Improved root penetration of soil hard layers by a selected genotype

Crops can be effectively grown on hardpan soils and water effectively used from deep in the profile if hard layers in soils can be penetrated or if th ey are broken up by tillage. Addition of gypsum to the soil or exploitation of genetic differences in root penetrability may help improve root penetra tion through hard layers with less need to depend on the energy requirement s of deep tillage. To test this theory, a single-grained Ap horizon of Norf olk loamy sand soil was compacted into soil columns to compare root penetra bility of soybean [Glycine max (L.) Merr.] genotypes Essex and PI416937 in the presence and absence of gypsum and at two soil compaction levels (colum ns with uniform compaction at 1.4 g cm(-1) and columns with increasing comp action with depth from 1.4 to 1.75 g cm(-1)). Compaction treatments were im posed by constructing soil columns composed of 2.5-cm-deep, 7.5-cm-diameter cylindrical cores compacted to predetermined bulk densities (1.40, 1.55, 1 .65, and 1.75 g cm(-3)). Soil penetration resistances were measured on dupl icate cores using a 3-mm-diameter cone-tipped penetrometer. Columns were no t watered during the study; soybean genotypes were grown in the columns unt il they died. Both genotypes lived one day longer in columns with lower bul k density and penetration resistance. Although root growth was more abundan t for Essex than for PI416937, root growth of PI416937 was not decreased by compaction as much as it was for Essex. These results suggest that PI41693 7 may possess the genetic capability to produce more root growth in soils w ith high penetration resistance. This study suggests that genetic improveme nt for root growth in soils with hard or acidic layers may potentially redu ce our dependence on tillage. Gypsum did not affect root growth in this stu dy.