Influence of humic substances on rooting and nutrient content of creeping bentgrass

Two greenhouse experiments were conducted to determine if application of humic substances to creeping bentgrass (Agrostis stolonifera L.) turf would improve root mass, root length, or nutrient uptake. A commercially mined granular humate, a commercial liquid humic acid (HA), and liquid HAs extracted from leonardite, peat, and soil were applied to creeping bentgrass growing in either sand or solution culture and maintained at a height of 6 mm. Foliar applications included monthly or biweekly applications of HAs at 0, 100, 200, or 400 mg HA L-1. In sand culture, humate incorporated to a depth of 10 cm stimulated a 45% increase in root mass at the 0- to 10-cm depth and a 38% increase in root mass at the 10- to 20-cm depth compared with the control. Incorporation of granular humate increased maximum root length 15% compared,vith non-treated turf in sand culture. No foliar applied HA source consistently provided rooting superior to the control in either sand or solution culture. Nitrogen, Ca, Mg, and Fe uptake were relatively unaffected by humic substance application. The phosphorous concentration of plants in sand culture was increased 3 to 5% by incorporated humate and foliar application of soil, peat, or Leonardite-derived HA. In solution culture, however, P uptake was unaffected by HA application. The lack of improved rooting or increased P uptake in solution culture supports the hypothesis that humic acids may have limited growth promoting effects on plants adequately supplied with nutrients.