of Cu in farmland soil. This excess corresponded to 0.379% of the amount of total Zn and 0.270% of the amount of total Cu in farmland soil. Even if all excess Zn and Cu was accumulated in farmland soil, it would take more than 190 years to reach the permissible upper level of Zn, based on the average Zn concentration in non-polluted farmland soil. However, the promotion of the use of livestock excreta, especially pig excreta that contain high levels of Zn and Cu, may shorten the time required to reach the level. Although there are some guidelines, the recommended dose level has not clearly determined. Conventional dose of Zn applied through chemical fertilizer and manure is lower than the recommended Zn application level. Doses may depend on plant diagnosis and we should monitor the contamination of soils and crops with Zn and Cu, or the application might depend on other indicators like the Zn/cadmium ratio.
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