Introduction

The risk or likelihood that soil will undergo breakdown is usually determined by measuring aggregate stability, the ability of soil aggregates, or a size range of

aggregates to withstand disaggregation when moistened under the laboratory wetting procedure. Aggregate stability may be determined by a number of procedures, most commonly by: wet sieving (WS), rainfall simulation (RS), and ultrasonic vibration (UV), as well as various measures of dispersion (D). Measurement of aggregate stability has attracted numerous reviews and comparative studies between procedures (Amezketa et al. 1996, Le Bissonnais 1996, Amezketa et al. 2003, Rohoskova & Valla 2004),

however guidelines for selection of procedures are lacking.

The selection of an appropriate method or procedure for determining aggregate stability is not straight forward. In theory, selection should be based on the purpose of the analysis, soil type, and the type and level of disruptive energy that soil aggregates experience in the field (Herrick et al. 2001). Ideally the selected procedure would be simple, inexpensive, easily replicated and have high coefficient of variation (COV)

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between sites to allow for discrimination between similar soils or treatments, and a low within site COV to reduce the need for replication.

Aggregate stability is commonly determined by wet sieving based on early work of Kemper and Koch (1966) and Kemper and Rosenau (1986). Aggregate breakdown during wet sieving is largely due to slaking and to a lesser extent the physical effects of water movement during lifting of the sieve and abrasion on the sieve surface (Emerson 1967). Slaking results from the compression of air trapped inside aggregates during rapid wetting, as occurs when dry aggregates are rapidly immersed in water (Truman et al. 1990). Slaking by air compression is influenced by the rate of water ingress, which in turn is influenced by soil porosity, pore connectivity, antecedent moisture content and the rate of wetting or immersion (Loch 1994). In soils containing 2:1 clays (vermiculite and smectite), slaking may also result from differential clay swelling (Le Bissonnais 1996, Ben-Hur & Lado 2008, Reichert et al. 2009). Swelling results from thickening of water held on the surface of clay colloids, which cause the clay platelets to be pushed apart. Differential rates of swelling lead to the formation of cracks in the areas of lowest shear strength.

Determination of aggregate stability by wet sieving has been criticised that it does not take into account all mechanisms responsible for aggregate breakdown in the field and tends to overemphasise slaking (Kemper & Rosenau 1986). In response, Le Bissonnais (1996) developed the ‘unified framework’ that combined use of both water and ethanol as wetting fluids together with slow and fast rates of wetting and mechanical energy (shaking after pre-wetting). By comparing values of aggregate stability, they were able to infer the proportion of aggregate breakdown resulting from different mechanisms. Amezketa (1999) added a further two treatments, in which a salt solution is used to

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identify the effect of water quality on aggregate breakdown by dispersion. The unified approach developed by Le Bissonnais (1996) and others is comprehensive and allows for results to be related to processes of aggregate breakdown. However, the procedure is complicated, involves multiple tests and does not appear to be widely adopted by

subsequent researchers. Furthermore, both the wet sieving procedures developed by Kemper and Rosenau (1986) and Le Bissonnais (1996) do not consider the effect of raindrop bombardment on disaggregation and are usually conducted over too short a duration to allow for dispersion and flocculation.

Aggregate breakdown by raindrop impact is closely associated with the formation of soil crusts (Awadhwal & Thierstein 1985). When a soil surface is exposed to rainfall, rain drops impact the soil surface causing soil aggregates to shatter into smaller particles. The amount of detachment and shattering generally increases with raindrop size and input energy (Furbish et al. 2007). Fine soil particles are detached and drawn into soil pores by capillary flow, which clog soil pores to form a soil seal or crust (Legout et al. 2005, Bu et al. 2013), resulting in ponding and potentially further aggregate breakdown by slaking (Terry 1992, Gholami et al. 2013).

A number of apparatuses have been developed to measure aggregate stability by rainfall simulation. Ideally rainfall simulation should apply droplets of similar size and level of energy as what aggregates are exposed to in the field. Review of the literature however indicates that a range of droplet sizes, drop heights and rainfall intensity have been used to determine aggregate stability (McCalla 1944, Low 1954, Morin & Benyamini 1977, Norton 1987, An et al. 2012).

Field aggregates may also break down by dispersion, especially if they contain sodic clays. Dispersion tends to occur over longer durations than most procedures used to

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measure aggregate stability and thus its importance may be underestimated by wet sieving and rainfall simulation tests. Dispersion is influenced by many factors including; (i) clay charge characteristics, (ii) cation ratios, especially the exchangeable sodium percentage (ESP), (iii) the electrolyte concentration of infiltration water (Emerson 1967, Agassi et al. 1981, Agassi et al. 1985, Römkens et al. 1990, Le Bissonnais 1996,

Marchuk et al. 2012), and (iv) attractive forces between colloidal particles (Emerson 1967, Sumner 1992). Clay dispersion may be measured using qualitative (Emerson 2002) or quantitative tests in which the amount of turbidity or suspended clay is compared between different fluids, dispersion test or levels of agitation (Rengasamy et al. 1984), as well as procedures that measure erosion such as the pinhole test (AS 1289.3.8.3 1997).

In addition to wet sieving, rainfall simulation and clay dispersion, sonication or

ultrasonic vibration have been used to measure aggregate stability (Imeson & Vis 1984, Mentler et al. 2004, Fristensky & Grismer 2008, Zhu et al. 2009, Rawlins et al. 2013). Sonication creates cavitation, the formation of an empty space or bubbles within soil aggregates and water. As these bubbles expand they rupture the soil along cracks and lines of weakness, leading to cleavage of aggregates into smaller fragments or primary particles. Whilst sonication is a very effective means of disaggregating soils, the procedure is not representative of the behaviour of field soils.

This study was conducted to; (i) explore effects of methodology on values of aggregate stability, (ii) estimate the effects of different wetting fluids on aggregate breakdown, and (iii) provide guidance on selection of methodology and procedures for the determination of aggregate stability of intensively cultivated sandy clay loam soils.

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