pH-Dependent surface properties of perlite: Effects of plant growth

Perlite is a glassy volcanic rock with a rhyolitic composition, traditionally used in the construction and chemical industries. In the last two decades perlite has become an important soilless growing medium, and is used for potting mixes in the horticultural industry. Plant growth induces significant changes in the medium and, therefore, growing plants in perlite poses an interesting dilemma regarding the applicability of the predetermined properties of the fresh material to the changing conditions as the plants develop. To fill this gap in our knowledge we have investigated the influence of plants on the surface properties of perlite, and the kinetics of release of pH- and ionic-strength-dependent elements from perlite, both in intact and used (after plant growth) systems. The effects of plants on the chemical composition of perlite were found to be significant, especially in enhancing the water-soluble P, Ca and Mg concentrations. Reaction kinetics were characterised by rapid H+ consumption and element releases during the first 24 h, followed by constant-rate (zero-order) H+ consumption and dissolution reactions. Release of nutritional elements such as Ca, Mg, K and P into solution was higher in used perlite (Up) than in new intact perlite (Np) and the rate decreased with advancing time and increasing pH in both Up and Np suspensions. The pH titration lines obtained with three different NaCl concentrations overlapped and it was not possible to detect any intersection point. The quantities of acid needed for reaching the desired pH were higher in Up, but most of this difference resulted from H+ consumption through dissolution reactions that enhanced release of ions into the solution. Solution-Si concentrations exhibited pH-dependent behaviour and, contrary to the trend for cations, decreased as ionic strength increased. Caution is advised when new perlite used as a growth substrate is maintained below pH 5, because the Al concentration could exceed toxic levels.