Total Potassium to Exchangeable Potassium Ratios as a Guide to Sustainable Soil Potassium Supply

Abstract

Many diagnostic soil-testing services only use measures of extractable and/or exchangeable potassium (exchangeable K) to provide guidance on the need or otherwise for applications of K fertilizer. Less commonly, soil tests for nonexchangeable, plant-available K are included, although this combination of soil K tests does not unequivocally detect situations where soil total K reserves are critically low. This article provides examples of how atom ratios between measured values of total K and exchangeable K in soils can alert crop advisors and growers of such situations. Specifically, total K / exchangeable K ratios at different depths in samples (136 sites) from canelands of northeastern Australia and from cropping lands (67 samples) in Fiji were found to vary from around 1.0 to 1,235. Lowest total K / exchangeable K ratio values were commonly recorded in soils from the Bundaberg/Maryborough region of northeastern Australia and from highly weathered ferruginous latosols supporting “talasiga” vegetation in Vanua Levu, Fiji. The Vanua Levu soils contained much lower total K reserves than did cropping soils near Nadi on Viti Levu. Summary details of total K / exchangeable K ratios for the five cane-growing regions of northeastern Australia are provided, together with soil K data to 800–900 mm in selected soils from the Fijian Islands. There is need to seriously consider additions of plant-available K when levels of exchangeable K are at or below accepted “critical levels” and corresponding total K / exchangeable K ratios are around <2 to 3. Total K and exchangeable K in the soils tested were not intercorrelated.

Keywords:

• Australia
• cropping soils
• Fiji
• potassium
• soil fertility
• soil testing
• sugarcane