Positive ions that are available in soils absorb on grain surfaces. The total sum of cations that can be absorbed bij a soil/sediment at a certain PH is defined by the cation-exchange capacity (CEC, in meq g-1: mol equivalents per gram). The uptake of cations is an important parameter in agriculture and the larger the CEC, the more cations can be absorbed to the soil. The CEC depends highly on the pH of soil and sediments, where the CEC decreases with decreasing PH (increasing acidity). The exchange of ions on sediments occurs commonly fast on geological time scales, but the kinetics of adsorption in natural environments is still poorly understood. The strength of the bonding between the cations and the sediments varies from weak Van der Waals bondings (physical adsorption) to strong chemical bonds. The CEC is widely used for agricultural assessment because it is a measure of general soil fertility as well as an indicator of structural stability because CED is capabel of enhancing development of shrinkage cracks. The list below shows the CEC for different types of minerals. The data indicate that the CEC can vary over 2 orders of magnitude for various types of , minerals and can vary one order of magnitude within one soil type. Cation exchange capacity for different types of sediment (Eisma, 1992; Locher and de Bakker, 1990):

Cation exchange capacity.

Summary
Crusting and erosion of cultivated soils result from aggregate breakdown and the detachment of soil fragments by rain, and the susceptibility of soil to these processes is often inferred from measurements of aggregate stability. Here, theories of aggregate breakdown are reviewed and four main mechanisms (i.e. slaking, breakdown by differential swelling, mechanical breakdown by raindrop impact and physico–chemical dispersion) are defined. Their relative importance depends on the nature of the rain, as well as on the soil's physical and chemical properties. The relations between aggregate breakdown, crusting and water erosion are analysed, and existing methods for the assessment of aggregate stability are reviewed. A unified framework for the measurement of aggregate stability is proposed to assess a soil's susceptibility to crusting and erosion. It combines three treatments having various wetting conditions and energies (fast wetting, slow wetting, and stirring after pre-wetting) and measures the resulting fragment size distribution after each treatment. It is designed to compare different soils, or different climatic conditions for a given soil, not to compare time-dependent changes in that soil.

Aggregate stability and assessment of soil crustability and erodibility: I. Theory and methodology

Soil aggregate stability is a crucial soil property affecting soil sustainability and crop production A broad outline of the processes and agents of aggregate formation and aggregate stabilization are presented and discussed in this review Aggregate stability is difficult to quantify and interpret The aim of aggregate stability tests is to give a reliable description and ranking of the behavior of soils under the effect of water, wind and management Numerous methods have been used to determine aggregate stability with varying success The different methodologies complicate the comparison among aggregate stability data It is also difficult to obtain a consistent correlation between aggregate stability and other important soil properties such as soil erodibility or crusting potential This paper reviews the different methods of measurement of soil aggregate stability used in the literature, paying attention to the conditions of sample collection in the field and sample preparation and treatmen

Soil aggregate stability: a review

The evaluation of soil susceptibility to runoff and water erosion in the field is often expensive or time-consuming. Several authors have reported that susceptibility is linked to aggregate stability, whose determination is far easier. However, this susceptibility has generally been deduced from rainfall simulation experiments on sieved soil samples, whose behaviour is not always representative of field-scale phenomena. Our aim was to extend the validity of relationships between soil aggregation and erosion through comparisons of topsoil aggregate stability and field-assessed susceptibility to runoff and erosion. Susceptibility to runoff and erosion was determined at several levels: first, on a southern French Regosol, through measurements of runoff and soil loss from 1-m2 microplots under simulated rainfall; second, from 100- to 800-m2 runoff plots on a Nitosol in Benin, a Ferralsol in Cameroon and a Regosol in Mexico (with additional data on Syria from the literature); and finally, soil susceptibility to erosion was determined through semi-quantitative assessment of the frequency of erosion features on vineyard hillsides in southern France. Aggregate stability was determined by immersion in water and wet-sieving of 2-mm sieved, air-dried 0- to 10-cm soil samples, which actually tests aggregate resistance to slaking. Under simulated rainfall, runoff depth and soil loss after 30 min were negatively correlated with topsoil content in stable macroaggregates (>0.2 mm). On runoff plots, 3-year runoff rate and soil loss were negatively correlated with topsoil aggregate stability, especially stable macroaggregate content; these correlations were improved when slope gradient and climate aggressiveness were considered in addition to aggregate stability. On vineyard hillsides, the frequency index of erosion features was negatively correlated with topsoil content in stable macroaggregates, especially in the absence of conservation practices. These results confirm that aggregate stability is a relevant indicator of soil susceptibility to runoff and erosion, especially in Mediterranean and tropical areas where intense rainfall is frequent. They also confirm that simple laboratory determination can provide data closely correlated with those resulting from field investigations.

https://horizon.documentation.ird.fr/exl-doc/pleins_textes/pleins_textes_6/divers1/010027383.pdf

Aggregate stability as an indicator of soil susceptibility to runoff and erosion; validation at several levels

Soil erosion is a global issue because of its severe adverse economic and environmental impacts. Economic impacts on productivity may be due to direct effects on crops/plants on-site and off-site, and environmental consequences are primarily off-site due either to pollution of natural waters or adverse effects on air quality due to dust and emissions of radiatively active gases. Off-site economic effects of erosion are related to the damage to civil structure, siltation of water ways and reservoirs, and additional costs involved in water treatment. There are numerous reports regarding the on-site effects of erosion on productivity. However, a vast majority of these are from the U.S., Canada, Australia, and Europe, and only a few from soils of the tropics and subtropics. On-site effects of erosion on agronomic productivity are assessed with a wide range of methods, which can be broadly grouped into three categories: agronomic/soil quality evaluation, economic assessment, and knowledge surveys. Agronomic me...

Soil Erosion Impact on Agronomic Productivity and Environment Quality

Previous studies about the effect of antecedent moisture content (AMC) on seal formation have shown contradictory results. We hypothesize that this controversy is related to differences in slaking during wetting. The objectives were to analyze the effects of: (i) clay content on aggregate stability and slaking; (ii) clay content and slaking on seal formation and interrill erosion under various wetting rates (WR) and AMC under simulated rain. Aggregate stability was determined on six smectitic soils from Israel with clay content from 80 to 630 g kg -1 . In the rain simulator, soils with 230, 410 and 620 g kg -1 clay were prewetted with WR = 1 and 5 mm h -1 to AMC = 0.25 and 0.5 of field capacity (FC), prior to the application of 80 mm of rain. Aggregate stability and slaking by fast WR increased with increase in clay content. In soils with 230 and 410 g kg -1 clay, raindrop impact was enough to disintegrate the aggregates and sealing was not affected by WR and AMC. Conversely, in the soil with 620 g kg -1 clay, seal formation increased with slaking caused by fast wetting. Thus, final infiltration rate of the clay soil with AMC = 0.5 FC and WR = 1 mm h -1 was 11.1 mm h -1 compared with 6.0 mm h -1 in the air-dry soil (fast wetting by rain). The effects of WR and AMC on soil loss were similar to their effect on runoff but more pronounced. The relation between wetting process and clay content should be considered when predicting soil erosion in smectitic soils.

Soil Wetting and Texture Effects on Aggregate Stability, Seal Formation, and Erosion

The effect of soil texture and CaCO3 content on water infiltration rate in crusted soil was studied with the use of a rain simulator Two types of soils with low exchangeable sodium percentage (ESP 400 mm winter rain Soil samples with clay percentages between 3 and 60 were collected in each region Distilled water (simulating rainfall) and saline water (simulating irrigation water) were sprinkled on the soil The soils were exposed to ‘rain’ until steady state infiltration and corresponding crust formation were obtained For both types of soils and for both types of applied water, soils with ∼ 20% clay were found to be the most sensitive to crust formation and have the lowest infiltration rate With increasing percentage of clay, the soil structure was more stable and the formation of crust was diminished In soils with lower clay content (< 20%), there was a limited amount of clay to disperse and, as a result, undeveloped crust was formed Silt and CaCO3 had no effect on the final infiltration rate for either type of applied water, whereas with saline water, increasing the silt content increased the rate of crust formation

Effect of soil texture and CaCO3 content on water infiltration in crusted soil as related to water salinity

Seals formed at the soil surface during rainstorms reduce rain infiltration and cause runoff and erosion. Surface application of anionic polyacrylamide (PAM) in solution has been found to be very effective in decreasing seal formation, runoff, and erosion. The objective of this study was to investigate the effect of surface application of granular PAM (10 and 20 kg ha -1 ) and gypsum (2 and 4 Mg ha -1 ) on the infiltration rate (IR) and soil erosion from a silty loam (Calcic Haploxeralf) and sandy clay (Typic Chromoxerert) during simulated distilled water rainstorms. Mixing dry PAM with the upper 5 mm of the soil surface reduced slightly the IRs, and reduced significantly soil erosion from the two soils. Spreading gypsum at the soil surface doubled the final IR compared with that of control and reduced erosion slightly. Spreading dry PAM mixed with gypsum was very effective in increasing the rain IR and reducing erosion. Mixture of 20 kg ha -1 PAM and 4 Mg ha -1 gypsum increased the final IR of the two soils by four times and reduced erosion to 30% that of the control. Gypsum added to the erosion benefits of PAM by increasing infiltration and decreasing runoff. However, mixing gypsum with dry PAM decreased the beneficial effect of PAM in reducing erosion in the silty loam soil. The mechanisms responsible for the specific effects of PAM (mixed with soil and gypsum) on rain infiltration and soil losses are discussed.

Infiltration and Erosion in Soils Treated with Dry PAM and Gypsum

The effect of clay mineralogy on seal formation, infiltration rate (IR) and soil loss was studied on 19 cultivated soils from South Africa (SA) using a laboratory rainfall simulator. Two soil treatments were applied: control and spreading phosphogypsum (PG) at the rate of 5 mg ha-1. The IR and soil

Clay mineralogy effect on rain infiltration, seal formation and soil losses

Surface sealing is determined by aggregate disintegration and clay dispersion, which in turn depend on aggregate wetting rate, and soil sodicity and texture. We hypothesised that soil susceptibility to seal formation increases when the aggregate wetting rate (WR) is increased, and that the effect of WR depends on soil texture and soil sodicity. The objective of this study was to investigate the effects of WR on seal formation, by observing infiltration rate (IR) and runoff, in cultivated soils varying in clay content and exchangeable sodium percent (ESP). Effects of 3 wetting rates (2, 8, and 64 mm/h) on IR and runoff from 6 Israeli soils exposed to 60 mm of simulated rain of deionised water were studied in the laboratory. The soils ranged in clay from 8.8 to 68.3% and ESP levels from 0.9 to 20.4. Effects of WR on soil infiltration rate and runoff depended on soil texture and soil ESP. In soils with low clay content (8.8%), the effect of WR on seal formation was negligible, whereas effect of ESP was significant. Conversely, in clay soils (>52.1%), WR had a predominant effect on IR and runoff, while the effect of ESP was notable yet secondary to that of WR. The soils with intermediate clay content (22.5–40.2% clay) were the soils most susceptible to seal formation, with WR and ESP having moderate effects on seal formation. Effects of WR on aggregate disintegration and seal formation increased with increasing clay content and aggregate stability. Conversely, the role of ESP in determining sealing decreased with an increase in clay content and in WR.

Wetting rate, sodicity, and soil texture effects on infiltration rate and runoff

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