Determination of dissolved reactive and dissolved total phosphorus in water extract of soils

The simplified acid peroxydisulphate digestion of soil water extract was evaluated for determination of total dis solved phosphorus by molybdenum-blue colorimetry in comparison with direct P-detection in water extract by the ICP-AES technique. The research was conducted on 79 agricultural soils with different agrochemical characteristics. The results of the colorimetric P detection in water extract without digestion and ICP phosphorus detection were different. The median of values determined by ICP-AES was 1.7 times higher than that of colorimetry, but the correlations between the two measurements were quite close (r = 0.993). Differences between the colorimetric phosphorus and ICP-AES phosphorus were irregular, increasing as the phosphorus level in soils decreased. The simplified procedure of acid peroxydisulphate digestion is useful for routine determination of total water-extracted phosphorus in soils when the soil testing laboratory is not equipped with the ICP-AES technique. The two-tailed paired t-test did not prove any difference in the values between the direct ICP-AES P-detection in water extract of soils and colorimetric P-detection in the acid peroxydisulphate digest.