SEEDLING EFFECT ON ROOT SUBSTRATE pH

Abstract

Paper used for seed germination tests may or may not contain limestone. The presence of limestone presented a pH buffer in this study that could interfere with studies addressing pH, calcium (Ca), or magnesium (Mg) effects on seedling development. Alternatively, pH of unbuffered chromatography paper was changed sufficiently by seedlings to confound research objectives that are pH dependent. Gradients across unbuffered substrate paper exceeded 1.5 pH units from a point under seedling roots to a distance 5 cm away. Precise measurements of effects of seedlings on substrate pH need to be taken in close proximity to the roots. Seedling effect on unlimed paper substrate pH varied across 25 taxa from a maximum rise of 1.4 units with zinnia to a decline of 1.1 units with tomato in the interim from sowing to separation of cotyledons. In general, pH levels in paper were higher when a basic complete nutrient solution with all nitrogen (N) in NO3 form was supplied compared to an acidic solution with N comprised of 40% NH4 + 60% NO3. In a peat moss:perlite substrate, 11 taxa altered substrate pH over a range of 0.6 units, much lower than on paper. While five taxa had similar pH enhancing or suppressing influences in the paper and peat moss: perlite substrates, six shifted the direction of the pH change. The effects of most taxa on pH were similar when fertilized with acidic or basic fertilizers. Exceptions included pansy (Viola x wittrockiana Gams.), petunia (Petunia x hybrida Vilm.-Andr.), snapdragon (Antirrhinum majus L.), and verbena (Veberna x hybrida Voss) that responded to basic fertilizer with higher substrate pH rises than anticipated. Only modest shifts occurred in the order of taxa in terms of their effects on substrate pH over time in the peat moss:perlite substrate. Taxa that tended to raise peat moss:perlite substrate pH regardless of fertilizer type or time included pansy, petunia, and vinca (Catharanthus roseus G. Don) while those that lowered pH were celosia (Celosia cristata L.), tomato (Lycopersicon esculentum Mill.), and zinnia (Zinnia elegans Jacq.).