o Background and aims
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 33P.
The SA in the plant shoot corresponded with that of the extracts of the different soil
tests, except CaCl2 and ammonium oxalate extracts, at the low P rate in the soil with
low P sorption capacity. 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 test. The SA in the soil extracts was significantly
smaller than that in the maize shoot for 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, in any of the treatments.
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.