Observations on the determination of the availability of potassium for rhodes grass grown in open-cut mine overburden

A potassium availability index, the value of the free energy of exchange of calcium and potassium, was determined when Rhodes grass growing in mine overburden and in topsoil exhibited visual potassium deficiency symptoms. The value was found to be -4060 � 100 cal equiv.-1. There was no significant difference in this value between the overburdens and topsoil studied. The potassium characteristics of the growth media changed form during the growth of the Rhodes grass.

Nutrient potential and capacity. III. Minimum value of potassium potential for availability to Trifolium subterraneum in soil and in solution culture

Plants of subterranean clover (Trifolium subterraneum L.) were grown in a range of soils until the supplies of potassium were exhausted. The potassium potential of each soil was then measured by determining the free energy of exchange of potassium for calcium plus magnesium. In soils of very high buffering capacity the potassium potential of the exhausted soil was about –6000 cal/equiv. In soils of lower buffering capacity the potential of the exhausted soil was higher but this may have been an artefact due to release of potassium from fine root material left in the soil at harvest. Subterranean clover plants were also grown in large volumes of well-stirred nutrient solutions which were held at a range of potassium concentrations. Again it was found that the plants were unable to take up potassium when the potassium potential was about –6000 cal/equiv. It is suggested that the potentials were similar because diffusion gradients were negligible in the exhausted soil and also in the swiftly flowing solutions. At potentials above –6000 cal/equiv. availability of potassium appeared to be greater in the solution cultures than in soils. It is suggested that this occurred because, in soils, the uptake of potassium causes the potential at the plant root to be lower than in the bulk of the soil.

Nutrient potential and capacity. II. Relationship between potassium potential and buffering capacity and the supply of potassium to plants

The potassium status of 22 soils with low reserves of non-exchangeable potassium was assessed by measuring: (a) the potassium potential (the free energy of exchange of potassium for calcium plus magnesium); (b) the soil's capacity to resist a change in potential ("buffering capacity"); (c) the amount of exchangeable potassium and its percentage saturation. The ability of these measures to predict the supply of potassium was tested by growing subterranean clover (Trifolium subterraneum L.) in pots and determining its potassium content at successive harvests. The soil's buffering capacity was found to affect the supply of potassium even when uptake was small. It is suggested that this was partly because buffering capacity affects the rate of supply by diffusion to plant roots. As the uptake of potassium became progressively larger, the potential became increasingly unsuitable as a single index of availability, but a multiple regression containing terms for potential and for buffering capacity continued to account for a large proportion (89%) of the variation. The relationship obtained can be used to indicate the amount of potassium required to bring a deficient soil to a desirable status.