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Potassium (K) fixation in Argentine soils and its effects on fertilizer K dose effectiveness remains an under-researched topic. The effects of K addition doses and clay type and content on the K fixation/release dynamics of added K in four soils, two Argiudols, an Ultisol and an Andisol were studied. Fixation responded to soil type, but not to K doses. From 5 to 24% of the applied K was fixed by the soils ranking from highest to lowest: Aquic Argiudol, Typic Palehumult, Typic Udivitrand and Typic Argiudol. The trend for K fixation relative to clay content was reversed. The soils fixed between 0.23 and 2.38 (cmol c kg 21 ) (g clay) 2 1 in increasing order: Typic Argiudol, Typic Palehumult, Aquic Argiudol and Typic Udivitrand. Potassium fixation by occlusion in clay pores could account for the high activity of the Andisol. The Q/I isotherms results were consistent with those from the fixation 2679 ORDER REPRINTS experiment. The residuality index, B, ranged from 0.65 (Aquic Argiudol) to 0.95 (Typic Argiudol), a trend consistent with K fixation by these soils. Exchangeable K measured in the fertilized soils and calculated through the residuality index was highly correlated (r ¼ 0:95; p , 0.01). This study shows that the B index is a useful tool in quantifying the potential of contrasting soils for net K fixation and in adjusting K rates to accommodate for this process.

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