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Summary Isotopically exchangeable cadmium and zinc (‘E values’) were measured on soils historically contaminated by sewage sludge and ones on zinc‐rich mine spoil. The E‐value assay involves determining the distribution of an added metal isotope, e.g. 109Cd, between the solid and solution phases of a soil suspension. The E values for both metals were found to be robust to changes in the position of the metal solid⇔solution equilibrium, even though the concentration of dissolved metal varied substantially with electrolyte composition and soil:solution ratio. Concentration of labile metal was also invariant over isotope equilibration times of 2–6 days. The use of a submicron filtration procedure, in addition to centrifuging at 2200 g, proved unnecessary if 0.1 m Ca electrolyte was used to suspend the soils. The proportion of ‘fixed’ metal, in non‐labile forms, apparently increased with increasing pH, although there was considerable variation in both sets of contaminated soil. Zinc and cadmium in the sludged soils were similarly labile. Several possible methods for the measurement of chemically reactive metal were explored for comparison with E values, including single extraction with 1 m CaCl2 and a ‘pool depletion’ (PD) method. The latter involves comparing solid⇔solution metal equilibria in two electrolytes with differing degrees of (solution) complex formation, 0.1 m Ca(NO3)2 and CaCl2. Both the single extraction and the PD method gave good estimates of E value for Cd, although the single extraction was more consistent. Neither technique was a useful substitute for determining labile Zn, because of weak chloro‐complexation of Zn2+. We therefore suggest that 1 m CaCl2 extraction of Cd alone be used as an alternative to E values to avoid the inconvenience of isotopic dilution procedures.

Phosphorus (P) is a finite natural resource and an essential plant macronutrient with major impact on crop productivity and global food security. Here, we demonstrate that time-resolved chlorophyll a fluorescence is a unique tool to monitor bioactive P in plants and can be used to detect latent P deficiency. When plants suffer from P deficiency, the shape of the time-dependent fluorescence transients is altered distinctively, as the so-called I step gradually straightens and eventually disappears. This effect is shown to be fully reversible, as P resupply leads to a rapid restoration of the I step. The fading I step suggests that the electron transport at photosystem I (PSI) is affected in P-deficient plants. This is corroborated by the observation that differences at the I step in chlorophyll a fluorescence transients from healthy and P-deficient plants can be completely eliminated through prior reduction of PSI by far-red illumination. Moreover, it is observed that the barley (Hordeum vulgare) mutant Viridis-zb(63), which is devoid of PSI activity, similarly does not display the I step. Among the essential plant nutrients, the effect of P deficiency is shown to be specific and sufficiently sensitive to enable rapid in situ determination of latent P deficiency across different plant species, thereby providing a unique tool for timely remediation of P deficiency in agriculture.

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