Cone index and root growth in surface and subsurface microirrigated hardpan soil

Restricted root growth caused by subsurface hardpans and low water holding capacity reduces crop yields in many United States southeastern Coastal Plain soils. With intensive irrigation it is possible to obtain suitable yields without deep tillage. The objective of this study was to find differences of root growth and cone indices between surface and subsurface applied sources of irrigation water. We measured root growth and cone index in intensively managed irrigation plots of green beans (Phaseolus vulgaris L.) in 1988 and 1989. Microirrigation tubes were placed on the surface or in the subsurface — buried at a depth of approximately 0.25 m — and irrigated either continuously or with intermittent pulses of water. Mean profile cone indices for the surface tube placement were significantly lower than for the subsurface tube placement. Mean profile cone indices for the continuous irrigation treatment were significant lower than for the intermittent irrigation treatment. However, no one treatment significantly reduced the cone indices within the hardpan. Total root count was significantly greater for the subsurface tube placement with increased root growth below the 0.1-m depth. There was a high density of roots next to the subsurface tube that would have been effective in uptake of water from the microirrigation tube. However, yield was significantly greater for the surface tube placement. For all treatments, the largest concentration of root growth occurred in the top 0.2 m of the soil within the row. We expected this for the surface irrigation treatment. However, we also measured this for the subsurface treatment, probably because of high rainfall and upwelling of water from the subsurface micro-irrigation tube.