The stage of growth at which three plant species, a grass, a legume and a cereal, absorbed initially non-exchangeable potassium (Kn) from two soils, a red podzolic and a krasnozem, was examined by sequential harvesting in a glasshouse experiment. Large amounts of Kn were removed from the red podzolic soil, which contained greater amounts of micaceous material, in the order setaria > maize > lucerne. Uptake of Kn appeared insignificant after exchangeable Kn as measured on the bulk soil, had been depleted by about 60%, and its contribution to total plant uptake thereafter was independent of species. For setaria and maize, plant K concentrations at this time were above the critical level, so that Kn represents a potassium resource that can be exploited during normal growth of an appropriate species. Uptake of Kn from the krasnozem was relatively small. The red podzolic soil yielded much more Kn than a similar soil in earlier experiments. This was attributed to differences in clay mineralogy, possibly associated with differences in the weathering stages reached by the two soils. Thus, it is not possible to infer the pattern of release of Kn simply from a knowledge of Great Soil Group and parent material.

Five soils were cropped with four plant species in pot experiments in the glasshouse, without addition of potassium, until growth virtually ceased, after which the depleted soils were replanted with either the same species, or a different one, to test the reproducibility of the 'exhaustion' conditions imposed. Uptake of potassium by the plants exceeded the changes in exchangeable potassium in four soils; the excess amounts taken up differed markedly between species and these differences were reproducible on replanting. Uptake by setaria (Setaria anceps) and siratro (Macroptilium atropurpureum) exceeded that by Rhodes grass (Chloris gayana) and lucerne (Medicago sativa), except for one swelling clay soil from which uptake by lucerne equalled that by siratro. Most of the differences between species were attributed to the uptake of initially non-exchangeable potassium, which was removed in significant amounts from three of the soils. For these three soils, uptake from non-exchangeable sources was detected only after about 80% of the exchangeable potassium had been removed. It is postulated that the efficiency of plant removal of non-exchangeable potassium from soil is directly related to the degree to which plant roots can lower the concentration of potassium in the soil solution.

Setaria (Setaria sphacelata cv. Nandi) was grown in 10 surface soils, of contrasting properties, in pots until (after repeated harvesting of tops) plant growth virtually ceased. Uptake of potassium by tops and roots was correlated with change in exchangeable potassium during cropping, initial exchangeable potassium, and with quantities of potassium predicted from the immediate Q/I relations of the soils. Some initially non-exchangeable potassium was absorbed from seven of the soils by the plants, such that these soil quantity parameters (which were statistically equivalent) predicted two-thirds of the total potassium uptake averaged over all soils. Potassium uptake was also correlated with soil potassium soluble in constant-boiling hydrochloric acid, and with potassium extracted by a cation exchange resin (both of which predicted about twice the plant absorption). For six of the soils, uptake of initially non-exchangeable potassium was correlated with the quantity of total potassium present in each pot in the 1 0 . 1 pm clay fraction. The intensity of soil potassium (measured in dilute calcium chloride solution from the immediate Q/I relation) was reduced on all soils by exhaustive cropping to a mean value for the equilibrium activity ratio (AR0) of 4.7 10-4 mole1/2 L.-1/2 (�G = -4500 cal equiv.-1); it was suggested that the observed variations in this value were unlikely to prohibit its use for practical soil testing, except for the prediction of the quantity parameter for those soils for which the form of the isotherm is altered by cropping. A tentative value for the potential of soil potassium at which setaria has an adequate average potassium concentration in its tops was - 3600 cal equiv.-1 (AR0 2.1 x 10-4 mole1/2 L.-1/2).

Highly coloured, organic-stained water (black water) occurs as springs, seeps and perched lakes in the Cooloola area of south-eastern Queensland (latitude 26� S.). Springs of clear, colourless water (white water) sometimes occur in close proximity to the black water. Field evidence suggests that black waters move laterally along semipermeable B horizons of humus podzols, whereas white waters are part of a much larger storage system in which the water has been decolorized by contact with the C horizons. Thus it appears that black waters are the active eluviating agent and white waters are the residual liquid phase of the podzolization process. Chemical analyses of black and white waters, and the results of a laboratory leaching experiment, support this view. Possible mechanisms and some implications for the genesis of podzol B horizons are discussed.

Exhaustive cropping of four soils by four species revealed only small differences in the limiting intensity (expressed as the equilibrium activity ratio, ARo) of soil potassium, for which the mean value was 5.0 x 10-4 mole1/2 L 1/2. Replanting of depleted soils led to slightly lower values. With a fifth soil, which contained significant amounts of potassium felspar in both clay and sand fractions, grasses reduced the intensity to a lower level (3.8 x 10-4 mole1/2 L 1/2) than did legumes (21.0 x 10-4 mole1/2 L 1/2), and these values were substantially reproducible with replantings of the same species. It was suggested that the peculiar behaviour of this soil may have been associated with the differential ability of the various plants to use potassium from orthoclase. The intensity at which potassium was first absorbed from sources not initially in equilibrium with 0.001M calcium chloride varied from 8 to 29 x 10-4 depending on the soil, and, in the case of the felspathic soil, depending on the plant.