Interactive effects of tillage system and soil water content on aggregate size distribution for seedbed preparation in Fluvisols in southwest Iran

A proper aggregate size distribution is an important factor in successful crop production in semi-arid region. Field experiments were conducted on both silty clay loam and loam soils (Calcaric Fluvisols, FAO) in semi-arid area of southwest of Iran to assess the effect of different tillage systems and soil water contents on aggregate size distribution in plowed layers (250 mm depth). Three tillage treatments, moldboard plowing followed by disc harrowing (MD); disc plowing using disc plow followed by disc harrowing (DD); disking using offset disc followed by disc harrowing (OD) were used. Each tillage treatment was implemented at four soil water contents, viz. 0.8, 0.7, 0.6 and 0.5w(PL) (plastic limit). The aggregate size distributions for each tillage-soil water content were determined of 0-50, 50-100, 100-150 and 150-250 mm soil layers. Tillage systems and soil water contents both showed significant effects on aggregate size distribution. In silty clay loam soil, the greatest proportion of <5 mm and the lowest proportions of >25 mm aggregates were obtained by MD and DD tillage treatments at 0.8w(PL), and for OD at 0.7w(PL). The percentage of <5 mm aggregates for MD, DD and OD tillage treatments for the aforementioned water contents were ranged 42-47, 33-35 and 22-45%, respectively. However, for loam soil, the greatest proportion of <5 mm and the lowest proportions of >25 mm aggregates were obtained by MD and DD tillage treatments at 0.7w(PL), and for OD at 0.8w(PL). The percentage of <5 mm aggregates for MD, DD and OD tillage treatments for the aforementioned water content were ranged 40-49, 22-47 and 37-51 %, respectively. Results indicated that aggregate size distribution of plowed layers under various tillage systems vary with soil water content and soil type. (C) 2004 Elsevier B.V. All rights reserved.