Tillage alters corn root distribution in coarse-textured soil

Root responses to tillage vary and the driving factors are not well understood. Characterization of root response is requisite to optimize fertilizer placement and to understand limitations to no-till production. Corn (Zea mays L.) root length and weight were measured in the top 0.3 m of coarse-textured soil (Psammentic Hapludalf) in southwestern Ontario, Canada after 5, 6 and 7 yr of conventional and no-till management. Root length density in the top 0.1 m was greater under no-till (17 km m(-3)) than under conventional till (7 km m(-3)) 2 yr out of 3. Root length density was 4 km m(-3) lower under no-till than under conventional till in the 0.15 to 0.3 m layer 1 yr out of 3, but otherwise root growth below 0.1 m was unaffected by tillage. Each year, root length and weight were distributed more horizontally under no-till than under conventional till. Corn grain yields did not vary with tillage, even though soil water content was often greater under no-till. The increase in soil water (of between 0.01 and 0.03 m(3) m-3) was partly due to increased water holdings capacity-water held between -8 and -200 kPa matric potential was usually greater under no-till (0.07 m(3) m(-3)) than under conventional till (0.06 m(3) m(-3)) in the top 0.15 m. The shift in root distribution was apparently driven by soil structure because variation in bulk density with tillage and depth followed the same trends as variation in root length. Bulk density was greater under no-till (1.5 Mg m(-3)) than under conventional till (1.4 Mg m(-3)) in the top 0.15 m. In the top 0.075 m, the proportion of the total space occupied by capillary pores (< 36 mu m diameter) was greater under no-till (17%) than under conventional till (15%), there were more dry-stable aggregates under no-till (9% of total soil in the 0.85-5.7 mm size fraction) than under conventional till (7%), and a greater proportion of these aggregates were water-stable under no-till (25%) than under conventional till (16%). Greater bulk density may trigger formation of lateral roots, and greater aggregation contribute to the more superficial development by deflecting roots from their gravitropic pathway. Given the more superficial root distribution under no-till, shallower placement of downwardly mobile nutrients such as nitrogen may be more efficient than knife-injection. (C) 1998 Elsevier Science B.V. All rights reserved.