Depth of Ion Exchange Resin Capsule Placement Impacts on Estimation of Nitrogen and Phosphorus Bioavailability in Semiarid Low-Fertility Soils

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Ion exchange resin capsules provide a possible alternative method to conventional soil testing procedures. Previous studies correlating fertilizer application rates with resin capsule nutrient concentrations in semiarid low-fertility soils found relationships that were not statistically significant with less mobile nutrients such as P. We propose that additional placement depths of resin capsules may improve those relationships. Nitrogen and P were broadcast applied at six rates and lightly raked into the soil at sites in Rush and Skull Valleys, Utah, in March 2010. Resin capsules were placed in the soils in June 2010 at depths of 0 to 5 cm, 5 to 10 cm, and 10 to 15 cm. The capsules were removed and replaced in September 2010, with final removal in May 2011. Soil samples were taken at the same depths and times whenever capsules were installed or removed. The determination of NH4-N was most precise at 5 to 10 cm, and resin capsules improved on soil test estimates. Resin capsule NO3-N was correlated to N applied regardless of depth, and resin capsules had a stronger relationship with N applied than the soil test 398 days after application. Resin capsule P was not related to P application at the first sampling but was significant 398 days after application in the 0- to 5- and 5- to 10-cm depths when soil moisture was higher. However, the shallower resin capsule placement improved estimates of bioavailability compared with a single placement depth. Soil moisture should be carefully monitored when using resin capsules to access P availability in semiarid soils.

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