COMPARISON OF SPURWAY AND SATURATED MEDIA EXTRACTS OF SOILLESS MEDIA

Seedlings of Catharanthus roseus 'Grape Cooler' transplanted into 120- cm3 cell packs of three commercial potting media were fertilized at ea ch irrigation in flood and drain trays with commercial 20N-4.4P-17K fe rtilizer at concentrations of 50, 150, or 250 ppm nitrogen (N). Media samples were obtained 2, 3, 4 and 5 weeks post transplant. The content s of four cells from each of two replications were airdried, mixed, an d sifted to pass a 2 mm sieve. Concentrations of calcium (Ca), magnesi um (Mg), potassium (K), phosphorus (P), ammonium (NH4+), and nitrate ( NO3-) were measured in Spurway (SPUR) and Saturated Media (SME) extrac ts. Electrical conductivity (EC) was measured in SME extracts. Leaf sa mples obtained four weeks post transplant were analyzed for all essent ial elements. Highly significant correlations were obtained for nutrie nt concentrations in SME and SPUR, but the coefficient of determinatio n (r2) for linear regressions ranged from 0.34 for Ca to 0.77 for NH4. Trends over time for increasing concentrations of Ca and Mg were det ected in SME, but not SPUR. SME and SPUR indicated generally similar t rends for NH4+ and NO3-. Both media extracts were correlated with plan t tissue for P, but not other elements. Media EC increased with time a nd with increasing fertilizer concentration. EC in all media fertilize d with 250 ppm N exceeded 4.5 dS/m at the final harvest. Plant growth responded positively to increasing fertilizer concentration to 150 ppm N in two media, probably due to increased accumulation of both N and P. Decreased growth at 250 ppm was related to high EC.