The colour in a sequence of soils formed on basalt in the Burnie area of Tasmania changes from red to red-brown near sea level to a yellow-brown in the more elevated areas. This colour change reflects the variations in the goethite/ haematite ratio which in turn has been positively conelated to the proportions of Ca plus Mg on the exchange complex. The results support the laboratory observations of Schellmann that the adsorption of Ca and Mg ions by freshly precipitated iron oxide gel directs the ageing towards haematite.
The relationship between vegetational type and a number of soil chemical factors was e.xamined in secondary successions from fire-maintained eucalypt/grass to clima.x rainforest communities growing on uniform granitic soil parent material. Canonical variates analysis, which utilized the following variables: pH; loss on ignition: total N. P. K. Ca. andMg: cation exchange capacity and exchangeable Ca. K. and Mg: and potentially mineralizable N, revealed close overall similarity between surface soils of adjacent types, and significant differences among those of types distant from each other in the successional sequences. Exchangeable Ca, mineralizable N, total N, F, and Mg, andpH all differed significantly among soils of the vegetational types. However, the only identifiable gradients in soil properties that were detected within a successional sequence were in total and mineralizable N. which tended to increase, and pH, which generally tended to decrease with progression towards the climax vegetation. Nitrification was promoted by the presence of Acacia dealbata and apparently inhibited by the presence of Leptospermum lanigerum; it was more rapid in soils beneath late successional vegetation than in those from climax vegetation or early stages of succession, and was inhibited in soil from old (> 200 years) grassland. It was concluded that differences among soils in chemical composition and rates of mineralization ofN were due to differences in species composition of the vegetational types that thev carried for the time being.
The potassium status of Frodsley sandy loam and some associated soils from the Fingal district, north-eastern Tasmania, has been investigated. Regularly spaced sampling of surface soils on a rectilinear grid pattern disclosed a variability of exchangeable potassium values in the field much greater than recorded for soils elsewhere; even for 3-ft spacings there was a sevenfold range. The need for adequate sampling of experimental areas is stressed. The median value for exchangeable potassium in the A1 horizon of Frodsley sandy loam was 0.31 m-equiv./100 g for "developed" areas and 0.35 m-equiv./100 g for "undeveloped" areas. Values for the A2 horizon were closely correlated with those for the corresponding surface horizon, but only about one-third as high. The amounts in the B horizon tended to approximate to those in the A1 horizon except for soils with high values in the surface. The potassium-supplying capacity of the soils was assessed by fractionation of the potassium into water-soluble, exchangeable, difficultly exchangeable, hydrochloric acid-soluble, and total potassium. In Frodsley sandy loam approximately 35 per cent. of the exchangeable potassium appeared in the water-soluble form, which suggested that there may be moderate losses from this soil by leaching. Ten minutes' boiling with normal nitric acid released only about 90 per cent. more potassium than was present in the exchangeable fraction. Boiling normal nitric acid extracted much less potassium from some samples of this soil type than did cold normal ammonium chloride from others, because of the great variability of exchangeable potassium. Concentrated hydrochloric acid dissolved a further 0.60.7 m-equiv./100 g on the average. These low values for the two latter fractions are taken to indicate the poverty of this soil type in reserves of potassium and, with the relatively low values for the exchangeable fraction, explain the widespread responses to potassium reported in field experiments. Examination of the minerals of the clay fraction of Frodsley sandy loam supported the chemical data in regard to the poor potassium status of these soils. Separation of sand, silt, and clay from the B horizons of two profiles showed that much of the total potassium was present in the coarser fractions of the soil. Type A, a soil associated with Frodsley sandy loam on river terraces, had a similar potassium status. Soils formed on dolerite were significantly higher in all categories of potassium.
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