Plant and Soil 160: 57-66, 1994. © 1994 KluwerAcademicPublishers.Printed in the Netherlands. PLSO 3195 Modification of soil aggregation by watering regime and roots growing through beds of large aggregates S.A. MATERECHERA l, J.M. KIRBY 2, A.M. ALSTON and A.R. DEXTER 3 Department of Soil Science, Waite Agricultural Research Institute, The University of Adelaide, Glen Osmond, SA 5064, Australia. Present addresses: ~Crop Science Department, Bunda College of Agriculture, University of Malawi, P.O. Box 219, Lilongwe, Malawi, eCSIRO Division of Soils, GPO Box 639, Canberra, ACT 2601, Australia and 3Silsoe Research Institute, Wrest Park, Silsoe, Bedford MK45 4HS, UK Received 10 May 1993. Accepted in revised form 2l October 1993 Key words: aggregation, large aggregates, plant species, root growth, soil watering regimes, wetting and drying Abstract The influence of root growth and soil watering regime on aggregation was studied under controlled conditions. The study examined the influence of pea (Pisum sativum cv Greenfeast), ryegrass (Lolium rigidum cv Wimmera) and wheat (Triticum aestivum cv Kite) roots on changes in aggregation and on the properties of the aggregates. The soil was a non swelling red-brown earth which was either kept wet or was allowed to wet and dry during the experiment. Root growth increased the percentage of small sized aggregates (<18mm diameter), organic carbon, tensile strength and stability of aggregates in comparison with a non planted soil. Changes in aggregate size distribution and properties of the aggregates were related to root length density of the species and also to the soil watering regime. Root length density was in the order ryegrass > pea > wheat. Wetting and drying of soil increased the strength and stability of aggregates. Incubating aggregates allowed some roots to decompose but did not increase the strength or stability of aggregates compared with unincubated soil. The results of this experiment are of practical significance in soil structural management, and in studies of soil aggregation dynamics. It may be possible to use plant roots to alter the size and properties of aggregates. Introduction Beds of sieved aggregates with a narrow dis- tribution of sizes are often used in experiments aimed at studying the dynamics of soil aggrega- tion because they represent a reproducible and well-defined structure (Braunack and Dexter, 1989). They also provide a means by which the results of experiments conducted in different places and times can be compared quantitatively. Plant roots are known to grow preferentially in weak aggregates or in voids between aggregates (Dexter, 1986; McSweeney and Nausen, 1984). When a plant root makes contact with the surface of an individual aggregate in a bed of aggregates larger than the diameter of the root, it may penetrate, displace the aggregate or be deflected by it (Misra et al., 1984; Whiteley and Dexter, 1984). Penetration is determined by the strength of the aggregate and by the angle of incidence at which the root tip contacts the aggregate (Dexter and Hewitt, 1978). The size of the aggregate has also been reported to influence the penetration of roots into aggregates (Logs- don et al., 1987). The understanding of the above processes has led to the development of