Addition of lime to increase pH is generally required 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 limestone in soilless media and to compare resistance to acidification in soilless media amended with conventional or finely ground limestone. Various soilless media were amended with each type of lime at rates from 1 to 16 kg/m3 and incubated eight weeks at 20 degrees C. Subsamples were removed and pH was measured in saturated slurries. About half as much finely ground limestone as conventional lime was required to adjust pH to 6.0 within one week after mixing and wetting. Neither initial nor final pH of unlimed peat-rockwool mixes could be predicted from pH of components. In successive experiments with poinsettias (Euphorbia pulcherrima) and New Guinea Impatiens (Impatiens xhybrida), three media were amended with each type of lime to obtain pH approximately six after four days from mixing, planted or left unplanted, and irrigated with fertilizer solution with and without 0.5 mM sulfuric acid (H2SO4). With poinsettias, final media pH was significantly affected only by addition of acid. With impatiens, final pH of media amended with finely ground limestone averaged 0.2 units lower than media amended with conventional limestone. 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.
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