Summary We have examined the effect that acid deposition and other sources of acidity have had over the last 110–140 years on soil under woodland (Broadbalk and Geescroft Wildernesses) and grassland (Park Grass) comprising some of the Classical Experiments at Rothamsted Experimental Station. Changes in soil chemistry have been followed by analysing some of the unique archive of stored samples for pH, water‐soluble and exchangeable base cations, aluminium, iron and manganese, exchangeable acidity, cation exchange capacity (CEC) and soluble anions. Proton balances and historical data show the importance of acid deposition to acidification and concomitant changes in the chemistry of the soil. The pH of the surface soil of Geescroft Wilderness has fallen from 6.2 to 3.8 since 1883. The decrease in the pH of the unlimed, unfertilized plot on Park Grass was less over a similar period (from pH 5.2 to 4.2), illustrating the significant effect of the woodland canopy on the interception of acidifying pollutants. The effect of increasing acidity on the soil chemistry of Geescroft Wilderness is seen in its decreasing base saturation and CEC, with base cations moving down the soil profile. Clay minerals are being irreversibly weathered, and Mn and Al progressively mobilized, so that today Al occupies 70% of the exchange complex in the surface soil. Even with present reductions in sulphur deposition critical loads for sulphur, nitrogen and acidity are still exceeded. Such semi‐natural ecosystems are unsustainable under the current climate of pollution.

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Summary The usefulness of stored soils from long‐term experiments is often questioned because of changes that might occur during storage. We examined changes during long‐term storage (8–69 years) in the chemical properties of soils with a range of pH values (3.4–8.1 in water) from woodland and grassland experiments at Rothamsted Experimental Station in the UK. No significant changes during storage were measured for total C and N. Large but erratic changes in exchangeable Na+ content between 1959 and 1991 were probably caused by contamination of the 1959 samples by perspiration and from sodium‐based glassware. Exchangeable K+ increased during storage but only by a small amount. Small changes in exchangeable Ca2+ and Mg2+ were measured in some samples but not in others. Generally the amount of exchangeable cations increased slightly during storage. This is probably linked to the decreases of  0.4 units in the pH of acid soils, which we attribute to the hydrolysis of approximately 0.25% of the exchangeable Al3+. A doubling of the amount of exchangeable Mn2+ during storage for 32 years was probably caused by re‐equilibration of Mn species. The most practicable way to prepare soil samples for long‐term storage is to dry them in air. However, those who study changes in soil by re‐analysing samples of the soil stored for a long time must (i) use the same methods of analysis, or (ii) demonstrate that different methods lead to the same results, and (iii) know what changes can arise during storage.

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