Selection of conditions for determining soil available phosphoru by dilute salt solution-malachite green method

【Objective】The method of available phosphorus characterized by phosphorus extracted in KCl dilute-salt solution was studied. 【Method】 By using standard solution and soil samples from different land use patterns, influences of different conditions during extracting and color-developing stages on the precision and accurancy of dilute-salt solution-malachite green method were studied. 【Result and conclusion】 The optimum conditions for extraction were as follows:1 g soil∶20 mL water for soil-water ratio, 0.5 h for oscillation, and 0.02 mol·L^-1 for concertration of KCl extracting solution; The optimum final concentrations of reagents for the color-developing system were as follows: H₂SO₄ 0.48 mol·L^-1，(NH₄) ₆Mo₇O₂₄·4H₂O 0.024 mol·L^-1, malachite green 0.06 g·L^-1. The association complex had a maximum absorption wavelength at 620 nm， and Beer’s law was obeyed in the P range of 0~0.2 mg·L^-1 with the linear regression equation being A =3.894 3C+0.001（R² =0.999^**）and P detection limit being 0.002 mg·L^-1， and the method had high accurancy and precision as well. Thus the method is appropriate to determine phosphorus amount in KCl extracting solution.