PH IN ROCKWOOL PROPAGATION BLOCKS: A METHOD TO MEASURE THE PH BUFFER CAPACITY OF ROCKWOOL AND OTHER MINERAL ROOTING MEDIA

Within the framework of the international KIWA quality label for rooting media representatives of plant propagators and the rockwool industry agreed to develop a method to characterize the influence of the rooting medium materials on the pH in cultivation. A method for the characterization of building materials was studied and the instrumental set up was replicated in a laboratory. In a series of experiments various parameters were optimized to characterize the pH buffer capacity of rockwool and perlite. In the final method a solution of pH 5 was recirculated at a set pace over a sample of rockwool. Dissolution of the material resulted in a gradual pH raise which was corrected by the addition of hydrochloric acid every hour for five minutes. After nine more cycles of pH rise and correction, the final consumption of acid per unit weight of sample was calculated. A norm proposal aims at characterizing the maximum pH level to be reached and the capacity of the material to influence the pH. Typical values were a maximum pH of well below 7.0 and a pH buffering capacity of 5-10 mmol hydrogen ions per kilogram of dry rockwool and 10-20 mmol of hydrogen ions per kilogram of perlite.