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Evaluation of Solid Artificial Media on Lettuce Seedling Germination and Growth.  J.A. Montgomery, H.C. Wien Department of Horticulture, Cornell Univ., Ithaca, NY.

   A study was conducted to compare three sterile, soil-less media (Agrifoam and Oasis, growth foams, and Grodan, an expanded rockwool substrate) to determine which media characteristics favor seedling development and establishment. These media commonly used in hydroponic systems were studied during the seed germination stage, days 7-10. One problem observed with the use of foam media is the occurrence of "pop-outs", a disorder in which actively growing roots do not properly penetrate and spiral on the media surface causing the root tip to die. High percentages of pop-outs cause poor seedling stand, and prevent the advancement of soil-less media as the sole source of seedling germination. The occurrence of pop-outs was more frequently observed in Agrifoam as compared to the other media. Both physical and chemical characteristics were analyzed in order to isolate those which lead to pop-outs. High soluble salts, particularly high magnesium in the root zone produced roots that were "burned" and did not grow. This in combination with low pH and a decrease in number of cation exchange sites lead to toxicity in the roots. When magnesium salts in concentrations equivalent to that found in Agrifoam were added to Grodan (control substrate), the number of pop-outs increased and the roots resembled those grown in Agrifoam. Root hairs did not form as readily in Oasis and Agrifoam as in Grodan, possibly leading to poor anchorage and increased pop-outs. The amount of watering also contributed to the severity of pop-outs. If foam sheets are watered too little, the newly exposed roots dry up and die before they are able to anchor into the medium. If the foams are too wet, pop-outs and spiraling roots occur to a greater extent. A flooded root zone might also allow more cations in the foam to go into solution and to be taken up by the roots.