Abstract
The traditional soil potassium (K) testing methods fail to accurately predict K requirement by plants. The Diffusive Gradients in Thin-films (DGT) method is promising, but the relationship between the DGT-measured K pool and plant available K is not clear. Wheat (Triticum aestivum L., cv. Frame) was grown in 9 Australian broad acre agricultural soils in a glasshouse trial until the end of tillering growth stage (GS30) with different plant K demands generated by varying plant numbers and pot sizes. Different K concentrations in soils were varied by 4 rates of K fertilizer application. The relative dry matter and K uptake were plotted against the soil K test value (CaCl2, Colwell and NH4OAc and DGT K measurements). To obtain 90% of maximum relative dry matter at low root density (closest to field conditions), the critical value of the NH4OAc K method was 91 (R2 = 0.56) mg kg−1. The DGT K method was not able to accurately predict relative dry matter or K uptake due to a weak extraction force for K from soils with high CEC values. Further endeavor on increasing K extraction force of the DGT method is warranted to obtain accurate plant available K results.
Highlights
Accurate soil testing methods for measuring “plant-available” K would maximize crop productivity and quality, and avoid long term K depletion of fertile farm lands such as what has occurred in grain cropping regions of western and eastern Australia[1]
Fusseder and Krauss reported that K uptake by maize in field conditions decreased from 50% to 12% of the possible theoretical uptake based on a mathematic treatment of data when root density varied from >2 cm cm−3 to
Tandy et al proposed that the Diffusive Gradients in Thin-films (DGT) method could be used for soil K measurement by using Amberlite IRP69 resin as the binding gel and found a similar accuracy to the NH4OAc K method to predict K concentrations in winter barley grown in pots[34]
Summary
Accurate soil testing methods for measuring “plant-available” K would maximize crop productivity and quality, and avoid long term K depletion of fertile farm lands such as what has occurred in grain cropping regions of western and eastern Australia[1]. The most common soil tests for predicting available soil K are soil solution extraction and various chemical methods that attempt to either just displace K from the cation exchange complex (exchangeable K), or in addition, measure some of the non-exchangeable K that may contribute to plant uptake during the season (extractable K). The latter tests generally require stronger extracts, and except for extraction with 0.5 mol L−1 NaHCO3 (the Colwell K soil test, reported by Rayment and Lyons to be the most widely used method for measuring available K in Australia11), are only used in specific industries (e.g. sugarcane) or regions (Victoria) where calibrations have been developed[12]. More extensive quantification of the effects of competing cations on K uptake by the DGT will facilitate assessment of the accuracy of the DGT method in predicting plant growth response to K application
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