Soils are identified as being “acid” by their low pH, although H per se is rarely the limitation to plant growth. Jenny [1] perceived that, during the 25 years that preceded his review in 1961, soil acidity research had come full-circle, from the identification by Veitch [2] that acid clays were Al clays to the 1960s interest in solution chemistry and exchange behavior of Al. Jenny felt that agronomists, who had once thought that plants in an acid soil were confronted by H ions, had received a new stimulus with the recognition of Al toxicity as a limitation to plant growth in acid soils. He conjectured that “Presumably the plants knew this all along.” Forty years after Jenny’s review, the challenge to soil researchers remains the same, namely to identify which of the many potential effects of soil acidity is limiting plant growth.