Three lettuce experiments were conducted in which raw seeds, seeds with 5-to-1 and 10-to-1 clay-to-seed coating ratios, and fully coated seeds were compared when planted under soil, and when planted under soil sprayed on the surface with petroleum mulch, and when planted under stabilized vermiculite, or petroleum coke. Stabilized vermiculite and petroleum coke increased the percentage of emergence and lowered the time required, regardless of seed coating. Petroleum mulch also improved emergence, but not as much as the other materials. Counts showed that both 10-to-1 and 5-to-1 coated seeds improved, to a considerable extent, on the slower and poorer emergence that had been observed with fully coated seeds. Both minimum seed coatings and replacement of the soil seed covering by vermiculite or coke offer real advantages for the precision planting of lettuce.

Germain's Inc. contributed seed for these tests; Spreckels Sugar Co. contributed land; Western Rain Bird contributed sprinklers; and Collier Carbon and Chemical Corp., California Zonolite Co., and Moyer Chemical Co. contributed the covering materials.