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Changes in the extractability and uptake by crops of sludge metals in a long‐term field experiment, started in 1942, were measured to assess whether Zn and Cd are either fixed by the sludge/soil constituents or are released as the sludge organic matter (OM) decomposes. Total and 0.1 M CaCl2‐extractable concentrations of Zn and Cd in soil and total concentrations in crops were measured on archived crop and soil samples. Extractability of Zn as a proportion of the total ranged from 0.5 to 3% and that of Cd from 4 to 18%, and were higher in sludge‐amended than farmyard manure or fertilizer‐amended soils. Over a 23‐yr period after 1961, when sludge was last applied, the extractability of both metals fluctuated, but neither decreased or increased consistently. The relationships between total soil and crop metal concentrations were linear, with no evidence of a plateau across the range of soil metal concentrations achieved. The slopes of the soil‐plant relationships depended on the type of crop or crop part examined, but were generally in the order red beet (Beta vulgaris L.) > sugar beet (Beta vulgaris L.) > carrot (Daucus carota L.) > barley (Hordeum vulgare L.). However, there also were large seasonal differences in metal concentrations in the crops. It is concluded from the available evidence that up to 23 yr after sludge applications cease, Zn and Cd extractability and bioavailability do not decrease.

The most widely used methods for determination of total phosphorus in soils are sodium carbonate fusion and perchloric acid digestion.The fusion method is unsuitable for routine analysis, whereas the perchloric acid digestion method is potentially hazardous and can give low recovery. This paper compares an alternative digestion method, using aqua regia, with Na 2 CO 3 fusion, and also compares inductively coupled plasma atomic emission spectroscopy (ICP) with colourimetry for the measurement of total P. For two internationally certified standard soils, results from fusion-colourimetry, fusion-ICP and aqua regia-ICP were in good agreement with the certified mean values and within the 95% confidence limits. Twenty-three soils were selected from England and Wales to represent a wide range of soil properties. Good agreement was obtained between aqua regia-ICP and fusion-colourimetry for these and the two certified soils. Agreement between aqua regia-ICP and fusion-ICP was poorer, and was probably due to lower sensitivity of fusion-ICP, resulting from the large dilution of the fusion solution, which was necessary to overcome the large concentration of Na present. Also, aqua regia-ICP gave better precision than fusion-ICP. 1358 CROSLAND ET AL.Aqua regia-ICP is suitable for analysis of a wide range of soils, it is easy to operate, rapid and relatively safe.

No abstract

The German limit for Cd in wheat grain of 0.1 mg kg-' fresh wt. (0.12 mg kg-' dry wt.) (BGA, 1986) has been mistakenly labeled as the WHO limit in the above paper. There is as yet no FAO/WHO Cd in wheat grain limit.