The physicochemical properties of the soil were
analyzed according to the methods described by LU [20]. Using a pH meter, the pH value of soil samples was determined by suspending them in distilled water at the soil to water ratio of 1:2.5 (w/v). Soil organic matter was oxidized with K
2
Cr
2
O
. Available N was extracted with 1 mol/L NaOH and titrated with 0.01 mol/L H
7
, and total N was determined by the Kjeldahl method. Available P (Olsen-P) was extracted using 0.5 mol/L NaHCO
with pH 8.5 and total P was determined by the Williams and Stewart method. Available K was extracted with 1.0 mol/L NH
3
4
2
OAc of pH 7.0. Total K and availble K were determined with atomic absorption spectrometry (AA6800, Shimadzu, Japan). Soil sample and amendments including sepiolite and phosphogypsum were digested with a mixture of HNO
3
H
2
O
[21], and plant samples were digested with a mixture of HNO
3
HClO
4
2
[20]. Concentrations of As, Cd and Pb in solutions were determined using an inductively coupled plasma optical emission spectrometer (ICPOES, IRIS Intrepid II XSP, USA). Blank and standard reference
SO
4