The stability of soil aggregates is closely connected with particle interaction determined by the combination of the van der Waals attractive force and electric repulsive force according to Derjaguin-Landau-Verwey-Overbeek (DVLO) theory. Recently, hydration force and dispersion force were put forward to explain the different behaviours of cations or anions of the same valence at the ion-surface interface, namely the specific ion effect, where the application of classical DLVO theory had failed. Here, we employed two cation species, potassium and sodium (K + and Na + ), to discover how the specific ion effect would influence clay aggregate stability. The stability of K + -and Na + -montmorillonite aggregates was determined under different electrolyte concentrations, indicated by the mass percentages of particles with diameters of < 10, < 5 and < 2 μm released after aggregate breakdown. There were large differences in the stability of the K + -and Na + -aggregates, and strong specific ion effects were shown. These effects could not be explained by the differences in ionic size, hydration and ion-surface dispersion forces between K + and Na + . We have proved that the difference in polarization between the K + and Na + at the charged clay surface was responsible for the specific ion effects. The difference in polarization observed between the adsorbed K + and Na + was hundreds to thousands of times larger than classical values; these results were also verified independently with different methods. The strong non-classical polarization of the adsorbed cation decreased the electric field and the electrostatic repulsion between adjacent particles in the aggregates, and thus strongly increased the aggregate stability.

Summary Specific ion effects are now thought to be important in nature. We studied the specific ion effects on soil particle transport during rainfall simulation (150 mm hour−1, 110 minutes) in sodium nitrate (NaNO3), potassium nitrate (KNO3) and caesium nitrate (CsNO3) solutions. The results showed marked differences in the intensity of soil particle transport in Na+, K+ and Cs+ systems. The differences increased sharply with the decrease in electrolyte concentration, which indicated strong specific ion effects on soil transport and suggested that the differences could not be explained by ionic size, hydration effect or dispersion force. The cationic non‐classical polarization in a strong electric field increases the Coulomb attractive force between the cation and clay surface, and further adversely decreases the strength of the electric field. With the absolute effective charge coefficients, γ, of Na+ (1.110), K+ (1.699) and Cs+ (2.506), we recalculated the true surface potentials of soil particles in NaNO3, KNO3 and CsNO3 solutions. The true surface potentials decrease sharply with the increase in ionic non‐classical polarization, and then the electrostatic repulsive pressure between particles in the soil should decrease sharply. Comparison of fitting the equation for transport intensity in NaNO3 solution with that in KNO3 and CsNO3 solutions showed clearly that the soil electric field controlled the aggregate breakdown and particle transport. The results suggested that the stronger the non‐classical polarization for cations in the soil, the weaker is the electrostatic field that forms and the soil erosion at the same solution concentrations.

It is well known that humus markedly increases soil aggregate stability, but at the same time strongly decreases the flocculation of clay particles in suspension. These seemingly inconsistent observations suggest the need for a deeper understanding of the physical mechanisms that govern clay–humus interactions. In this research, soil samples from an Entisol were used to explore the role of cationic polarization in humus‐increased soil aggregate stability and sodium (Na+) and potassium (K+) ions were used to characterize weak and strong polarization, respectively. The results showed that strong cationic polarization has a critical role in increased soil aggregate stability in the presence of humus. We concluded that, without cationic polarization, the effects of humus alone on soil aggregate stability were weak in the presence of monovalent metal cations. When we compared the individual contributions of humus and strong cationic polarization, the latter proved much more important than humus in increasing soil aggregate stability. The strongest increase in soil aggregate stability occurred when strong cationic polarization was coupled with humus. The combined analysis of activation energy of soil flocculation in suspension and soil aggregate stability in the presence of humus indicated that humus increased the long‐range electrostatic repulsive force of soil particles, and increased the short‐range attractive force of soil particles. Consequently, humus decreased the flocculation of soil particles in suspension but increased soil aggregate stability; all effects were adjusted by the strength of cationic polarization. Highlights Elucidation of physical mechanisms of cation–surface interactions that determine clay–humus interactions. Cations at the particle surface of the clay–humus complex are strongly polarized. Cationic polarization has a critical role in humus‐increased soil aggregate stability. Without cationic polarization, the effect of humus alone on soil aggregate stability was weak.

Summary Particle aggregation and aggregate breakdown are important processes that frequently occur in soil under natural conditions. However, how these two opposing processes are affected by the forces that govern soil particle interaction remains unclear. Thus, in this research, we aimed to: (i) investigate the relation between particle aggregation and aggregate breakdown and (ii) probe the mechanism underlying particle aggregation and aggregate breakdown under the influences of soil particle interaction forces. Specifically, we investigated particle aggregation and aggregate breakdown in a permanently charged clay‐rich soil in solutions with different electrolyte (NaNO3 and Mg(NO3)2) concentrations. We used the fast wetting method in aggregate breakdown experiments and the dynamic light‐scattering method in aggregation experiments. For soils in either NaNO3 or Mg(NO3)2 solution, the critical coagulation concentration obtained through particle aggregation experiments was equal to the critical breakdown concentration from aggregate breakdown experiments. This result indicated that the net force, which is defined as the sum of the van der Waals, electrostatic and surface hydration forces, is attractive for aggregation but is repulsive for aggregate breakdown. Although several interaction forces were involved in soil particle interactions, we found that the repulsive electrostatic force solely determines whether the net force is attractive or repulsive and thus determines whether aggregation or breakdown would occur. For a given soil, non‐classical cationic polarization in cation–surface interactions strongly influenced the repulsive electrostatic potential energy of soil particles, thus influencing the occurrence of aggregation or breakdown. Our results suggested that adjusting soil internal forces is a feasible approach to regulate particle aggregation and promote aggregate stability. Highlights The process of soil aggregate breakdown and particle aggregation were evaluated quantitatively. There was equality in the opposing forces for particle aggregation and aggregate breakdown. Electrostatic repulsive force between soil particles controlled processes of aggregation and aggregate breakdown Cationic non‐classic polarization in cation–surface interactions has an important effect on aggregation and aggregate breakdown.

Summary Soil water infiltration is closely associated with the transport of nutrients and contaminants in soil. The physical mechanism by which electrolyte concentration influences water infiltration is not clear, however. Recent studies have shown that interaction forces between soil particles play a crucial role in the stability of soil aggregates and pores, which further affects soil water infiltration. In this research, column experiments were used to elucidate the effects of soil particle interaction forces on water infiltration in a permanently charged and a variably charged soil using different concentrations of KNO3 solutions (0.0001, 0.001, 0.01 and 0.1 mol l−1), and some interesting results on soil water infiltration were discovered. They indicated that: (i) four forces in soil (long‐range van der Waals attractive, electrostatic repulsive, hydration and osmotic repulsive forces) co‐determined the rate of soil water infiltration for both soil types tested, (ii) at smaller soil water content (large electrolyte concentration) the osmotic repulsive force became more important in soil water infiltration, and yet with a larger water content (small electrolyte concentration) the electrostatic repulsive force became more important, and (iii) the osmotic force affected water infiltration through its effect on particle interaction forces rather than its influence on the osmotic potential of water. This paper presents a quantitative description of how particle interaction forces affect water infiltration into soil. Highlights Elucidation of physical mechanisms of effects of clay charge and electrolyte concentration on soil water infiltration. Quantified effects of interaction forces of soil particles on soil water infiltration. Electrostatic repulsive and osmotic forces play a critical role in water infiltration. DLVO, hydration and osmotic forces among soil particles co‐determined soil water infiltration rate.

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This study aimed to evaluate the acaricidal activity of lettuce (Lactuca sativa) extracts against carmine spider mites (Tetranychus cinnabarinus Boisd.) and isolate the acaricidal components. Acaricidal activities of lettuce extracts isolated from different parts (the leaf, root and seed) using various solvents (petroleum ether, acetone and methanol) were evaluated with slide-dip bioassay and relatively high median lethal concentration (LC50) values were detected. Acetone extracts of lettuce leaves harvested in July and September were fractionated and isolated with silica gel and thin-layer chromatography. Consequently, acetone extracts of lettuce leaves harvested in July exhibited higher acaricidal activity than those harvested in September, with an LC50 value of 0.268 mg ml-1 at 72 h post-treatment. A total of 27 fractions were obtained from the acetone extract of lettuce leaves harvested in July, and mite mortalities with the 11th and 12th fractions were higher than those with the other 25 fractions (LC50: 0.751 and 1.258 mg ml-1 at 48 h post-treatment, respectively). Subsequently, active acaricidal components of the 11th fraction were identified by infrared, nuclear magnetic resonance and liquid chromatography/mass spectrometry. Five components were isolated from the 11th fraction, with components 11-a and 11-b showing relatively high acaricidal activities (LC50: 0.288 and 0.114 mg ml-1 at 48 h post-treatment, respectively). Component 11-a was identified as β-sitosterol. In conclusion, acetone extracts of lettuce leaves harvested in July might be used as a novel phytogenic acaricide to control mites.

BackgroundPepper blight, caused by Phytophthora capsici, is a destructive soilborne disease, which poses a serious threat to pepper, Capsicum annuum L., production. Chemical fungicides, which mainly are used to control pepper blight, have a negative effect on the environment, rendering biological control as a promising alternative to maintain the balance between ecology and pest management. The purpose of this study was to screen the biocontrol bacteria, reduce the dosage of fungicides and increase the stability of biocontrol bacteria, and to find the mixing ratio of biocontrol bacteria and fungicides giving the best control effect.ResultsWe isolated actinomycetes strains from the soil surrounding the roots of healthy pepper plants amongst field‐grown plants infected with P. capsici. Of these, Streptomyces albus XJC2‐1 showed a strong inhibition effect on the growth of P. capsici, with an inhibition rate of ≤85%. XJC2‐1 effectively inhibited the formation of sporangium and release of zoospores of P. capsici as well as directly destroyed its hyphae, to achieve the inhibitory effect. Transcriptomic profiling of pepper leaves, postirrigation of plants with the XJC2‐1 fermentation broth, revealed upregulation of genes related to the photosynthesis pathway in pepper. Furthermore, XJC2‐1 treatment improved the net photosynthetic rate and intercellular CO2 concentration, thereby improving the pepper plant's resistance to pathogens. The combination of XJC2‐1 with the fungicide dimethomorph (8 μg mL−1) displayed strong synergism in inhibition of P. capsici infection, with a control efficiency as high as 75.16%, thus providing a basis for its application in the field.ConclusionOur study demonstrated that S. albus XJC2‐1 inhibited Phytophthora pathogens from infecting pepper plants and enhanced plant host resistance. The combination of XJC2‐1 and dimethomorph displayed a more stable and stronger control effect on pepper blight, showing potential for the future application of XJC2‐1 in the field of biological control. © 2023 Society of Chemical Industry.