Isotopic exchange studies were applied to the laboratory and glasshouse measurement of labile zinc in 25 soils from nine Great Soil Groups. The laboratory equilibration procedures worked well with acidic and most near neutral soils, but may overestimate labile zinc values for the lateritic podzolic soils. The values for some acidic soils were also compromised because of lack of isotopic equilibrium. The laboratory method gave erratic and unrealistic data when applied to alkaline soils due to fixation of the added zinc. The procedures based on the specific activity of zinc absorbed by plants from soils equilibrated with carrier-free 65Zn gave reproducible values of the total amount of plant available zinc for all soils. These values agree well with the corresponding laboratory data for acidic soils. Furthermore, the specific activity data showed that magnesium chloride and EDTA extractions had equilibrated with the same chemical form or forms of zinc as that absorbed by the plants.
Representatives of nine soil groups were extracted with reagents that have been used to predict zinc deficiency. The amounts of soil zinc removed were discussed in terms of specific and non-specific bonding in relation to the reagent used. The desorption of natural zinc was also described in terms of the quantity/intensity (Q/I) relation and an equilibrium zinc concentration (ZnQ) at natural pH. The ZnQ values varied from 1 to 4 �gI. for the alkaline soils and 8-190 pg/l. for the acidic soils. The Q/I ratio was derived by radioisotopic and chemical isotherm procedures. Log Q/I (I = total soluble zinc) approximated closely a linear relation of unit slope with pH. This was ascnbed to a common reation of zinc with all soils by specific sorption dominated by ZnOHA ions such that Q/I� = constant where I' = (ZnOH+aq). Deviations from this relation are discussed. The relations between soil and plant (clover and wheat) variables were studied by simple and multiple regression analysis. Single values of intensity variables, and, to a lesser extent, Q/I variables, correlated well with plant data but not the quantity variables. The improved correlations of Q variables when combined with Q/I variables accorded with published work. The problem of predicting zinc availability on alkaline soils which are dominated by very high Q/I values, is discussed.
The residual value of superphosphate on a red-brown earth naturally deficient in phosphate has been determined by means of a field experiment. pot trials. and chemical extractions . In a field experiment on a site where 24½ cwt of superphosphate per acre had been previously applied over a period of 23 years, it was found that wheat. in a wheat-barley-fallow rotation. could be grown for 5 years before a clearly defined response developed to superphosphate currently applied at 11 lb (P1) or 22 lb (P2) phosphorus pentoxide (P2O5 ) per acre . Stubble-sown barley showed a response after the first year.
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.
In a glasshouse experiment, lettuce was grown on a market garden soil near Adelaide, South Australia, to investigate how yields and foliar concentrations of Cd, Cu, Ni, Mn, Pb, and Zn are affected by applications of sewage sludges of different origin. One sludge came from a modern treatment plant and the other from an abandoned treatment farm now within the Adelaide suburban area. Heat sterilization of one of the sludges decreased the uptake of Cd, Cu, and Zn, and especially of Ni and Mn. Fertilizing with sludges alone gave poor yields thus indicating the need for supplementary nutrients. High rates of sludge application gave marked increases in some metal concentrations in the lettuce. The ease with which metals in the sludges could be absorbed by lettuce decreased in the following order: Cd > Zn > Ni > Cu >> Pb. The effect of pot size is discussed. Monitoring of produce may be required if sludge is used at high rates in intensive crop production on a similar soil.
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