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— In urban conditions, the soil is exposed to a number of adverse factors that have a great impact on its hydrophobic and hydrophilic properties. The water-repellent properties of urban dust and soils were determined with the water drop penetration time (WDPT) test. Hydrophobization of Albic Retisols in the conditions of a megalopolis was modeled, and its rate was estimated. Three dust samples with different values of the WDPT test from 420 to 850 seconds were studied. According to the results of the model experiment, with an increase of solid atmospheric fallout in the Retisols, the level of their hydrophobicity also increases. The rate of the increase depends on the water-repellent properties of the dust. During pollution with the most hydrophobic dust, the maximum hydrophobization of the humus-accumulative soil horizon is achieved upon a 70-year-long impact. For other dust samples, an increase in the time of absorption of a drop was observed up to the maximum period of aerial soil contamination within the model experiment (200 years). Values of the WDPT test for the studied soil horizons ranged from 2.4 s to 1493.5 s for background soil and urban soil forming near a major highway for 90 years, respectively. In the soil of forty-year-old residential area, the value was 237.1 s. The correspondence of the levels of hydrophobicity, the degree of anthropogenic load and the residence time in the urban environment in the model experiment and in the samples of real urban soils indicates that solid atmospheric fallouts are a component of humus-accumulative horizons of urban soils and have a significant impact on their water-repellent properties.

During studying water stability of soils, spherical particles of several hundred nanometers in size were found to be released from the soil into the water at capillary contact of soils with water. Such particles were shown to pass into water from any humus-containing objects—soils, peats, humic acids, humates, and fulvic acids. Elemental microanalysis of such particles released from soddy-podzolic soil showed that they consist mainly of organic matter. These particles represent previously detected supramolecular formations (SF) from specific organic matter of soils. Humic substances of soils are known to be fractally organized, and in water they exist as fractal clusters of several hundred nanometers in size (F-clusters) consisting of particles–molecules of humic substances of about 10 nm in size. This allowed us to assume that the supramolecular formations released from humus-containing samples are F-clusters. Based on the high stability of supramolecular formations of humic substances to decomposition into particles–molecules, it follows that humic substances in soils should have a fractal–cluster organization.

One of the methods to increase the water stability and erosion resistance of soils is the use of structure-forming polymers. It is believed that the mechanism of their action is based on strengthening the bonds between particles of clay minerals. This approach ignores the existence of organomineral gels on the surface of mineral particles, which can affect the effectiveness of polymers. The purpose of this work is to investigate the interaction of a number of structure-formers used to increase water stability and erosion resistance with soil components. In model experiments on the interaction of polymers with soil components, suspensions of humate and bentonite are used. The results are verified on leached chernozem. The effectiveness of polymers is evaluated by the blade method used to determine the water stability of soils, and the interaction of particles in suspensions in model experiments is evaluated by laser diffractometry. It is established that, in solutions of humates with polymers, the size of particles formed in solutions increases with increasing hydrophobicity of the polymers. No such unambiguous relationship is found in bentonite suspensions with polymers. Verification of the results of model experiments on chernozem has shown that the water stability of the aggregates increases with an increase in the hydrophobicity of the polymer used for treatment. To further verify the role of organic matter in ensuring soil water stability, the possibility of using oppositely charged humic substances and iron sol to increase soil water stability is evaluated. The experiments have shown that the use of iron sol increases the water stability of chernozem. Moreover, at an increase in the pH of the iron sol solution from 1.7 to 6.1, the effect value increases from 11 to 59%. The results of this study suggest that shifting focus from strengthening the adhesion between mineral particles to strengthening organic and organomineral interactions should be considered as a reserve for increasing the effectiveness of compositions to maintain the soil structure.

Humic substances influence a number of soil properties: structure, cation exchange capacity, water retention capacity, etc. At the same time, in soils and solutions, humic substances exist not in the form of individual molecules, but in the form of supramolecular formations having a fractal cluster organization (F-clusters). Consequently, F-clusters should exert their influence on soil properties. As F-clusters are tightly interconnected, to assess their influence on soil properties it is necessary to separate them. This can be done by mechanical activation—increasing the reactivity (activity) of substances during their mechanical treatment. We studied the influence of mechanical activation on some soil properties and on the development of plants in activated soils. It has been shown that the water retention capacity of soil samples from the main types of zonal soils increases by up to 35% of the initial value under the impact of mechanical activation. This can be explained from the standpoint of a decrease in the mobility of gravitational water by F-clusters in macrocapillaries. The optical density of water extracts from chernozem increased by 75% and the viscosity of soil pastes increased by 57% due to an increase in the number of F-clusters in the soil solution. Activated soils stimulated the germination of wheat seeds by 26%. This effect may be associated with the formation of films of F-clusters on the surface of seeds, which fix soil allelotoxins that slow down seed development.

In modern soil physics soil stability as a concept is divided into two directions, namely water stability and mechanical resistance to compression and wedging. Both soil properties in water-saturated soil are based on the rupture of interaggregate interparticle bonds, however, no standard of physically based parameters have been proposed to characterize the aggregate stability. The purpose of the article is to substantiate the physical concept of stability of soil aggregates and to propose a single methodological method for quantifying stability as a normative soil characteristic. A high-performance method has been developed based on the dissection of linearly arranged water-saturated aggregates with blades under controlled load. The main stages of the technique are vacuuming of aggregates to eliminate the uncontrolled influence of trapped air, saturation of aggregates with water in vacuum, and subsequent determination of the aggregate stability to penetration of blades. Experimental stability values (mN/aggregate) were obtained for 17 soils, which made it possible to form normative ranges for arable loamy soils, namely 17–19, 27–29, and 34–37 mN/aggregate for soddy-podzolic, gray forest soils, and chernozem, respectively, and a number of other soils, which makes it possible to apply the obtained value as a soil characteristic of the aggregate stability. The possibility of using the stability values as a methodological basis for monitoring soil stability and degradation and for quantitative directions for assessing of physical characteristics of soil aggregates (firstly, their main parameter—stability) is discussed. Taking into account the highly correlative dependence of the proposed stability characteristic on the water stability values obtained by the Savvinov method (>85%) and the high performance of the stability determination method (the proposed method is about 20 times more productive than the Savvinov method), the possibilities of using the method and the obtained values of the stability of aggregates as a general physical characteristic and a separate one for quantifying water stability are discussed.

Crop exposure to stress during cultivation is known to reduce the yield and to cause the release of allelotoxins from plants into soil. It was assumed that allelotoxin release may considerably affect the vegetable growth in greenhouses and that a decrease in the allelotoxin concentration in greenhouse substrates may improve the plant growth. To verify the assumptions, allelotoxicity and microbial contents were determined in greenhouse substrates in which cucumber, tomato, and pepper plants grew well or poorly. The allelotoxin content was found to be higher and the prokaryote content, lower in the substrates of poorly growing plants. The finding confirmed the assumption that allelotoxins significantly influence the cultivation of vegetables in greenhouses. Treating the plant root zone with humate solutions having a high allelotoxin absorption capacity appreciably improved the cucumber plant growth and was assumed to provide a promising means to increase the vegetable yields in greenhouses.

The point of limited availability of water (PLAW) characterizes the lower boundary of the area with the most productive moisture for plants. The analysis of the experimental methods used to determine PLAW is indicative of their high labor intensity and low efficiency. The objective of this study is to develop a high-performance and accurate method to determine PLAW. In the work 18 samples from various soils were used. It is proposed to determine PLAW by the following method: soil samples are placed in a Schott funnel, moistened with excess water, and then water is pumped out using a water jet pump. As water is removed from the sample, the interval between drops falling from the funnel increases. A jump in the intervals between drops is considered as an indicator of experiment completion. According to the experimental results, the soil moisture content obtained by vacuuming correlates with the values calculated for the lowest soil moisture capacity (according to Dolgov) by 87%. The PLAW values obtained by the secant method (according to Voronin) for some soil samples do not fall out of the dependence obtained. This method makes it possible to demonstrate that soil drying leads to a decrease in the measured PLAW value. An explanation of these results from the standpoint of the presence of organomineral gels in soils is proposed.

It is common to consider the experimental soil physics results from the standpoint of a three-phase soil model. Along with the three-phase model, a gel model of soils is used. These models are based on different principles: in the three-phase model, solid phase constancy and liquid mobility; in the gel model, the ability of soil gels to swell, harden, and reduce water mobility. The purpose of this work is to assess the applicability of three-phase and gel soil models to analyzing the study results of some physical properties of soils. The studies were carried out with the zonal soil series: sod–podzolic, gray forest, chernozem, and chestnut soils. The following methods were used in this work: vibration viscometry, laser diffractometry, and electrical resistivity of soils. Unexpected results were obtained in the study of the soil’s physical properties. Firstly, the curve of the relationship between the moisture content of soil samples and the viscosity of pastes prepared from them reached the maximum at the point of limited availability of water (PLAW). Secondly, under the increased mechanical action on soil pastes, the particle size increased rather than decreased in them. Thirdly, the soil electrical resistivity–moisture relationship maintains a uniform course in the PLAW area. Meanwhile, at this water content, the continuous liquid phase framework providing moisture and electrical conductivity disappears in the soils. Fourthly, moist soils dry out in a desiccator over water. It is not possible to explain these results from the standpoint of the three-phase soil model generally accepted in soil science. For this reason, the gel model of soils was used to analyze and explain all the results obtained.

Based on earlier study results, the drying process changes the soil properties and, in particular, the characteristic features of a specific soil organic material such as a humic substance (HS). HS is the basis of soil organomineral gels that cover and bind soil particles. When water is removed from soil, gels are subjected to hydrophobization and compression resulting in changes in properties of soil samples. The recovery of soil gels of air-dried samples should reduce the discrepancy between the study data obtained on the soil properties of dried and non-dried soil samples. The study objective is to find ways to recover the structure of soil gels. Samples of six soil types were studied. Vibration viscometry, laser diffractometry, scanning electron microscopy (SEM), photocolorimetry, and conductometry were used in this work. The drying of soil samples increases the size of supramolecular formations (SMFs) in the soil and reduces the soil paste viscosity, a parameter characterizing the structure and the ability of gels to swell. To recover the structure of soil gels, it is proposed to reduce the size of SMFs from HSs to the initial level. SMFs of air-dried samples were separated by soil moistening and subsequent treatment with various temperatures, by ultrasound, and by freezing. Based on the SEM data, heating and ultrasound treatment do not reduce, but enlarge SMFs. Humidification of air-dried soils, exposure to moisture for two weeks, and subsequent freezing bring the paste viscosity of a number of studied soils closer to the condition of samples that were not dried. This process is due to the return of SMFs to size values of the initial soils, as evidenced by the laser diffractometer data on the suspended particle size distribution. Hence, a method for recovery of gel structures in dried soils to the initial state is proposed.

The opinion exists that water stability is provided by hydrophobic bonds between organic soil particles; however, there are works in which the main role in the occurrence of this property is assigned to the presence of hydrophilic organic substances in soils. The goal of this study is to clarify the nature of the bonds (hydrophilic or hydrophobic) that ensure the water stability of soils. We used samples of sod-podzolic and gray forest soils, as well as leached chernozem. Experiments to assess water stability were carried out using the method of “blades.” It is based on the dissection of linearly arranged aggregates, which were preliminarily moistened in vacuum to values close to saturation. The energy of hydrophobic bonds depends on the temperature; therefore, the influence of temperature on the value of the determined water stability was studied. Experiments showed that, as the temperature increases, the water stability of aggregates stored in the wet state increases from the moment of selection and decreases as the temperature increases. This indicates the leading role of hydrophobic bonds in the formation of water stability. As for the samples dried to an air-dry state, moistened again, and kept wet for more than two weeks, no temperature dependence of the water stability has been found. Taking into account that the strength of hydrophobic bonds increases with increasing temperature, while that of hydrophilic bonds decreases, the obtained data immutability of water stability can be explained if we assume the joint participation both hydrophobic and hydrophilic bonds in water stability of soil samples that have passed through the stage of drying to an air-dry state. In fact, these results indicate a strong change in the structural organization of soils during drying.