The performance of DGT versus conventional soil phosphorus tests in tropical soils - An isotope dilution study uri icon


  • A soil test that samples nutrients only from fractions that are accessible to plants will predict availability and uptake more robustly than empirical tests. This can be tested by comparison of the isotope ratios (specific activity, SA) of the nutrient between plant and the soil extract. This study was set up to assess this requirement for the diffusive gradients in thin films technique (DGT), recently proposed as a soil P test, in comparison with conventional soil P tests viz. Olsen, Colwell, Bray-1, Mehlich-3, ammonium oxalate, anion exchange membranes (AEM) and 0.01 M CaCl2 solution.
  • Maize (Zea mays L.) was grown in two P-deficient soils from western Kenya with contrasting P sorption characteristics, amended with a low and a high P rate and labelled with P-33.
  • Most conventional soil tests can extract a fraction of P which is not available to maize. The DGT technique, however, only samples P from the plant-accessible pool.
  • The SA in the plant shoot corresponded with that of the extracts of the different soil tests, except for CaCl2 and ammonium oxalate extracts, at the low P rate in the soil with low P sorption capacity, Teso soil. For the high P rate on this soil, differences in SA between maize shoot and soil test were small for all established soil tests, but significant for the Colwell, Bray-1, Mehlich-3 and AEM tests. The SA in the soil extracts was significantly smaller than that in the maize shoot for Sega the strongly P-sorbing soil at both P rates for all conventional tests, including AEM. This indicates that these tests extracted P from a pool that is not accessible to the plant. For the DGT test, however, there was no difference in SA between the maize shoot and the soil test, for any of the treatments.

publication date

  • 2012
  • 2012