Temporal changes in chemical properties of air-dried stored soils and their interpretation for long-term experiments

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 less than or equal to 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.