Some thoughts on soil potassium with special reference to two North Canterbury soils : A dissertation submitted in partial fulfilment of the requirements for the Diploma of Agricultural Science in the University of Canterbury

Abstract

Although, in the form of some of its simpler chemical compounds, it is one of the most soluble elements, in the soil-plant system potassium behaves with extreme differences of solubility and mobility. Thus, while its absorption from solutions and soils is highly efficient, and its movement through plants, very rapid, its net retention in individual cells may be quite strong. Potassium is essential for the growth and reproduction of higher plants for which it is required in larger quantities than any other metallic cation.

Although many studies have been made of the metabolic role of potassium in plants, few specific functions have been established for this nutrient. This, in part, is due to our inability to isolate discrete organo-metallic compounds as they occur in the plant, because of their instability after death of the plant. Some correlations have, however, been observed (Sutcliffe, 1967). Potassium ions have been found to activate a number of enzymes, a role in which it is usually irreplaceable by sodium or magnesium: often, activation can also be achieved by ammonium or rubidium ions. It is prob­ably through such inactivated enzyme systems that lack of potassium has its other harmful effects on metabolism. Such effects are seen as decreased photosynthesis, and changes in carbohydrate, sugar and protein metabolism. As a cation in the sap of plant cells, potassium has the role of a natural osmotic regulator: to some extent this function can be ful­filled by other easily-absorbable cations, such as sodium or magnesium. Further roles of potassium in plant nutrition are discussed by Fowler (1963).

The nature of potassium in the soil is little simpler. It exists, like other plant nutrients, in a series of forms (arbitrarily divided) of widely varying solubility and plant availability. These forms, their assessment and the equil-ibria existing between them will be discussed.