Studi bioavailabilitas logam berat (Cu dan Zn) dalam tanah secara ekstraksi bertahap dan diffusive gradient in thin films (DGT) serta penyerapannya pada kangkung darat (ipomea reptans poir.) = Study of bioavailabilty heavy metals (Cu and Zn) in soil by mean of sequential extraction and diffusive gradient in thin films (DGT) and Its absorption into ipomea reptans poir