Assessment of P availability in heavily fertilized soils using the diffusive gradient in thin films (DGT) technique

Abstract

Phosphorus-availability tests typically provide an indication of quantity of P available (Colwell bicarbonateextractable P), or of the intensity of supply (0.01 M CaCl 2 -extractable P). The soil's capacity to buffer P is more difficult to assess, and is generally estimated using a P-adsorption curve. The diffusive gradient in thin films (DGT) approach may provide a simpler means of assessing a soil's ability to maintain soil solution P. Optimal extraction conditions were found to be 24 h exposure of DGT samplers to saturated soil. The DGT approach was evaluated on a range of 24 soils, some of which had high Colwell-(>100 µg g −1 ) and Bray 1-(>30 µg g −1 ) extractable P content, but showed a tomato (Lycopersicon esculentum Mill.) yield response to the addition of P fertilizer. The DGT approach provided an excellent separation of soils on which tomato showed a yield response, from those where fertilizer P did not increase dry-matter yield. Phosphorus accumulation was strongly correlated with soil solution P concentration and anion exchange resin-extractable P, but showed poor correlation with Colwellor Bray 1-extractable P. The DGT P accumulation rate of 3.62 × 10 −7 to 4.79 × 10 −5 mol s −1 m −3 for the soils tested was comparable to the uptake rate of roots of tomato plants that were adequately supplied with P (2.25 × 10 −5 mol s −1 m −3 ).Abbreviations: DGT -diffusive gradient in thin films; ICPAES -inductively coupled plasma atomic emission spectroscopy.

Predicting availability of mineral elements to plants with the DGT technique: a review of experimental data and interpretation by modelling

“…[73,74] Ferrihydrite is then used as binding agent instead of Chelex. Ratios of c DGT to P concentration in a stirred solution were unaffected by varying pH (pH 3-9) and by increasing concentrations of chloride, sulfate and nitrate, while bicarbonate decreased c DGT , but only at very high concentrations that are unlikely to occur in soil solutions.…”

“…[75] Anions are, therefore, not expected to have an interfering effect on DGT measurements of P in soils, whereas the resin method with an anion exchange membrane [76] is more subject to anionic interferences. The capacity of the Fe oxide gel is ∼2.5 µg P cm −2 , which corresponds to a c DGT of 450 µg L −1 for a 24 h deployment with a standard DGT device at 25 • C. [72,73] Saturation (i.e. a non-linear response to the solution concentration) may, therefore, start to occur under these conditions for c DGT larger than ∼300 µg L −1 or 10 µM.…”

“…The calculations show that DGT measurements indeed should narrow the difference between soils: the P concentration at which a 50% reduction in yield as a result of P deficiency was predicted differed 29-fold between scenarios (small or large K d ) when expressed as total While the use of DGT as an indicator of deficiency has not yet been assessed for micronutrients, experimental evidence that DGT may indeed be a good predictor for P deficiency has been obtained. Menzies et al [73] tested the yield response of tomato to the addition of P fertilizer for 24 soils in a glasshouse experiment. The DGT-measured concentration provided an excellent separation of soils on which tomato showed a yield response from the soils with no response.…”

“…Triticum aestivum Cd, Zn, Pb: shoot concentrations well correlated with DGT and soil solution [3] concentrations; Cu: shoot concentration best correlated with total Cu, weakly correlated with DGT concentration P 2 4 Lycopersicon esculentum, Triticum aestivum P deficiency assessed by yield response to P fertilizer; DGT provided [73,74] excellent separation of responsive and non-responsive soils A The 28 samples from four sites at various Zn levels (field contaminated or laboratory spiked).…”

Determination of Plant Available P in Soil

“…Most studies which investigated the suitability of DGT to assess the soil P status for different soils reported significant correlations between the amount of plant available P in the soil determined by DGT and plant response (e.g. dry matter yield or P uptake), while other standard extraction methods (Olsen,Colwell, predominantly displayed weaker or no correlations at all (Menzies et al 2005;Mason et al 2010;Tandy et al 2011;Six et al 2013a, b). In contrast to this, however, the yields of rice in a pot trial were best predicted by Mehlich III, Bray-1, Olsen and resin P in comparison to DGT and CaCl 2 .…”