489 PB 137 LIME REACTION AND pH BUFFERING IN SOILLESS MEDIA

Abstract

Addition of lime to increase pH is generally essential for soilless media based on acidic organic materials. Media pH may decrease over time as the result of addition of acidic fertilizers. The objectives of this research were: to characterize reactions of conventional or finely ground lime in soilless media; to compare resistance to acidification in soilless media amended with conventional or finely ground lime; and to evaluate, for media containing rockwool, an equation to predict H+ activity (HA) of binary mixtures from HA of components. Various soilless media were amended with each type of lime at rates from 1 to 16 kg m-3 and incubated 8 weeks at 20 C. Subsamples were removed and pH was measured in saturated slurries. Finely ground lime was about twice as effective as conventional lime in adjusting pH to 6.0 within 1 week after mixing and wetting. Neither initial nor final HA of unlimed peat-rockwool mixes could not be predicted from HA of components. Three media were amended with each type of lime, planted or left unplanted, and irrigated with fertilizer solution with and without 1.0 N H2SO4. Final pH of media amended with finely ground lime averaged 0.2 units lower than media amended with conventional lime. Final pH of unplanted media averaged 0.5 units lower than planted media. Final pH of media fertilized with solution containing H2SO4 averaged 0.6 units lower than without. Addition of H2SO4 to fertilizer stimulated growth of New Guinea Impatiens (Impatiens xhybrida).