Summary Most of the Cu previously added to humic acid derived from a sedge fen peat was easily removed by washing with acid, but the strength of binding increased as the Cu content decreased. The capacity to retain firmly bound Cu was reduced to about half by treating the humic acid to block either carboxyl or hydroxyl groups alone or to block both groups together. The retaining sites blocked therefore probably involve both carboxyl and hydroxyl groups acting together, but alternatively could be groups such as hydroxy‐quinone, affected by both types of treatment. Retention of Cu by peat differs from retention by the extracted humic acid.

Ideally crop protection should prevent damaging effects of pests, diseases and weeds economically, safely and without harming the environment or inducing subsequent control problems. Present methods, based mainly on pesticides and resistant crop varieties, control many damaging organisms effectively but have important limitations. Vulnerability to the emergence of tolerant strains of pest or pathogen is probably the most severe; chemical methods are also often insufficiently selective and very wasteful. Dependence on these methods will continue, however, and it is therefore essential to seek ways of minimizing their deficiencies. The prospects for improvement are discussed in relation to the need for better intelligence about infestations and their consequences, the need to ensure that control measures remain effective and the need to improve the efficiency of utilization of crop protection agents. Implementation of the suggestions for improvement could require fundamental changes in the organization of crop protection practices.

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Adsorption of the systemic insecticide disulfoton (diethyl S-[a-(ethylthio) ethyl] phosphorothiolothionate) by soil was studied using a wet slurry technique. Extraction of soils and solutions after equilibration showed that more disulfoton was lost from solution than could be extracted from soil, principally because of microbial alteration and adsorption by glass. In two contrasting soils equilibration was complete by 3 h and air-dry soils in the laboratory adsorbed similarly to the moist field soils from which they were derived. Adsorption was fully reversible if desorption took place immediately after uptake when soils were still wet, but the release was modified when the soils were allowed to dry thoroughly between adsorption and desorption. The empirical Freundlich isotherm fitted adsorption results for 17 different soils well. The isotherms had different curvatures but deviations from linearity were small so that linear isotherms provide good approximations. Comparison of the slopes of the best-fitting linear relationships showed that adsorption was closely related to the amount of organic matter in the soil.