HUMIC-ACID RELIEVES PH-INHIBITION OF NITRIFICATION IN CONTINUOUS-FLOW COLUMNS

Humic acids obtained from a commercial source and isolated from mushroom wheatstraw compost inhibited nitrification by Nitrosomonas europaea and Nitrobacter agilis in a continuous-flow column at concentrations in excess of 100-mu-g cm-3. Lowering the pH of the input medium also reduced the rate of nitrification but in the presence of the compost, and to a lesser extent the commercial humic acid, pH-inhibition was relieved. With the commercial humic acid, this effect can be explained in terms of the buffering capacity of the extract.