Soil samples collected from the Breton plots, which had received various treatments and had been cropped to a 5-yr rotation of cereals and forages over a 40-yr period, were analyzed to determine the effects of the treatments on soil acidity. Treatments included applications of fertilizers at low rates, infrequent applications of lime, applications of manure and applications of various combinations of these. Those receiving NS, NPS and NPKS were more acidic than the check plots and those receiving lime, lime + NPKS and P as 0–45–0 were less acidic. Plots treated with manure or manure + NPKS were not acidified. Since 1967, a brome–alfalfa mixture has been seeded and on the more acidic plots the established stand contained less than 30% alfalfa as compared to greater than 70% in the lime or P (0–45–0) treatments. Liming one half of each plot in each series in 1972 significantly increased the stand and reduced the aluminum content of alfalfa, especially in the more acidic plots. Alfalfa grown on these more acidic plots had, in general, higher Mn and Al and lower N contents than did alfalfa in the limed portions of the plots. The poor growth of alfalfa on plots receiving NS, NPS and NPKS is attributed to the acidifying effects of the fertilizers being sufficient to inhibit nitrogen fixation and induce some toxic concentrations of Al and Mn in the soil.