The availability of potassium in some Tasmanian soils. II. Exhaustive cropping in relation to potassium reserves in the soil

Abstract

Five soil samples from Fingal, Tas., representing Frodsley sandy loam of varying exchangeable potassium status, were subjected to exhaustive cropping in pots and the changes in their potassium status and that of the crops measured. Three of the samples represented an area carrying improved pastures while two were from a less intensively managed property. The results were contrasted with those from two soils from South Australia (Urrbrae loam and Seddon gravelly sandy loam). The progressive removal of potassium by cropping was determined by analysis of each crop and of the soil at the end of the experiment. Uptake and removal of potassium was greatest from the soils of highest exchangeable potassium status. Potassium applied to the soils was also rapidly taken up by the crops. For the Tasmanian soils, applications of potassium gave no responses in yield until the level of exchangeable potassium in the control pots fell to 0.10-0.15 m-equiv./100 g. The higher the initial level of exchangeable potassium, the greater the number of crops before a yield response to applied potassium was obtained. In contrast, yields were maintained at a high level on Urrbrae loam and, even in the fourth crop, applications of potassium gave only a barely significant response. From Frodsley and Seddon soils only small amounts of potassium were taken up from non-exchangeable sources. From Urrbrae loam, in the absence of applied potassium, slightly more than half of the potassium taken up by the crops was derived from other than exchangeable sources. These differences were in keeping with the different amounts of potassium released to boiling nitric acid by these soils. After four crops exchangeable potassium was reduced to very low levels (0.05-0.14 m-equiv./100 g) in all five Tasmanian soils. These values were directly related to the amounts of difficultly exchangeable potassium in these soils. Exchangeable potassium in Urrbrae loam was not reduced below 0.33 m-equiv./100 g, again reflecting the higher reserve of difficultly exchangeable potassium in the illitic clay of this soil and its greater capacity to replenish the exchangeable potassium fraction. The percentage of potassium in the dry matter of the crop was closely related to the level of exchangeable potassium or exchangeable plus added potassium in the soil. Subterranean clover showing severe leaf necrosis due to potassium deficiency contained 0.55-0.63 per cent. potassium. Plants showing less severe deficiency symptoms or symptoms developing at a later stage showed 0.84-0.98 per cent. potassium. Plants with 1.20 per cent. potassium were free from symptoms and gave no further yield responses to applied potassium. Additions of potassium to the soil led to higher values in the plants, and luxury uptake was common. At least 84-95 per cent. of the applied potassium was taken up by the crops on four of the Tasmanian soils.