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Nonacid Cation Bioavailability in Sand Rootzones

Woods, M.S. 2006. Ph.D. Dissertation. Dept. Horticulture, Cornell University, Ithaca, NY

Abstract

Soil nutrient analyses are used as indices of nutrient availability to plant roots. The 1 M NH4OAc, Mehlich 3, Morgan, 1:5 H2O, and 0.01 M SrCl2 extracting solutions were evaluated for measurement of extractable nonacid cations in a calcareous sand rootzone. The 1:5 H2O and 0.01 M SrCl2 tests adjusted to sample pH during the extraction process, but the 1 M NH4OAc, Mehlich 3, and Morgan tests did not adjust to sample pH. When comparing the extraction methods for their ability to detect K-induced changes in extractable Ca or Mg from a calcareous sand, the methods that adjusted to sample pH were sensitive to the changes, but the non- adjusting methods were not. The 0.01 M SrCl2 method also predicted cation exchange capacity (CEC). In a selection of 37 sands and 17 soils, CEC was estimated by summation of the nonacid cations extracted by soil nutrient analyses. These CEC estimates were compared to CEC measured by compulsive exchange of Mg2+ for Ba2+ . In sand samples, the 0.01 M SrCl2 estimates of CEC were very similar to measured CEC, but the 1 M NH4OAc, Mehlich 3, and Morgan estimates of CEC were larger than the measured CEC. The nonacid cations extracted by 0.01 M SrCl2 can be used to estimate CEC in calcareous and non-calcareous sands and soils. All extracting solutions were able to detect increased K availability to creeping bentgrass [Agrostis stolonifera var.palustris (Huds.) Farw.] in field and greenhouse experiments. Cation exchange membranes detected increased K supply rates in field plots to which K fertilizer had been applied. However, leaf K content varied between sampling dates, so although leaf K was related to soil K at individual dates, it was difficult to predict creeping bentgrass K content from soil nutrient analyses of sand rootzones. By expressing leaf K as the concentration of K in tissue water (Kw), variability associated with changes in leaf water content between sampling dates was reduced. Performance of L-93 creeping bentgrass in a calcareous sand classified as low in K was not affected by K fertilizer application or by changes in soil K, Ca, and Mg. These results suggest that current interpretations of nonacid cation soil test sufficiency levels should be re- evaluated for sand rootzones. Under greenhouse conditions, A-1 creeping bentgrass grown in sands with pH ranging from 5.0 to 8.5 had leaf Ca, Mg, and K content within sufficiency levels, even in sands classified as low in Mehlich 3 extractable Ca, Mg, and K. Testing methods that adjust to sample pH were suitable for assessing nonacid cation availability in calcareous and non-calcareous sands. Future research should more clearly identify the relationship between extractable nonacid cations and turfgrass growth.