Impact of pre-plant root substrate amendments on soilless substrate EC, pH, and nutrient availability

Pre-plant root substrate amendments have a great impact on the soluble salt level of plug seedling substrates. Nine common pre-plant amendments alone and a typical commercial combination of some of these showed that the greatest contributors to seedling substrate (2 sphagnum peat moss: 1 perlite: 1 vermiculite by volume) electrical conductivity (EC) level were gypsum, calcium nitrate, and potassium nitrate. Moderate contributions were realized from triple superphosphate, Epsom salt, and two commercial micronutrient mixes. The salt contributions from dolomitic limestone and wetting agent were small and of little commercial concern. At the most common application rates for seedling substrate, additions of gypsum, calcium nitrate, potassium nitrate, and the mixture raised the total substrate EC (saturated media extract procedure) by 1.47, 1.23, 1.09, and 1.57 dS m(-1), respectively, from an initial no amendment level of 0.84 dS m(-1). The resulting EC levels were unacceptable according to the current maximum standards. Seedling efficacy tests should be conducted with special attention paid to gypsum, calcium nitrate, and potassium nitrate for possible reduction or elimination from the pre-plant nutrient charge. During the 12 days following sowing, mean substrate EC for all amendments in the 0% leach treatment declined 19% while EC in the 20% leach treatment declined 36%. Of all the pre-plant amendments tested, the only single amendment that increased substrate pH over the control treatment was dolomitic limestone. All other amendments lowered substrate pH by 0.1 to 0.24 units when applied at standard commercial rates. When dolomitic limestone was incorporated in combination with other amendments in the mixture treatment, the substrate pH rise was not as great. The impacts of rate of nutrient amendments on nutrient availability are presented.