MODIFICATION OF SUBSOIL CHARACTERISTICS RESULTING FROM DRAINAGE TILE INSTALLATION

Modification of subsoil characteristics caused by drainage tile installation was examined on loamy glacial till soils in Nova Scotia and New Brunswick, Canada. The objectives were to evaluate the effects of different drain line installation techniques, trenchless vs. trenching, and system age on subsoil physical characteristics. The area of disturbance caused by trenching (both machine and hand dug) tends to be uniformly 30-40 cm wide in the subsoil portion of the profile, whereas a zone of disturbance 7-17 cm wide was created by the trenchless plow. Although both methods result in bulk density of the disturbed subsoil being significantly lower than the undisturbed subsoil, disturbance caused by trenching exhibited lower bulk density than that of the trenchless plow. Percent reduction in bulk density did not significantly change over time periods of up to 30-40 years. Increase in macroporosity of disturbed subsoils correlates significantly with reduction in bulk density. With increase in macropores, saturated hydraulic conductivity of the disturbed subsoil was significantly higher than found originally, except in newly drained (1 and 2 years) fine-loamy till soils. Saturated hydraulic conductivity of disturbed subsoils resulting from trenching was significantly higher than that of the trenchless plow. Although not always statistically significant, organic carbon content of the disturbed subsoil was considerably higher than that of the undisturbed. The difference in organic carbon between the disturbed and the undisturbed subsoils decreased with increasing time. A significant correlation was found between the logarithm of the rate of organic carbon decomposition and average age of installation (r(2) = 0.9). This indicates that the shallow rooted crops being grown (corn and forages) did not, or could not, possibly because of low pH values, take advantage of the modified subsoil conditions created since little organic matter from decaying roots was added to the disturbed soil mass. As a result of subsoil modifications, surface soil moisture content immediately above the drain line was lower than the adjacent surface soil.