Soil-aggregate formation as influenced by clay content and organic-matter amendment Stephen Wagner 1 , Stephen R. Cattle 2 , and Thomas Scholten 3 * 1 Institute of Soil Science and Land Evaluation, University of Hohenheim, Emil-Wolff-Str. 27, 70599 Stuttgart, Germany 2 Faculty of Agriculture, Food & Natural Resources, The University of Sydney, NSW 2006, Australia 3 Institute of Geography, Chair of Physical Geography, Eberhard-Karls-University Tübingen, Rümelinstraße 19–23, 72070 Tübingen, Germany Accepted May 5, 2006 Summary Naturally occurring wetting-and-drying cycles often enhance aggregation and give rise to a stable soil structure. In com- paratively dry regions, such as large areas of Australia, organic-matter (OM) contents in topsoils of arable land are usually small. Therefore, the effects of wetting and drying are almost solely reliant on the clay content. To investigate the relations between wetting-and-drying cycles, aggregation, clay content, and OM in the Australian environment, an experiment was set up to determine the relative influence of both clay content (23%, 31%, 34%, and 38%) and OM amendments of barley straw (equivalent to 3.1 t ha –1 , 6.2 t ha –1 , and 12.4 t ha –1 ) on the development of water- stable aggregates in agricultural soil. The aggregate stability of each of the sixteen composite soils was determined after one, three, and six wet/dry cycles and subsequent fast and slow prewetting and was then compared to the aggregate stabilities of all other composite soils. While a single wet/dry cycle initiated soil structural evolution in all composite soils, enhancing macroaggregation, the incorporation of barley straw was most effective for the development of water-stable aggregates in those soils with 34% and 38% clay. Repeated wetting-and-drying events revealed that soil aggregation is primarily based on the clay content of the soil, but that large straw additions also tend to enhance soil aggregation. Relative to untreated soil, straw additions equivalent to 3.1 t ha –1 and 12.4 t ha –1 increased soil aggregation by about 100% and 250%, respectively, after three wet/dry cycles and fast prewetting, but were of less influence with subsequent wet/dry cycles. Straw additions were even more effective in aggregating soil when combined with slow prewetting; after three wet/dry cycles, the mean weight diameters of aggregates were increased by 70% and 140% with the same OM additions and by 160% and 290% after six wet/dry cycles, compared to samples without organic amendments. We suggest that in arable soils poor in OM and with a field texture grade of clay loam or finer, the addition of straw, which is often available from preceding crops, may be useful for improving aggregation. For a satisfactory degree of aggre- gate stability and an improved soil structural form, we found that straw additions of at least 6.2 t ha –1 were required. How- ever, rapid wetting of straw-amended soil will disrupt newly formed aggregates, and straw has only a limited ability to sustain structural improvement. Key words: clay / barley straw / wet/dry cycles / soil aggregation / aggregate stability / agricultural management practices  2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1436-8730/07/0102-173 J. Plant Nutr. Soil Sci. 2007, 170, 173–180 DOI: 10.1002/jpln.200521732 173 1 Introduction The interaction of environmental and biotic agents influences the physical condition of the soil, particularly through soil structural evolution. Wetting-and-drying cycles are important environmental processes known to enhance aggregation in some soil types (Pulleman and Marinissen, 2004), while clay minerals, sesquioxides, and soil organic matter (SOM) are the soil solids most involved in soil structural development (e.g., Sullivan, 1990; Le Bissonnais and Singer , 1993). While it is practically impossible to alter the clay mineral or sesqui- oxide content of soil for large areas of land, it is both possible and feasible to increase the SOM content in the field in as lit- tle as 4 y (e.g., Aulakh et al., 2001). Well-established strate- gies for increasing SOM contents in agricultural production systems include the production and incorporation of green- manure crops (e.g., Aulakh et al., 2001; Gerzabek et al., 2001), application of livestock manure (e.g., Gerzabek et al., 1997, 2001), and the incorporation into the topsoil of straw from a previous crop with a subsequent increase in minerali- zation and microbial biomass (e.g., Bird et al., 2001). The latter strategy in particular is one which has obtained increas- ing popularity in broadacre cropping regions where annual crops are produced. The improvement of structure by such strategies will ideally result in a soil which exhibits a high degree of aggregation, a large proportion of soil aggregates in the 1–10 mm–size range which remain stable when wetted (Tisdall and Oades, 1982) and a moderate to large porosity. This kind of structure generally confers suitable soil physical properties for plant establishment and growth, including a large water-holding capacity, moderate saturated hydraulic conductivity, and suffi- cient aeration (Tisdall and Oades, 1982). Depending on the extent of disruption caused by agricultural practices such as tillage, the incorporation of organic matter (OM) into soil aggregates can provide physical protection of that SOM, retarding its decomposition (Pulleman and Marinissen, 2004). Furthermore, a favorable soil structure greatly in- creases soil resistance to both wind and water erosion (e.g., Le Bissonnais and Arrouays, 1997; Barthès et al., 2000). * Correspondence: Prof. Dr. Th. Scholten; e-mail: thomas.scholten@uni-tuebingen.de