A STUDY ON SOIL SERIES: THEIR CORRELATION WITH THE INTENSITY AND CAPACITY PROPERTIES OF SOIL POTASSIUM

The Intensity (I) or availability of soil K exhibits considerable variations within a Soil Series, presumably as a result of local differences in management and fertilization. The contrast is most marked between woodland and cultivated examples of the same Series. Earlier work had shown similar variations in the Quantities (Q) of labile K. Those Capacity properties (dQ/dI) of soils for labile K which derive from the planar surfaces of the clay crystals in the mineral skeleton of the soil show rather less variability. The Capacity properties of different samples of soil from one Series are not equal, but become increasingly so as the non-clay fractions, the amphoteric oxides, and the organic matter held in the < 2μ fraction, are successively removed. It is inferred that the active surfaces available for K–(Ca + Mg) exchange in the soils studied are fundamentally similar for the soils in any one Series, but that they may be reversibly blocked by organic and amphoteric cations, to varying extents according to their previous management. The differences are greater than the differences commonly observed between different experimental plots in the same field. Duplicate samples of three Series out of the four examined showed close or moderate agreement in their K-fixation properties. While soil organic matter has some influence on dQ/dI, the fraction with most effect is not that which makes the most substantial contribution to the CEC of the soil.