EFFECT OF PROLONGED CROPPING ON EXCHANGE SURFACES OF CLAYS OF BROADBALK FIELD

Earlier studies have shown that the K‐(Ca+Mg) exchange isotherms of field soils are not greatly changed by cropping, by additions of K‐fertilizer, nor by moderate release of K from fixed forms. The present work shows that very prolonged K‐enrichment or depletion in the Broadbalk trials has led to substantial differences between the exchange isotherms of soils from adjacent plots in proportion to the severity of their K‐depletion. It is suggested that the changes result from the removal of inter‐lamellar K and the consequent increases in the inter‐lamellar spacings of parts of the clay crystals, which have increased the extent of the surface areas accessible to K‐(Ca+Mg) exchange. It is possible that increases sufficient to cause substantial changes in the exchange isotherms may not be apparent in X‐ray diffraction diagrams. Treatment of the K‐depleted clays with m CaCl2 appears to contract the newly expanded parts of the clay crystals, so that exchange on the occluded surfaces is once again too slow to be observed by the procedures used. Copyright © 1969, Wiley Blackwell. All rights reserved