BULK-DENSITY VALUES DIAGNOSTIC OF RESTRICTED ROOT-GROWTH IN COARSE-TEXTURED SOILS

Bulk density values diagnostic of both moderate and excessive levels of soil compaction have not been conclusively identified. This study estimated diagnostic bulk density values related to conditions where root elongation was virtually stopped due to high mechanical impedance, and also to the less compact condition where root growth was occurring but where subsoiling could be used to increase root growth and yield. Undisturbed cores were collected from Ap, E-B, and B horizons of 24 Paleudults and Hapludults. Each core was equilibrated to a soil water potential of -0.1 MPa and mechanical impedance was determined and expressed as a cone index (CI). Bulk density and particle size distribution were also measured. A model was developed to estimate CI from bulk density (.rho.b) and percentages of sand, clay, and very fine sand (0.125-0.050 mm diam). Using the CI model and previously published soil, tobacco (Nicotiana tabacum L.) root, and tobacco yield data, diagnostic CI values related to zero root growth and significant subsoiling effects on both root growth and yield were estimated. The combinations of .rho.b and texture that produced the diagnostic CI values were then computed for four textural classes having very fine sand percentages of 0, 10, and 20%. The minimum bulk density values associated with where subsoiling increased both root number and yield over that of a moldboard plow/disk treatment were 1.66, 1.61. 1.60, and 1.58 Mg m-3 for the sand, loamy sand, sandy loam and sandy clay loam textural classes, respectively, having 10% very fine sand. It was estimated that root concentration would be zero at bulk densities of 1.85, 1.82, 1.81, and 1.80 Mg m-3 for these same textural classes.