Restrictions to root growth limit the yield of shoots of irrigated white clover

Pasture yields in irrigated northern Victoria are low. This experiment examined the degree to which restrictions in root growth explained the low shoot yield of white clover (Trifolium repens) grown in irrigated soils. Soils were collected as intact cores from various field sites producing from well above to well below average pasture growth. A vegetative cutting of white clover was planted into each soil core and the growth of the shoot and root systems was monitored over a 38 day period by a series of destructive harvests. All plants were managed to minimize water, oxygen or nutrient stresses. There were large effects of treatments on the productivity of plants with growth in the soil cores reflecting the field productivity of each soil. This suggested that despite managing the cores carefully, it was not possible to overcome the limitations to plant yield that occur in these soils in the field. The most productive soil was one for which the profile had been physically modified. Measures of soil physical properties (bulk density, air-filled porosity, volumetric water content, penetrometer resistance) were collected from the field sites when the soil was at field capacity, but it was not possible to identify which of these were associated with the differences in productivity between soil core treatments. No measures of the soil physical conditions were made in the cores, but it is possible that the field data did not accurately reflect conditions in the soil cores, particularly during each daily watering. There were strong correlations between shoot and root production whether measured in terms of dry weight or morphological characteristics. The most important characteristic of highly productive soil was the capacity to support the rapid proliferation of a large root system. Improved forage yield will only be possible if the potential for white clover to produce roots in irrigated soils is increased.