(195) Evaluation of Solid Artificial Media on Lettuce Seedling Growth and Anchorage

Abstract

A study was conducted to compare three sterile, commonly used soil-less media (Agrifoam™ and Oasis™, growth foams, and Grodan™, an expanded rockwool substrate) to determine which media characteristics favor seedling development and establishment in hydroponic systems. These media were studied during days 7–10 of the seed germination stage, when one often observes with the use of foam media the occurrence of “pop-outs,” a disorder in which actively growing roots do not penetrate and spiral on the media surface causing the root tip to die. High percentages of pop-outs cause poor seedling stand, and discourage the use of soilless media for seedling germination. Pop-outs were more frequently observed in Agrifoam (50%) compared to the other media, with Oasis intermediate (15%), and Grodan least affected (>5%). Both physical and chemical characteristics were analyzed to isolate the causes of pop-outs. Oasis and Agrifoam both have higher water retention, and pop-outs increased as the water content of the foam increased. High soluble salts, particularly high magnesium in the root zone, produced roots that were “burned” and did not grow. When magnesium salts in concentrations equivalent to that found in Agrifoam were added to Grodan (control substrate), the number of pop-outs increased. The roots were stunted with little or no root hairs, resembling roots when grown in Agrifoam. We conclude that the high incident of pop-outs in Agrifoam, and to a lesser extent, in Oasis, is caused by high-medium water content and toxic levels of magnesium ions in solution.