Humic acid seed and substrate treatments promote seedling root development

Seed of Pelargonium xhortorum L.H. Bailey 'Freckles' (geranium) and Tagetes patula L. 'Bonanza' (marigold) were soaked for 12, 24, or 48 h in solution s containing 0 (deionized water), 5000, 10,000, or 15,000 mg.L-1 humic acid (HA) or nutrient controls (NC) containing similar levels of nutrients prio r to planting. Soaking in deionized water (DI) and NC treatments had no sig nificant effect on root fresh weight. However, several of the HA treatments increased root fresh weight of marigold seedlings, and all increased geran ium root fresh weight. Percentage of germination and shoot fresh weight wer e not significantly affected by treatment. Seed of Cucumis sativus L. 'Sala d Bush' (cucumber), Cucurbita pepo L. 'Golden Summer Crookneck' (squash), ' Freckles' geranium and 'Bonanza' marigold were sown into 15-cell plug trays (5 mL, volume), and the substrate was drenched with DI, 2500 or 5000 mg.L- 1 HA, or 2500 or 5000 mg.L-1 NC. DI and NC treatments did not affect root f resh weight. However, cucumber, squash, and marigold seedlings germinated i n substrate drenched with 2500 and 5000 mg.L-1 HA and geranium seedlings ge rminated in substrate drenched with 2500 mg.L-1 HA had significantly higher root fresh weight than did seedlings from all other treatments. Percentage of germination and shoot fresh weight were not significantly affected by t reatment. 'Salad Bush' cucumber and 'Golden Summer Crookneck' squash seedli ngs germinated on germination towels soaked with 2500 or 5000 mg.L-1 HA, ha d significantly higher root fresh weight than did seedlings germinated on t owels soaked with DI or NC solutions. Treatment with HA did not affect shoo t fresh weight or the number of lateral roots. However, HA treatment increa sed the total length of lateral roots. The increase in lateral root growth occurred primarily in lateral roots developing from the lower hypocotyl.