Soil

SOIL, a complex of organic and mineral compounds that arose on the surface of the earth's crust as a result of physical, chemical, and biological processes. The study of soil is of interest to the hygienist, sanitary physician, and epidemiologist, since soil plays a huge role in matters of sanitary life: pollution and infection of the soil, and thereby of soil waters, entails the development of epidemics. Knowledge of the properties of soil is essentially necessary when erecting buildings, setting up camps, laying water supply and sewage networks, and when establishing cemeteries, irrigation fields, etc. In addition, the close interrelationships between soil and the climate of a given locality, and between soil and vegetation, further increase the importance that the study of soil has for the hygiene of populated places, in particular in the matter of building cities, settlements, etc. At the end of the 19th century, the study of soil, largely thanks to the works of Russian scientists (Dokuchaev, Sibirtsev, Vinogradsky, Omelyansky, and others), acquired the character of an independent scientific discipline ('soil science' or 'pedology'), the development of which is closely connected with the successes of geology, microbiology, and chemistry. Soil-forming processes consist mainly of two moments: 1) weathering of rocks, in which both mechanical and chemical factors participate, and 2) the formation of the organic component of soils, in which the animal world living in the soil (rodents, reptiles, worms, insects, etc.) participates, as well as the plant world, which provides material for the transformations of organic matter, and the world of microbes, which processes this material into peat, humus, and mold (or mull). Information about the chemistry of various types of soil is essential both in the matter of the life of microbes in the soil, and for understanding the complex biochemical processes occurring in it, and finally in the matter of developing criteria for its sanitary assessment.

The first question, insofar as it concerns pathogenic bacteria, is of great importance in epidemiology; it received special development in the so-called soil, or localistic, theory of Pettenkofer, who considered certain types of soil as an environment particularly favorable for the reproduction of disease-causing bacteria (for example, the typhoid bacillus, the cholera vibrio, etc.). Some soils, such as clay or peat, he, on the contrary, considered 'immune.' A student and follower of Pettenkofer, Emmerich, explains the lack of 'predisposition' in clay and peat soils by their ability to adsorb organic substances and thereby cause a depletion of nutrient material in the water permeating the soil. The same role, according to Pettenkofer's thought, is played by the level of standing soil waters, which is why his theory is sometimes called the 'groundwater theory'; when this level drops, the soil layers, drying out, become an environment unsuitable for bacterial life; when the level rises, the dilution of organic substances in the water lowers the nutrient value of the solution, by virtue of which reproduction slows down; therefore, the most favorable period for bacterial life is the period immediately following the drop in the level of soil waters. A large statistical-epidemiological body of material collected by Pettenkofer pointed to the coincidence in time of cholera and typhoid fever epidemics and periods of falling soil waters. According to Emmerich, the reaction of the soil also plays a significant role in the question of the reproduction of pathogenic bacteria in the soil. In soils possessing an acidic reaction, reproduction does not take place, just as it does not in soils rich in silicic acid, alumina, and iron, since the colloidal solutions of these substances act bactericidally. At the present time, the Pettenkofer soil theory, despite its apparent coherence and the abundance of statistical material with which the Munich school tried to substantiate it, is recognized in its original form by only a few. On the one hand, the epidemiological material on which it is mainly based can easily be explained from the points of view of contact and water theories. On the other hand, its weak point is the lack of direct experimental evidence of the reproducibility of pathogenic bacteria in the soil. Indeed, the lowering of the level of soil waters, as Kruse correctly notes, entails the drying up of drinking groundwater, which is usually cleaner, and thereby the mass consumption by the population of contaminated surface waters. Likewise, the lack of sanitary protection of the soil and its infection with disease-causing microbes increases the possibility of indirect contact infection. As for the reproducibility of pathogenic bacteria in the soil, this fact cannot be established directly; these groups of bacteria, in terms of nutritional conditions and temperature, turn out to be in less favorable conditions in the soil than saprophytic species, which are represented by millions of individuals in 1 gram of soil, and are usually more or less quickly suppressed by the latter. The same is true for the conditionally pathogenic species B. coli. Thus, in non-sterilized soil extract inoculated with this species, one can observe a strong drop in the coli-titer within two weeks, whereas in sterilized extract, the coli-titer remained unchanged during the same period. The survival rate of individual pathogenic bacteria in the soil can be quite significant; not to mention spore-forming ones, such as anthrax bacilli and various anaerobic causative agents of wound infections, the spores of which are preserved in the soil for years, there are data in the literature on the preservation of viability by typhoid bacilli in moist soil for up to 6 months (Grancher, Deschamps). But even if the soil theory has not retained the significance in modern epidemiology that was attributed to it before, the role of contaminated soil in the spread of epidemics is nevertheless undeniable, and from a sanitary point of view, the examination of soil for contamination with animal waste is of great importance. Already, the usual sanitary survey of populated places to a certain extent reveals the picture of soil contamination at a given moment. The presence or absence of proper sewage, this or that arrangement and maintenance of cesspools and garbage pits, and the systems used for the removal and neutralization of sewage allow one to form a general idea of the degree of soil contamination. For an accurate judgment about the degree of contamination of soil, in which processes of pollution and self-purification have been going on for centuries, an in-depth chemical-bacteriological study is also necessary.

A complete analysis of soil for sanitary purposes usually includes a characterization of its mechanical structure and physical properties. The mechanical structure of soil is characterized by the ratio between the amount of large soil particles, which constitute, in the expression of Knopp, the 'soil skeleton,' and small soil particles ('soil body'), as well as their mutual arrangement. To sort soil particles, the soil under study, dried at room temperature (the so-called 'air-dry' soil), is sifted through a set of Knopp sieves (Fig. 1). The first of these, located at the top, has holes with a diameter of 7 mm and retains only coarse gravel, the second—4 mm—medium gravel, the third—2 mm—fine gravel, the fourth—1 mm—coarse sand, the fifth—0.3 mm—medium sand; finally, in the bottom compartment, fine sand or dust with particles with a diameter smaller than 0.3 mm collects. To sort soil particles by their specific gravity, one of the methods of elutriation proposed by Nobel, Schöne, Wagner, Orth, Sobanin, and others is used. The Nobel apparatus (Fig. 2) consists of four pear-shaped vessels made of thick glass, the volumes of which relate to each other as the cubes of the first four numbers, i.e., as 1 : 8 : 27 : 64, the total volume being 4 liters; the vessels, connected to each other by tubes, are mounted in a stand, with the smallest vessel connected to a bottle located above it containing 9 liters of water, and the outlet tube of the extreme largest vessel is lowered into a 5-liter beaker. 30 g of the air-dry soil under study, sifted through a sieve with holes of 3 mm in order to separate the smallest particles, is boiled in water for several hours, and the cloudy sediment is poured into the second vessel from the bottle, and the soil itself is transferred to the first and then a current of water from the bottle is passed for 20 minutes, after which the particles that have settled in each vessel are collected on weighed filters, dried at 25°, and weighed in the same way as the evaporated residue from the beaker; the figures obtained are converted into percentages.

Between the soil particles there are free spaces, or pores, the total volume of which depends on the arrangement, shape, and size of the soil particles and constitutes from 29% to 84% of the apparent volume of the soil for various soils, with the highest figures relating to fine-grained soils, such as clay or peat, with organic impurities. To determine the total volume of pores in the soil, the Renk method is usually used, which consists of pouring a measured amount of soil B into a measured amount of water A; in view of the penetration of water into the pores of the soil, the resulting volume C in the cylinder will be less than the sum of A + B; from this, the volume of pores in volume B of soil is calculated = A + B - C; when expressed as a percentage, it is equal to (A + B - C) * 100 / B.

Nebel's apparatus for decanting soil. The same value can be obtained by determining the true volume of the soil, i.e., the volume of the soil particles themselves; subtracting this figure from the apparent volume gives the total volume of pores. The true volume of the soil is determined by dividing the weight of the soil by its specific gravity. The specific gravity of the soil usually ranges from 2.5 to 2.8 and is determined with the help of a pycnometer. If the weight of the pycnometer with water is P, the weight of the pycnometer with a suspension of A grams of soil (previously boiled to remove air and dried at 100°) is P_h, then the weight of water displaced by A grams of soil is (P + A)-P_h, and the ratio A / ((P + A) - P_h) will express the specific gravity of the soil. Related to the porosity of the soil is its air permeability. To detect this property and study the conditions affecting it, the apparatus of Renk is usually used, which consists of several tall cylindrical tubes communicating at their lower ends with a horizontal tube. The soil under investigation is poured into the vertical tubes up to a certain level, while the horizontal tube, closed at one end, communicates at the other with a source of illuminating gas, which passes through the columns of soil. By igniting the gas at the upper end of the vertical tubes, one can judge the permeability of one soil or another for gas by the speed of appearance of the flame and its height; at the same time, it is easy to verify that it depends mainly on the size of the soil particles and increases with their size, while the total volume of pores has no effect on it. According to Renk's data, if the volume of air passing in unit time through fine sand (with a particle diameter of 0.3 mm) is taken as unity, then for medium sand S29 (0.3-1 mm in diameter) under the same other conditions, 84 is obtained, for coarse sand (1-2 mm)-961, for fine gravel (2-4 mm)-5,195, for coarse gravel (4-7 mm)-11,684; while the total volume of pores in the last three cases hardly changes. Another factor affecting the passage of air through the soil is pressure, and at low speeds the amount of air passing through a given soil is proportional to the pressure and inversely proportional to the thickness of the soil layer.-Thanks to the air permeability of the soil, its pores are always filled with air, which differs significantly in composition from atmospheric air, since on the one hand it gives up its O₂ for the oxidation of organic substances in the soil, and on the other hand it is enriched with various gaseous impurities arising from the decomposition of these organic substances.' Research by Fodor in Budapest and Fleck in Dresden showed that as one goes deeper into the soil, the content of O₂ in the soil air decreases (18.8-21.3% at a depth of 1 m; 16.33-19.4% at a depth of 2 m; 15.7-16.8% at a depth of 4 m; 14.2-14.95% at a depth of 6 m), while the content of CO₂ increases (0.9-1.04% at a depth of 1 m; 2.9-3% at a depth of 2 m; 4.1-5.6 at a depth of 4 m; 4.2-7.96% at a depth of 6 m). In soils rich in organic substances, sharp changes in composition compared to atmospheric air are already observed at a shallow depth: thus, Boussingault and Levy, investigating the soil air taken from the soil of a cultivated field at a depth of 0.3-0.4 m, found in it 10.35% O, 9.74% CO₂ and 7-9.91% N. As gaseous impurities in the soil air, NH₃, H₂S, volatile hydrocarbons, and sometimes CO (e.g., in case of leakage of illuminating gas from the gas pipeline network) can be detected; usually the soil air is saturated with water vapor. The question of the composition of soil air is by no means indifferent from a sanitary point of view, since there is a constant exchange between the soil air on the one hand, and the atmospheric air and the air of living quarters on the other. Thus, cases of poisoning with illuminating gas are known (in Breslau, Budapest, Moscow, Kharkov, etc.), and the houses where the victims lived were located 4, 7 and even 27 m from the places of damage to the gas pipeline network. In connection with these facts, the question of soil ventilation has significant sanitary importance. Soil ventilation occurs continuously under the influence of various factors, such as fluctuations in atmospheric pressure (emergence of soil air to the outside at reduced pressure, penetration of atmospheric air into the soil at increased pressure), fluctuations in the level of groundwater (displacement of soil air during rise, suction of atmospheric air during descent), draft caused by the difference in temperature between soil and atmospheric air, etc. Coarse-grained dry soils are ventilated most intensively; in fine-grained moist soils, air exchange is negligible. No less important from a sanitary point of view is the attitude of the soil to water (hygroscopicity, capillarity, water-holding capacity, water permeability), since it affects both the climate, conditions and the regime of groundwater and underground structures. A hygroscopic soil, attracting water vapor from the air and condensing it, easily becomes damp in the surface layers. Capillarity, which, like hygroscopicity, is more significant in fine-grained soils, causes the rise of groundwater into the overlying layers of soil. In the soil in general, one can distinguish, going from top to bottom, three zones (Hofmann): 1) the evaporation zone, 2) the water filtration zone, 3) the capillary rise zone. Below these zones lie impermeable layers of soil, on which the groundwater is located. The latter fill the pores of the overlying loose layers, such as alluvial, and sometimes form underground ponds, streams, and rivers.-Hygroscopicity of the soil is measured by the amount of water in it, determined by drying the soil at 105° for 5 hours.-To determine the capillarity of the soil, it is poured to a certain level in glass tubes 1-1.5 m high and 1.5-2 cm in diameter with a bottom of gauze or mesh; the tubes are immersed to the same depth in water and the height of water rise is measured after a certain period of time in different soils, and conversely, the time required for water to rise to the same height in different soils.-Under water-holding capacity is understood the ability of the soil, after wetting it and free drainage of water, to retain a certain amount of water in its pores. To determine water-holding capacity by Renk's method, soil is poured into a cylinder with a mesh bottom, which is then immersed in water; after water appears above the level of the soil and subsequent free drainage (after raising the cylinder above the water in a dish), the cylinder is wiped on the outside and weighed. The difference in weight after and before wetting indicates the so-called maximum or full water-holding capacity. By determining the volume of pores remaining unfilled with water (or the total volume of pores of dry soil), one can calculate what percentage of the total volume of pores constitutes the maximum water-holding capacity.-Measurement of the level of groundwater and its fluctuations is produced in co

In wells or boreholes using the so-called Pettenkofer tape with 10 cups on the lower metal rod (Fig. 3). When lowering, it is noted at what depth the uppermost cup, which scoops up water, is located.-Among other physical properties of S., one should also consider its relation to heat; in particular, questions about the freezing of surface layers of S., the permafrost zone, etc., have important practical significance when developing water supply and sewerage projects in connection with the question of the required depth of laying these pipes. Pettenkofer. To measure the temperature of S. at various depths, special thermometers in a metal casing are used, which are buried in the ground at the desired depth, while the scale remains above the ground surface. The question of the chemical-bacteriological characteristics of S. is of even greater importance for sanitary purposes, as it allows one to judge the degree of contamination and self-purification of the latter, i.e., to give it a sanitary assessment. S. of populated centers, continuously contaminated, in the absence of proper sanitary protection, is also the arena of a continuous and complex process of self-purification, which consists of physicochemical processes, such as the adsorption of suspended and dissolved substances, especially colloidal ones, and gases, and biochemical processes in which the most diverse microorganisms participate. The cycle of nitrogen-containing organic substances has been studied most fully; they are decomposed in S. by the activity of numerous proteolytic and peptolytic bacteria into amino acids with their subsequent deamination. The resulting ammonium salts then pass under the influence of nitrifying bacteria (see Nitrification) into nitrites and finally into nitrates. Another group of bacteria ferments carbohydrates, a third - lipolytic species - breaks down fats. Special species carry out such specific functions as the decomposition of cellulose, pectin substances, urea, etc. Mineral substances, like organic ones, undergo various changes in S.; thus, nitrates can be converted back into nitrites by the activity of an extensive group of denitrifying bacteria; sulfates can be reduced to sulfites, hyposulfites, and even to H₂S, and this function can be performed, in addition to the special species described by Beijerinck, by representatives of the subtilis group (Horowitz-Vlasova). In recent years, bacteria of S. that reduce phosphates have also been described (Rudakov, Horowitz-Vlasova). With the question of self-Propetrovsk proposed an approximate scheme for assessing the degree of contamination of S. depending on the above chemical indicators, as well as the total number of bacteria and coli-titer (Table 1). Table 1. Composition and properties Total N (in 100 g of soil)......... Organic NH₃ (in 100 g of soil)..... Organic C (in 100 g of soil)...... P₂O₅ (in 100 g of soil) Number of bacteria in 1 g Coli-titer ...... Heavily contaminated > 200 mg > 50 » > 500 » > 60 » millions 1-2 mg Moderately contaminated > 100 mg > 25 » > 300 » > 50 » hundreds of thousands > 50 mg Relatively clean < 100 mg < 25 » ; <

300 » I < 50 » | < 10 000 I > 1 000 mg. The following summary of data on soil from various investigated cities, compared with data on river sediment (Table 2), gives an idea of the absolute values of the main chemical indicators. The figures express milligrams per 100 g of soil. Table 2. Chemical composition of soil. Place and author of study: Moscow (Lyalin, 1895), Warsaw (Savchenko), Budapest (Fidor, 1893), Dnepropetrovsk (Gorovits-Vlasova, 1925). Bottom sediment: Neva (Zalessky, 1912), Dnieper (Gorovits-Vlasova, 1925). Total N, Mineral NH₃, Organic C, Loss on ignition in %, P₂O₅. The question of the fate of pathogenic microorganisms in the soil is closely connected with the purification of the soil. Contrary to the views of Pettenkofer and his student Emmerich, who considered the soil to be the most favorable environment for the reproduction of a number of pathogenic species, at the present time it can be considered established, as already mentioned above, that pathogenic bacteria in the soil die off more or less quickly in the struggle with soil saprophytes, which are better adapted to the environment. To judge the degree of pollution and self-purification of the soil, in addition to everyday information about its pollution, data from chemical-bacteriological research are necessary. Of the various chemical indicators, the most valuable for sanitary assessment are the figures for total N, NH₃, C, and phosphoric anhydride. From the analyses of urban soils available in the literature—for Moscow, Dnepropetrovsk, Warsaw, Budapest—it is evident that the average figures for N fluctuate from 85 to 339 mg per 100 g of soil; average figures for organic C—from 185 to 3,500; P₂O₅—from 18 to 1,600, etc. Gorovits-Vlasova, on the basis of a detailed chemical-bacteriological study of the soil of various districts of the city of Dnepropetrovsk, gives an idea of the fluctuations of these figures in different districts of the same city depending on household pollution with the following figures in the soil of an old garbage dump in Dnepropetrovsk. The content of total N fluctuates from 297.6 to 378 mg per 100 g of soil, organic NH₃—about 115 mg, organic C—from 2,545.4 to 3,712.0 mg, P₂O₅—from 54.1 to 243.0 mg. For the soil of courtyards, the corresponding series of figures are: 35.7–526.4; 18.97–167.5; 695.4–1,641.0; 44.4–367.7. In other words, the soil of courtyards can be more polluted with nitrogen-containing organic substances than the soil of an old garbage dump. At a depth of 1 m (on a city square) we have for total N—91.0–96.7 mg; organic NH₃—43.6–56.1 mg; organic C—1,540 mg, i.e., figures smaller than on the surface. Soil outside the city limits gives: 59.7 mg for total N, 24.37 mg for organic NH₃, 185.4 mg for organic C, i.e., figures significantly lower than anywhere within the city limits. Bottom sediment, even with the systematic pollution of our rivers, turns out to be less polluted than urban soils. The indicated indicators, especially the first three, i.e., total N, organic NH₃, and organic C, are very valuable criteria when monitoring the process of self-purification on sewage fields, irrigation fields, etc. Thus, at a small experimental sewage installation in Dnepropetrovsk (Gorovits-Vlasova), the following series of figures was obtained for total N: before loading 204 mg, after loading January 12—425 mg, February 14—328 mg, March 26—154 mg, April 18—140 mg, June 26—112 mg. For organic NH₃ the corresponding series of figures is—34.85; 76.84; 62.56; 48.96; 48.28. For organic C—1,585.6; 2,820.0; 1,161.4; 764.7; 700; in June 267.3. In other words, the soil after half a year not only processed all the introduced sewage, but by the end of this period the effect of self-purification was more significant than at the beginning of the experiment. Other chemical data (e.g., the amount of mineral NH₃ and mineral C, the amount of Cl, the oxidizability of soil extracts, etc.) do not reveal any definite patterns during the systematic study of urban soils and are therefore of little use as criteria for the sanitary assessment of soil. When monitoring the processes of humification of plant residues, e.g., at garbage dumps, useful indications can be expected from determinations of cellulose, pentosans, pentoses, and humic substances. A successful attempt of this kind was made by Drachev and Skopintsev when studying the soil of dump sites in Moscow, but there is still little data on this issue. Bacteriological research of the soil consists of 5 different moments: 1) searching for pathogenic bacteria in the soil, 2) quantitative research, 3) determination of the titer of indicators of pollution by animal waste, 4) full qualitative analysis of species growing on ordinary media in both aerobic and anaerobic conditions, 5) research on bacteria with special functions that play a role in the cycle of matter. Among the pathogenic species that have been found in the soil, the first place is occupied by spore-forming anaerobes, such as B. perfringens Veillon and Zuber, Vibrion septique Pasteur, B. anthracis symptomatici Chauvoei, B. tetani Nicolaier, B. botulinus van Ermengem, and their distribution in urban soils to a certain extent coincides with the distribution of B. coli and allows one to think that they also enter the soil with human and animal excrement; the difference is apparently only that B. coli testifies to relatively fresh pollution, whereas spore-forming anaerobic species, persisting in the soil for months, can be indicators of older pollution. Minkevich comes to the same conclusions (based on studies of the soil of chernozem fields in the vicinity of Pyatigorsk, where anaerobes were detected in 100% of samples, while B. coli was absent). During the bacteriological study of the soil of the Donskoy Cemetery in Moscow at a depth of 180–300 cm, the number of anaerobic bacteria exceeded the number of B. coli by thousands of times, which again speaks in favor of the significance of anaerobes as indicators of long-standing pollution. Vibrion septique, or the bacillus of malignant edema, is also a frequent inhabitant of the soil. According to Mace (Mace), it is detected in almost 80% of soil samples from streets and gardens. Lortet, Arloing, and Roux found it in bottom sediment. B. anthracis symptomatici and B. enteritidis sporogenes Klein have also been found in the soil. To this list of pathogenic anaerobes found in the soil, one can add B. histolyticus. In 1919, its presence in the soil was proven by the work of the British Medical Commission; in 1923, Zeissler and Rassfeld and in 1925 Chiari found this species in soil samples taken from various sections of the military front; in 1923, Peterson and Hall discovered it in the soil of California fields. The tetanus bacillus has also been found in the soil of streets and gardens, especially in the surface layers. Roux found it in the silt of the Rhone River; Corte in the silt of the Dead Sea; Hall and Peterson found this species in the arable land of California (1924). It is hardly possible, however, to consider it universally distributed in the soil. B. putrificus coli was also repeatedly found in the soil of streets and courtyards. B. botulinus is apparently very common in the soil, as shown by the latest research in Belgium and America, where it was found in 60–90% of samples. Among aerobic pathogenic species, anthrax bacilli, typhoid, plague, and cholera vibrios have been found in the soil. Anthrax bacilli were found by Pasteur in the soil of so-called 'cursed fields' (see Anthrax), and he was able to see for himself the role of earthworms in transferring the infectious agent from deeper layers of the soil, where the corpses of animals that died from anthrax were buried, to the surface. Diatroptov managed to detect the presence of this species in the silt of a well on a farm where there was an anthrax epizootic. Cholera vibrios have been found in the soil of cities during epidemics; for example, in St. Petersburg in 1909–1910, they were found in 3 samples out of 61 taken near sewer pipes, as well as in 14 out of 110 samples of river silt. Typhoid bacilli have been found in arable land and in the silt of lakes. Occasionally, in the upper layers of the soil, or more precisely in its dried and pulverized particles, TB bacilli, pyogenic cocci, B. septicus agrigemis Nicolaier, which is very close to the chicken cholera bacillus, B. pyocyaneus, B. pneumoniae, and plague bacilli have been found. During the bacteriological study of the soil, one of the useful criteria for sanitary assessment is the prevalence of B. coli in it. This species was often found in the soil of streets, roads, arable land, meadows, forests, and in river silt. It was already pointed out above that the degree of its prevalence in the soil is connected with the degree of its pollution and that the coli-titer of the soil deserves to be introduced into the practice of sanitary-bacteriological research of the soil, as has long been done in relation to water.

The coli-titer of soil to a certain extent coincides with the anaerobic titer, at least in cases of fresh contamination, since, due to the competition of a huge number of soil saprophytes, B. coli, as already indicated above, quickly dies off in the soil. For quantitative determinations of bacteria in soil (as well as for qualitative bacterial investigations of deeper layers of soil), the latter is taken at the desired depth with the help of a Frankel soil auger, consisting of a hollow cylinder with a pointed end adapted for drilling. In the wall of the cylinder there is a window covered by a door, which, when the auger is turned to the left, automatically opens, allowing the soil to penetrate into the cavity of the cylinder (Fig. 4). In quantitative bacterial investigation of soil, it is necessary to keep in mind that on ordinary meat-peptone media and, moreover, under aerobic conditions, only an insignificant percentage of the bacterial population of the soil grows. In connection with this, Winogradsky in recent years has put forward the question of the so-called "microbial landscape," i.e., the directly observed picture of a soil extract under a microscope (fixed on a glass slide with 1/1000 agar and stained with 1% eosin or erythrosin): the number of bacteria counted in this way exceeds by hundreds and thousands of times the number of colonies growing on meat-peptone agar.

times the number of colonies growing on meat-peptone agar.

on agar. The cultures are made from a water extract (approximately 30-40 mg of soil in 5 cm³ of sterile water), obtained by 10-minute shaking with glass beads, under which conditions up to 97% of all bacteria capable of growing in the above-mentioned culture conditions pass into the extract (Gorovits-Vlasova). The surface layers of urban soils give hundreds of thousands, millions, and tens of millions of colonies per 1 g; in deeper layers their number gradually decreases, and at a depth of 3-4 m the soil proves to be very poor in microbes or even sterile. The following figures of Reimers and Kramer give an idea of the distribution of microbes in various layers of soil (Table 3). Table 3. Number of colonies Number of colonies Reimers' data in 1 g Kramer's data in 1 g Soil on the surface of a field . . 2,564,800 Soil on depth of 2 m (clay) 23,100 mixed with humus: 20 cm depth 650,000 Soil on depth of 60 cm (gravel) 500,000 70 cm depth 276,000 Soil on depth of 1 m 36,000 4.5 m (sand) 1.2 m depth 5,600 Soil on depth of 1.4 m 1.65 m depth individual colonies Similar results have been obtained by a number of authors. Among the microbes that do not grow on ordinary media and deserve attention both for the peculiarities of their biochemical functions and because of their practical importance in the processes of the cycle of substances, and therefore have significance for microbiology, agronomy, and sanitation, one can name nitrogen-fixing, nitrifying, and cellulose-decomposing species. Some nitrogen-fixing and cellulose-decomposing species can develop on ordinary media. As for other groups of bacteria participating in the cycle of substances, such as putrefactive species, species decomposing carbohydrates and alcohols, fats, urea, denitrifying, reducing sulfates, sulfites, phosphates, etc., many of these species grow on ordinary meat-peptone media. For the study of soil for nitrogen-fixing species, the methodology developed in 1893 by Vinogradsky is used, based on the use of nitrogen-free liquid and solid media. The anaerobic nitrogen fixer described by Vinogradsky under the name Clostridium pasteurianum is apparently very close to the ordinary Clostridium butyricum, which grows well on ordinary media and is widely distributed in urban soils. The anaerobic titer in urban soil is usually 1-2 g of soil. The aerobic nitrogen fixer, Azotobacter chroococcum, according to some authors, is extremely widespread in various soils; according to others, it is found mainly in uncontaminated soils. Nitrifying bacteria, Nitrosomonas and Nitrobacter of Vinogradsky, are detected by sowing soil into appropriate media devoid of organic substances with ammonium salts as the source of N for the first species and nitrite salts for the second. These species are extremely widespread in soil and are found even when extracts of the corresponding soils do not contain nitrites and nitrates, for example, when there is an abundance of organic substances, at low t° and other conditions that inhibit the vital activity of these species. Species decomposing cellulose are discovered in soil by Omelyansky's method, i.e., by sowing soil into a mineral liquid medium containing filter paper as the only source of C. Besides B. cellulosae methanicus and B. cellulosae hydrogenicus, discovered by Omelyansky, at present a whole series of aerobic species decomposing cellulose are known. These species are apparently not universally distributed in soil and are found mainly in soils rich in plant residues. In view of the great importance of these three groups of bacteria, especially the last two, in the processes of self-purification of soil, it is useful when studying soils of dumps, septic fields, irrigation fields, biological filters, etc., to systematically determine the corresponding titers; such attempts were made earlier by Remy, Giltay and Stermer and others. To judge the richness of soil in species capable of causing decomposition and mineralization of proteins, one can also determine their 'titer', i.e., the smallest amount of soil which, when sown into meat-peptone broth or peptone water, causes the appearance of putrefactive decomposition products, such as NH₃, H₂S, indole, skatole, mercaptan, etc. This group includes the most diverse species, both aerobic (Proteus, B. cloacae, etc.) and anaerobic, which have already been mentioned above, namely: B. putrificuscoli, B. cadaveris sporogenes, B. perfringens, then the extensive group of aerobic spore-formers, numbering many dozens of species, such as B. subtilis, B. mesentericus vulgatus, B. megatherium, B. ramosus liquefaciens, B. cereus Frankland, B. loxosus Burchard, B. hyalinus Jordan, B. pseudoanthracis Burri, B. corrugatus Flugge, B. vitreus Lembke, B. mycoides Flugge, B. implexus Zimmermann, B. anthracis simulans Wahrlich, B. casei Adametz, B. subsetosus Henrici, B. filamentosus Burchard, B. flexilis Burchard, B. aerophilus Liborius, B. tomentosus Henrici, B. filiformis Tils, etc. For the study of soil for ureobacteria, i.e., species decomposing urea, soil in descending quantities is sown into liquid media [broth or Sohn-gen's mineral medium] with the addition of urea, followed by sowing onto Sohn-gen's or Beijerinck's solid media. Various species of this group are not uncommon in soil, such as Micrococcus ureae Miquel, Sarcina ureae, Urobacillus Leube, Urobacillus Pasteur, etc. For the study of soil for denitrifying species, one can use broth or various mineral media, for example, Giltay's medium with the addition of 0.1-0.2% KNO₃; the appearance of nitrites in them serves as proof of the presence of bacteria capable of causing phase I of denitrification; to detect agents of phase II or so-called true denitrification, the appearance of bubbles of gaseous nitrogen serves as a criterion, and for greater clarity it is useful to increase the KNO₃ content to 0.5-1%. One can also successfully replace it with a nitrite salt by introducing 0.5-1% NaNO₂ into the medium. By sowing descending amounts of the soil under investigation into these media, one can determine the corresponding denitrifying titers. The number of denitrifying bacterial species in soil is very great. Phase I of denitrification is caused by representatives of the groups Proteus, B. coli, B. fluorescens, numerous cocci, such as Micrococcus albus and M. flavus liquefaciens, M. aquatilis, M. concentricus, etc.; among agents of true denitrification one must name B. pyocyaneus, B. fluorescens liquefaciens, B. denitrificans agilis; some races of B. coli also possess this property. For the study of soil for desulfurizing species, it is sown into Beijerinck and van Delden's protein-free medium with 0.1% MgSO₄; on the corresponding solid media with the addition of iron salts, colonies of species reducing sulfates to hydrogen sulfide are colored black due to the formation of FeS; by replacing sulfate salts with sulfite or thiosulfate salts in the medium, one can detect species reducing these salts; among the last two groups there are also species that do not reduce sulfates. Wilson and Blair proposed for the detection of desulfurizing species an agar with 1% glucose, 0.08% FeCl₃, 0.06% NaOH and 0.2% Na₂S₂O₃ (the last salt can also be replaced here, if desired, with Na₂SO₃ or Na₂SO₄); to prevent rapid oxidation of the H₂S formed, agar is layered over the cultures, under which colonies of desulfurizing bacteria retain their intensely black color for several days. Among species with these functions are numerous, often occurring in soil species. They include the already mentioned anaerobes, such as B. perfringens, B. Chauvaei, B. sporogenes, B. botulinus, also pathogenic ones, such as B. typhi, B. Gartneri, then putrefactive ones, such as Proteus vulgaris, finally species from the extensive group of aerobic spore-formers, such as B. subtilis, B. mesentericus, B. corrugatus, B. loxosus, B. terminalis, etc. In soils where putrefactive processes occur, and in the bottom silt of rivers, ponds, etc., sulfur bacteria may also be present, for the detection of which soil is sown into media poor in organic substances and containing about 0.3% sulfur salts. In recent years, the doctrine of the transformation of substances in soil has been supplemented by a new chapter regarding the transformations of phosphates. Rudakov in 1927 observed the reduction in soil of phosphates to phosphites, hypophosphites and even phosphine under the action of one bacterial species of the paracolonic type. Gorovits-Vlasova, having noted the decrease in phosphates in the soil of an experimental septic installation, isolated from it 2 species-Clostridium butyricum aerobicum and B. glycogenicum, which in pure culture caused in media containing phosphates the reduction of the latter.

As for the species that decompose carbohydrates, they are very numerous in soil; for example, polysaccharides of the starch and dextrin type are decomposed by most of the aforementioned anaerobic and aerobic spore-forming, mono- and di-saccharides are decomposed by lactic acid, butyric acid, group B. coli, yeast, molds, etc. Pentoses are decomposed by the activity of certain Clostridia, B. solaniperda, and others. The decomposition of cellulose was discussed above. The banal microflora of soil, growing on meat-peptone media, is represented by many dozens of species, also found in water and in the air; some of them, such as B. mycoides, B. violaceus, Actinomyces, due to the frequency of their finding specifically in soil, attract special attention as typical 'soil species'. Regarding the halophilic flora of soil, there are no data in the literature, although such flora undoubtedly exists in solonchak soils. Rubenchik found halophilic, i.e., salt-loving species, and even halobes, i.e., those that cannot grow on ordinary media without the addition of salt, in the silt of Kuyalnitsky Liman. It is highly probable that this long list of bacterial species with various biochemical functions by no means exhausts the diversity of soil flora as an active factor in the cycle of substances, and will be increased over time; however, the data available at the present moment are sufficient to characterize the sanitary-epidemiological significance of soil, as well as the enormous role of its microflora in the processes of self-purification of soil, which underlies a whole range of sanitary-technical installations. In view of the great importance of soil purity in the issues of sanitation of populated areas, its sanitary protection is a matter of paramount importance for the improvement of all populated areas. The basic prerequisite for the sanitary protection of soil in populated areas is the radical removal of sewage and refuse. With the presence of proper sewerage and garbage incineration systems, systematic pollution of soil is largely eliminated. Cremation of corpses eliminates the introduction of putrefying material into soil. Paving city squares and tar-coating roads are also highly appropriate ways to protect soil from pollution and infection. In special cases, the question of disinfection of soil during epidemics of cholera, typhoid fever (e.g., around cesspools) may arise; in these cases, lime milk, bleaching powder, etc. are usually resorted to. Gas warfare has recently raised a new question about the decontamination of soil when using mustard gas and other poisonous substances (see Decontamination). The question of the role of soil in human health poses to the sanitary physician the task of sanitary assessment of soil both in populated areas and in areas intended for settlement. The most favorable for settlement are dry sandy soils, excluding the possibility of marsh formation and allowing good ventilation, as well as filtration of atmospheric waters. To assess the degree of soil pollution, it is first necessary, as in the sanitary assessment of bodies of water, to take into account the living conditions, i.e., its previous use (old garbage dumps, etc., presence or absence of vegetation cover, methods used for removal of sewage and refuse, etc.). The most reliable criterion here, as in the sanitary assessment of water, is the chemical-bacteriological investigation. From what has been said above and from the sanitary assessment given in Table 1, it is clear that among the chemical indicators, the most reliable in this matter are total N, organic C and P2O5; among the bacteriological ones - the total number of bacteria growing on meat-peptone media, coli-titer and anaerobic titer. These indicators allow both to assess the degree of soil pollution at any given moment and to monitor the process of its self-purification on dumps and sewerage fields; in the latter case, the figures of nitrites and nitrates also give valuable indications, allowing to monitor the intensity of the nitrification process.

L. Gorovits-Vlasova. Soil as a source of human infection with parasites plays a very important role. Its significance in this respect is twofold: 1) Soil is a substrate in which part of the parasite's life cycle takes place and the infective forms of the latter develop; 2) Soil serves as an indirect, accidental source of infection with parasites that are in the infective state in other habitats. In some cases, parasites, being in soil, develop all the time under the shell of the egg, in others - larvae come out, which live in soil for some time and then penetrate the host's body. When assessing the significance of soil in the epidemiology of parasitic diseases, it is necessary to take into account the physical and chemical properties of soil, its qualities from the point of view of soil science, humidity, its structure, the state of groundwater, the presence of soil derivatives in the form of vegetation cover, the presence of various animal inhabitants in soil, etc. In addition to these general data, it is absolutely necessary to study this or that territory of soil against the background of the geographical landscape and as a station of the economy, subject to the powerful influence of man. In light of these viewpoints, one can approach the establishment of the parasitological significance of soil as a real segment of territory with all its natural and artificial features. In particular, the study of the microclimate, i.e., climatic factors of the immediate surroundings of the parasite, when it is on the surface of soil or in its thickness, is of exceptionally important significance. Thus, the value of insolation will be different depending on whether the parasites are in direct sunlight or are on the same soil in the shade of vegetation. The temperature effects on parasites in terms of the amplitude of temperature fluctuations by hours of the day and by seasons of the year decrease depending on the depth of the parasites' occurrence in soil. Similarly, the degree of humidity in the capillary spaces between soil particles as an element of microclimate will differ from the fluctuations in humidity recorded at the same geographical point by a meteorological station. Many parasites inhabit burrows of various animals; the structure of these burrows, their depth, the nature of the bedding and supplies collected by the burrow owners, and the species position with the biological features of the burrow builders - all this affects the fauna of parasites. Soil can serve as a permanent or temporary shelter for ectoparasites. In relation to endoparasites, it naturally plays a role as a stage in their life cycle. Endoparasites enter soil from their hosts through contamination of soil with feces and excretory products. If in relation to animal hosts of parasites such contamination is typical for each species of host, then the analogous role of man in this respect varies greatly depending on his cultural development and the level of sanitary-hygienic state of labor and living conditions (sanitary skills, types of toilets, etc.). In relation to parasitic protozoa, soil plays the role of a source of infection with various intestinal parasites, such as: dysentery amoeba, intestinal amoeba, various flagellates, etc. Infection occurs through garden greens contaminated with cysts of protozoa when soil is contaminated with feces as fertilizer. A significant role is played by contamination of soil waters and the transfer of cysts by them to other areas of soil. Some folk remedies, such as applying raw earth to the genitals of women, can lead to the entry of some organisms as false parasites into the genital tract. Many of the parasitic worms need soil as a temporary habitat. Such are the human ascaris and whipworm, whose eggs require a certain time for the development of the worm-like embryo in them. Here the properties of the soil into which the eggs of these parasites have fallen, its humidity, shading, etc., are manifested. In relation to hookworms - Ankylostoma duodenale and Necator americanus - the significance of soil as a substrate for development increases even more, because the hatching of larvae from eggs occurs in soil; here the further life of the larvae takes place, leading to the development of infective forms; an extremely important moment is the fact that infection of a person with hookworms occurs through direct contact of the human body with infected soil, from which the infective larvae of hookworms actively burrow into the human's coverings. Similar routes of infection are also noted for Strongyloides stercoralis, whose life cycle is even more complex than that of hookworms. On the other hand, such worms as pinworm, dwarf tapeworm, armed and unarmed tapeworm, give eggs with an already developed embryo; in connection with this, soil for the aforementioned worms has a limiting significance, mainly in relation to the conditions determining the duration of preservation of egg vitality (resp. infectivity for the host). For external parasites, soil is important as a habitat for the larvae of fleas, mosquitoes (Phlebotomus), some horseflies and flies. In addition, on this same substrate, the pupation of larvae of botflies (family Tachinidae, previously separated into family Oestridae) and parasitic flies (Wohlfart's fly, etc.) occurs. Finally, in soil live the larvae of beetles that are intermediate hosts of various parasitic worms.