Amending soil with sludge, manure, humic acid, orthophosphate and phytic acid: effects on aggregate stability A. I. Mamedov A,B,C,D , B. Bar-Yosef A , I. Levkovich A , R. Rosenberg A , A. Silber A , P. Fine A , and G. J. Levy A A Agricultural Research Organisation, Volcani Center, POB 6, Bet Dagan, 50250, Israel. B Sel¸ cuk University, Ardı¸ clı Mh., 42250 Konya, Turkey. C USDA-ARS, 1515 College Avenue, Manhattan, KS 66502, USA. D Corresponding author. Email: amrakh03@yahoo.com Abstract. Recycling of organic wastes via their incorporation in cultivated lands is known to alter soil structural stability. Aggregate stability tests are commonly used to express quantitatively the susceptibility of soil structural stability to deformation. The objective of this study was to investigate the effects of biosolids addition, namely composted manure (MC) and activated sludge (AS), and spiking of the soils with orthophosphate (OP), phytic acid (PA) or humic acid (HA), on soil aggregate stability of semi-arid loamy sand, loam and clay soils before and after subjecting the soils to six rain storms (each 30 mm rain with a break of 3–4 days). Aggregate stability was determined from water-retention curves at high matric potential. The effects of the applied amendments on pre- and post-rain aggregate stability were inconsistent and soil- dependent. For the pre-rain state, all of the tested amendments improved aggregate stability relative to the control. For the post-rain condition, aggregate stability was lower in the MC, OP and PA treatments and higher in the AS and HA treatments than in the control. The coarse-textured loam and loamy sand soils were more affected by the soil amendments than the clay soil. For the pre-rain state, addition of organic matter significantly improved macro-porosity and hence the stability of apparent macro-aggregate (>250 mm). Our results indicate a possible advantage for separation of aggregates into macro- and micro-aggregates for more precise evaluation and understanding of the effects organic amendments might have on aggregate stability. Additional keywords: aggregate stability, biosolids, high-energy moisture characteristic, humic acid, manure, slaking, sludge. Received 20 November 2013, accepted 11 February 2014, published online฀29฀April฀2014 Introduction Wet aggregate stability is increasingly being used to evaluate cohesion of aggregates, and the dynamics and nature of the bonding between particles (Kay and Angers 2002), all of which are major contributors to soil structural stability. Stability of wet aggregates is reduced by clay swelling and dispersion (Quirk and Panabokke 1962; Abu-Sharar et al. 1987). Conversely, soil aggregate stability is enhanced by the cementing effects of soil organic matter (OM) (Metzger and Yaron 1987; Le Bissonnais and Arrouays 1997; Lado et al. 2004; Wallace et al. 2009). Known organic cementing substances are polysaccharides, which have a transient binding effect, fungal hyphae, which have a temporary effect, and carbonaceous macromolecules (e.g. humic acid, HA), which have a persistent effect (Tisdall and Oades 1982; Metzger and Yaron 1987). The use of biosolids and urban organic wastes in agricultural soils as a nutrient resource and as a soil amendment is common practice, especially in arid or semi-arid zones, where low OM content and degraded soil structure frequently occur in cultivated lands (Khaleel et al. 1981; Haynes and Naidu 1998; Graber et al. 2006). Some common organic wastes, e.g. manure compost (MC) and activated sludge (AS), have been shown to positively affect soil aggregates and structural stability (Debosz et al. 2002; Whalen et al. 2003), partially due to adhesive HA films, which form bridges between soil clay platelets (Sander et al. 2004), the hydrophobicity which the HA in the organic matter may impart to the aggregates (Piccolo and Mbagwu 1999; Chenu et al. 2000). However, some studies report results that contradict those abovementioned, and show that the presence of dissolved fractions of soil OM and especially humic substances in the clay-size fraction of soils can, under certain conditions, enhance clay dispersion and aggregate breakdown (e.g. Narkis et al. 1970; Goldberg and Forster 1990; Frenkel et al. 1992; Tejada and Gonzalez 2007; Furukawa et al. 2009). The disagreement regarding the impact of soil OM may stem from the fact that part Journal compilation Ó CSIRO 2014 www.publish.csiro.au/journals/sr CSIRO PUBLISHING Soil Research, 2014, 52, 317–326 http://dx.doi.org/10.1071/SR13334