STUDIES ON THE POTASSIUM-SUPPLYING CAPACITIES OF SOUTHERN ONTARIO SOILS. II. NITRIC ACID EXTRACTION OF NONEXCHANGEABLE K AND ITS AVAILABILITY TO CROPS

A previous study showed that large differences in K uptake by alfalfa (Medicago sativa L.) grown in the greenhouse occurred on soils which had similar ammonium-acetate-extractable K (NH4OAc-K) concentrations at seeding. Differences in total K uptake appeared to be closely related to the uptake of nonexchangeable K (NEK). The objective of this study was to determine what fractions or forms of NEK in the soils were responsible for the large differences in total K uptake noted in the greenhouse experiment. The total amount of NEK extracted in seven sequential extractions with boiling 1 M HNO3 (Mactotal) varied widely among soils and was significantly related to the total amount of K (r = 0.69; P < 0.05) and NEK (r = 0.75; P < 0.05) taken up by eight crops of alfalfa. Mactotal was separated into two fractions of differing solubility and the most soluble fraction, StepK, was highly correlated with NEK uptake (r = 0.85; P < 0.01) and total K uptake (r = 0.86; P < 0.01). The least soluble fraction, CRK, was not ...

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