Maintaining soil fertility and sustaining or increasing crop yield is of worldwide importance. Many factors impact upon the complex biological, chemical and physical processes which govern soil fertility. Changes in fertility caused by acidification, declining levels of organic matter, or P and K status may take many years to appear. These properties can in turn be affected by external influences such as atmospheric pollution, global change, or changes in land management practice. Long-term experiments provide the best practical means of studying changes in soil properties and processes and providing information for farmers, scientists and policy makers. This paper shows how the experiments run at Rothamsted in southeast England continue to provide data which are highly relevant to today's agriculture and wider environmental concerns. Examples are given of how crop yield is affected by soil organic matter, by pests and disease and by P nutrition. The effect of atmospheric pollution on soil acidity and the mobilization of heavy metals are also examined. The need for making better use of existing long-term experiments is stressed. Key words: Soil fertility, sustainability, long-term experiments, global change