Availability of macrocations in perlite and pumice as influenced by the application of nutrient solutions having different cation concentration ratios
Unused pumice and perlite substrates were placed in 4 litre pots and washed (2 litres per pot) with 2 nutrient solutions differing in the K: Ca: Mg ratio (A = 2.8:3:1, B = 5.6:3:1; ratios on equivalent basis). After application of the nutrient solutions, substrate samples were collected and sent to chemical analysis to determine the water extractable and the exchangeable Ca, Mg, K, and Na contents. In addition, samples of pumice and perlite not supplied with nutrient solution were also collected and analyzed. The results revealed a cation exchange capacity (meq per 100 g of substrate) of 2.2-2.4 in perlite, 5.6-6.0 in unsieved pumice (0-8 mm) and 7.3-7.9 in sieved pumice (0-2 mm). Washing perlite with nutrient solution significantly increased both the water extractable and the exchangeable Ca, Mg, K and Na contents, to levels depending on the K: Ca: Mg ratio in the supplied nutrient solution. Unlike in perlite, the exchangeable Ca, Mg, and Na contents in pumice were not significantly altered by the addition of nutrient solution. The water extractable Ca and Mg contents were somewhat higher only in the 0-2 mm fraction of pumice after washing the substrate with nutrient solution, whilst in the unsieved samples (0-8 mm) no significant differences could be found. In contrast to the other macrocations, the addition of nutrient solution increased both, the water extractable and the exchangeable K in pumice, to levels clearly related to the K: Ca: Mg ratios in the supplied nutrient solution.