THE USE OF FINE-GRADE PERLITES IN LETTUCE BAG CULTURES. II. PHYSICAL PROPERTIES, RHEOLOGIC EFFECTS AND PRODUCTIVITY.
Physical properties, were analysed for unused, once and twice used perlites at the end of each crop in order to evaluate changes through use. Results showed that the small variations observed in these properties are not the main cause of the significant depression of bioproductivity associated with re-used perlites. Other reasons possibly related to "re-used depression" have been suggested.
Mechanical properties and air/water contents along the bag during drying/rewatering cycles were measured and related to root morphology, root density and bioproductivity of lettuce grown on each type of perlite. Higher water-holding capacity, more favourable mechanical properties and a near steady state of moisture along the profile of the bag give highest yield and bioproductivity parameters using fine-grade perlites rather than coarse-grade types.