Acidification of Soilless Substrate by 13 Floricultural Crops

Impacts on substrate pH by 13 major floricultural crops fertilized with a neutral fertilizer were evaluated. Plants were grown for 78 days in 16.5 cm diameter (1.8L) plastic pots filled with 3 sphagnum peat moss: 1 perlite (v:v) substrate amended with CaCO₃ powder to establish an initial target pH of 6.5. Plants were watered with deionized water upon transplanting. Beginning with the second irrigation at 4 DAT, a neutral 17N-2.2P-14.1K fertilizer dissolved in deionized water was applied with each irrigation at 100 or 200 mg·L^-1 N.  Substrate pH was monitored and ΔpH, the difference between initial pH measured 4 DAT and final pH measured 78 DAT, was used to determine the acidification levels of crops. Petunia (ΔpH=0.14), begonia (0.19), and osteospermum (0.43) were very light acidifier-species (ΔpH <0.5). Pansy (ΔpH=0.51), impatiens (0.79), New Guinea impatiens (0.89), and geranium (0.97) were light acidifier-species (ΔpH=0.5-1.0). Vinca (ΔpH=1.00), tomato (1.17), and Reiger begonia (1.46) were medium acidifier-species (ΔpH=1.0-1.5). Pot mum (ΔpH=1.56), sunflower (2.44), and kalanchoe (2.45) were heavy acidifier-species (ΔpH=1.50-2.50). However, the former three acidification groups of plants were fertilized at the rate of 100 mg·L^-1 N while this final fourth group was fertilized with 200 mg·L^-1 N to comply with fertilization recommendations. Overall, substrate pH declined rapidly for the initial 24 days in all crops. Plant nutrient uptake effect on substrate was probably minimal due to low biomass during this initial period allowing the acidic fertilizer solution (pH 4.6-4.8) to have the predominant effect. After 25 DAT the physiological pH effect of nutrient uptake from the neutral fertilizer became more important due to rapid plant growth. During the post 25 DAT period, pH effects of species were clearly seen. Begonia and petunia caused a rise in substrate pH from 6.4 and 6.2 to 6.7, respectively while pH remained essentially constant for osteospermum. The net pH effect in the light and medium acidifier crops was a decline, but there was a substrate pH plateau after 39 DAT for New Guinea impatiens and pansy and a similar plateau between 24 and 55 DAT for tomato and vinca. The rapid pH decline after 25 DAT for the heavy acidifier crops could be due in large part to the higher 200 mg·L^-1 N fertilizer concentration.