Additions of several categories of organic materials including fresh plant materials, compost, manure, coal derived products, and peat have been shown to ameliorate soil acidity, The main effect is increased soil pH and the corresponding decrease in soil solution and exchangeable aluminum (Al). The organic matter addition is expected to decrease the solubility of soil aluminum so that lower activities are maintained at a given pH value, but this effect has not been tested. Aluminum solubility was investigated after addition of brown coal and peat to an acid soil. Root length was used as a bioassay for the acid ameliorating properties of the organic materials. Addition to soil of brown coal and peat resulted in changes in Al activity in the soil solution. The negative log of Al activity (pAl) was directly proportional to the soil solution pH. A single pAl-pH line could describe the control, calcium chloride and organically treated soil samples. This line was parallel to that of gibbsite. The intercept was greater suggesting undersaturation with respect to that mineral. The coal and peat applied at the rate of 1 and 2% had little effect on the solubility of soil Al. Any decrease in Al activity was solely dependent on increased soil pH. Relative root length was inversely related to Al activity. This relationship was improved by including base cations in the index of Al phytotoxicity. This index could provide the basis for evaluating the value of brown coal and peat in ameliorating soil acidity.