Wednesday, November 7, 2007
271-20
Soil Bulk Density and Strength Effects on Seedling Growth in Annual Ryegrass.
Paul Bartholomew, USDA-ARS Grazinglands Research Laboratory, Langston University, Langston, OK 73050 and Robert Williams, PO Box 1730, USDA-ARS Grazinglands Research Laboratory, Langston University, Langston, OK 73050.
No-till seeding is commonly found to delay establishment and inhibit early growth of forage grasses, compared with sowing after tillage. Among several possible causes of retarded growth following no-till planting of forage species, the effects of soil compaction have received little attention. Studies were carried out in controlled environment to assess the effects of increased soil bulk density of two soils, a Coyle series clay loam (fine-loamy siliceous thermic Udic Arguistoll) and a Stephenville series fine sandy loam (fine-loamy siliceous active thermic Ultic Haplustalfs). Soil was packed into plastic pipe of 75mm diameter, to make soil columns of 60 x 75mm at three packing densities for each soil type. A maximum packing density was established for each soil and reduced bulk densities were calculated to 87.5 and 75% of this maximum value. Maximum dry bulk densities, and corresponding soil strength at 25% soil moisture, were 1.68 and 1.47 g cm-3 and 0.08 and 0.40 MPa for Stephenville and Coyle soils, respectively. Ryegrass seeds were placed on the surface of the soil and covered with 5mm layer of soil compacted to the same bulk density as the remainder of the column. Soil moisture was monitored gravimetrically and was returned to 25% volumetric water content every 24 h. Increased bulk density had no significant (P>0.05) effect on leaf and tiller appearance rates or on plant size at harvest in the Stephenville soil. In the Coyle soil leaf numbers on the mainstem and T1 tiller, the number of higher-order tillers and seedling size at harvest were all reduced as compaction was increased to 87.5 and 100% of maximum packing density. Although soil compaction may inhibit growth of grass seedlings, effects are likely to be dependent on soil type. The contribution of soil compaction to reduced growth following no-till establishment may be small under normal field conditions, as even in the Coyle soil, average bulk density under long-term pasture was only 80% of the maximum packing density tested in experiments reported here.