CHAPTER XXXIII.

THE SOIL.

=The Varieties of Soil.=—The following facts summarise what is regarded as the relative healthiness of various sites for dwellings. The differences between different sites may, however, be reduced to a minimum by having the dwelling well above the ground-level and by protecting it from dampness.

1. =Granitic=, =Metamorphic=, and =Trap Rocks= usually form healthy sites for houses. The slope is generally great, and the ground consequently dry.

2. =Clay Slate= resembles the last in its effects on health. Water is, however, often scarce, owing to the impermeability of the rocks, and for the same reason occasional floods occur.

3. =Limestone= and =Magnesian Limestone Rocks= resemble the last in possessing considerable slope, so that the water passes away quickly. The hard oolite is the best formation under this head, and magnesian limestone the worst.

4. =Chalk= is a healthy soil when unmixed with clay, and permeable. Goitre is not so common as in limestone districts. If the chalk be mixed with clay, it is often damp and cold.

5. =The Sandstones= are healthy, soil and air being dry. If mixed with clay, or if clay lie under a shallow layer of sand-rock, the site may be damp. The hard millstone grit is a healthy formation.

6. =Gravels= of any depth are healthy, unless they are water-logged, as near rivers. Then a house on impervious clay may be drier than one on gravel.

7. =Sands= are healthy when of considerable depth; they may be unhealthy when shallow, and lying on a clay basis; or when the ground water rises through them from ground at a higher level.

8. =Clay=, =Dense Marls=, and =Alluvial Soils generally=, are apt to be cold and damp. Water is retained in them, and is often very impure. Thorough drainage improves a clay soil, and a house on a clay soil may be so constructed, as not to be damp.

9. =Cultivated Soils= are not necessarily unhealthy; but

10. =Made Soils= are always to be carefully avoided, as sites for houses. The materials with which inequalities have been filled up are commonly the contents of dust-bins, or some other refuse. The gradual putrefaction of organic matters renders the air about the houses impure. Such soils require free subsoil drainage, in order to keep them dry. It appears that the organic matters in soil are gradually removed by oxidation and bacterial purification. At least three years should be allowed before any such site is built on.

The following table places different geological formations in their order of healthiness for the purposes of a site (Parkes):—

┌──────────────────────────────────────────┬────────────┬─────────────┐ │ │PERMEABILITY│ EMANATIONS │ │ │ OF WATER. │ INTO AIR. │ ├──────────────────────────────────────────┼────────────┼─────────────┤ │1. _Primitive rocks, clay slate, millstone│ │ │ │ grit_ │ Slight. │ None. │ │2. _Gravel and loose sands, with permeable│ │ │ │ subsoils_ │ Great. │ Slight. │ │3. _Sandstones_ │ Variable. │ Slight. │ │4. _Limestones_ │ Moderate. │ ── │ │5. _Sands with impermeable subsoil │Arrested by │Considerable.│ │ │ subsoils. │ │ │6. _Clays, marls, alluvial soils_ │ Slight. │Considerable.│ │7. _Marshes, when not peaty_ │ Slight. │Considerable.│ └──────────────────────────────────────────┴────────────┴─────────────┘

The general geological conditions have an important bearing on the choice of a site for a house in so far as they affect the local climate, and the difficulty of keeping the house warm and dry. Pettenkofer expressed this in his dictum, that we take holiday for change of soil, rather than for change of air. The character of a soil has an important influence on humidity, radiation, evaporation, and in fact most of the factors going to make up “climate.” The immediate local surroundings of a house (page 201) have an even greater influence on its salubrity than the underlying geological formation.

The soil consists of mineral and organic matters. On the amount and character of the animal and vegetable matters (along with the condition of moisture and aeration), the healthiness of a given soil depends. The presence of vegetable matter, subject to alternate wettings and dryings, and to heat, has until recently been regarded as the condition on which malaria depends; but it is now known that malarial places owe their character to their being favourable to the growth of the larvæ of certain mosquitoes (page 307); and that drainage of the soil cures malaria by removing the ponds in which these develop. The two chief agencies at work to rid the soil of organic impurities, are nitrification and the influence of growing plants. The organic matters become oxidised into ammonia, nitrites, and nitrates, and these are eagerly assimilated by vegetation.

=Nitrification= is effected by micro-organisms in the soil. Ordinary garden mould and agricultural humus contain large numbers of micro-organisms. Their number diminishes with the depth of the soil, and below 12 to 15 feet there are few. Apart from the occasional presence of pathogenic (disease-producing) micro-organisms, the most important are those producing oxidation of organic matter, especially _nitrification_. This occurs at a less depth than 4 feet from the surface of the ground. The operation of these micro-organisms is necessary to convert sewage and other impurities into harmless nitrites and nitrates, and it is regularly going on in all normal soils. That the power of purification of sewage by soil is due to the micro-organisms in the latter, can be proved by the fact that when the soil is baked, it loses for a time its purifying power.

=The Air contained in the Soil= varies greatly in amount with the character of the soil, and with the level of the ground-water. As the ground-water rises, the ground-air is driven out. Thus, after a heavy rainfall a large proportion of this air will be displaced. Variations in barometric pressure, and a rise or fall of temperature, cause movements in ground-air. A house artificially warmed is liable to receive air from underground, unless means are adopted to make the floors impervious. The warmth of the house acts as an air-pump, aspirating the colder air into its interior. The air from cesspools or defective drains may be similarly aspirated into the house; and the same cause particularly explains the unhealthiness of houses built on “made soils”. Coal gas has occasionally made its way into houses when not laid on to them, by the gas escaping from leaky pipes in the street often following the track of water or drain-pipes until it is aspirated from beneath the house into its interior. This has resulted in one instance in an explosion, and in others in poisoning by the gas.

The occurrence of currents of air in soil may be illustrated by a simple experiment. In Fig. 42 B is filled with fine sand in which is imbedded the tube A with its open end F at the bottom of the sand C. The upper end of A is connected by the rubber tubing D with the U-shaped tube E, in which is inserted some coloured water. When the experimenter blows on the surface of the sand at A, the impulse passes through the sand up the tube from F, and deflects the water in the syphon bend at E.

The _amount_ of ground-air varies greatly. Loose sands often contain 40 to 50 per cent., soft sandstone 20 to 40 per cent., and loose surface-soil many times its own volume.

The _nature_ of the air is not accurately known. It is, however, extremely rich in carbonic acid, of which it contains from 1 to 10 per cent. or even more. The carbonic acid is derived from the organic matter in the soil, by the action of bacteria, in a manner analogous to nitrification.

=The Water contained in the Soil= is divided into moisture and ground or subsoil-water. When air is present in the soil as well as water, the soil is merely moist. Pettenkofer defines the ground-water as that condition in which all the interstices are filled with water, so that, except in so far as its particles are separated by solid portions of soil, there is a continuous sheet of water.

The =Moisture= in the soil varies in amount. Open gravel will absorb from 9 to 13 per cent. by weight of water; gravelly surface soil 48 per cent.; light sandy soils from 23 to 36 per cent.; loamy soil 43 per cent.; stiff land and clay soils from 43·3 to 57·6 per cent.; sandy and peaty soils from 61·5 to 80 per cent.; peat 103 per cent. (B. Latham). The moisture being derived from the rainfall on one side, and the ground-water on the other, will vary with the amount of these. Some soils are practically _impermeable_ to water, such as trap or metamorphic rocks, unweathered granite, hard limestone, and dense clay; while others, such as chalk, sand, sandstone, vegetable soils are _permeable_. Commonly the metamorphic rocks and hard limestones present fissures, which render them pervious. The rainfall which does not penetrate the soil flows into the streams and rivers at once, or is re-evaporated. The amount of _percolation_ of rainfall is estimated by an artificial soil-gauge. Most percolation and least evaporation of rainfall occurs from October to March inclusive. The difference between the percolation and rainfall is the loss caused by evaporation and vegetation.

The =Ground-water= forms a subterranean sheet of water, which is in constant motion. There is first of all, an irregular rise and fall of the water, according as it receives new additions from the rainfall, or loses a certain amount of its substance by percolation and evaporation; and there is, secondly, a constant movement towards the nearest water-course or the sea. Many towns derive their drinking-water from the ground-water, especially that in the chalk. Thus in Brighton there are no streams; but wells are dug in the South Downs about 150 to 180 feet deep down to the level of the subterranean water. Then long adits are tunnelled, parallel to the coast at or near the level of this water, which is thus intercepted on its way towards the sea, and pumped up to supply the town. In Munich, Pettenkofer reckoned the rate of movement of the ground water towards the outlet as 15 feet daily. It is impeded by impermeability, or a deficient slope of the soil. The roots of trees also greatly impede its flow.

_The level of the ground-water_ is constantly changing (see Fig. 7). The alteration in level may be only a few inches either way, while in some parts of India it is as much as 16 feet. The level is generally lowest in October and November, highest in February and March.

A _fall_ in the level of the ground-water may be due to a dry season, or to improved subsoil drainage. A _rise_ in its level is due to an increase in the rainfall, or some obstruction in the outflow, as from a swollen river. The tide may influence the level of the ground-water at a great distance. A sudden alteration in the level of the ground-water is a common cause of floods in mines.

The distance of the ground-water from the surface may be only two or three feet, or several hundred feet, the difference being due to the varying level of the nearest impervious stratum of soil. Its distance below the surface of the soil can easily be measured by ascertaining that of the water of a shallow well in the neighbourhood. It should preferably not be nearer the surface than five or six feet. Sudden changes in the level of the ground-water from inundations render any soil unhealthy, and are even more objectionable than a persistently high level. This is especially true in the case of permeable soils. A sudden rising of ground-water expels the air in the soil, together possibly with particles which may comprise infectious material; it also washes similar impurities out of the subsoil, and carries them into neighbouring wells. Numerous epidemics have been traced to this source.

=The Temperature of the Soil= varies greatly with its geological character, as well as with the temperature of the atmosphere. The daily changes in the temperature of the atmosphere do not affect the soil beyond a depth of about three feet. The annual changes in the atmosphere will affect the soil in a varying degree, the amount being dependent on the character of the soil as regards conductivity and retentiveness for heat. Such annual variations do not penetrate below forty feet, and are very small below twenty-four feet. The temperature of the earth increases with its depth, the rate of increase in England being stated to be about 1° Fahr. for every 54½ feet.

In England the water of permanent springs has a fairly constant temperature of 49° to 51° Fahr., which is the temperature of the deeper part of the subsoil. The method of taking the daily temperature of the subsoil at a depth of 4 feet is described on page 240.

Although the average temperature of any soil depends on the climate, soils conduct heat in a very varying degree, and therefore absorb unequal quantities. This has an important bearing on the comfort of those living on a particular soil. Schübler’s experiments give the absorbing power of the chief kinds of soil, 100 being taken as the standard.

┌───────────────────────────────┐ │ _Sand, with some lime_ 100·0 │ │ _Pure sand_ 95·6 │ │ _Light clay_ 76·9 │ │ _Gypsum_ 73·2 │ │ _Heavy clay_ 71·1 │ │ _Clayey earth_ 68·4 │ │ _Pure clay_ 66·7 │ │ _Fine chalk_ 61·8 │ │ _Humus_ 49·0 │ └───────────────────────────────┘

It is evident from this table that sand is very retentive of heat, while clays and humus are very cold. Green vegetation lessens the absorbing power of the soil, and radiation of heat is more rapid, evaporation occurring constantly from the herbage. The influence of trees on the temperature of the soil is considered on page 228.

Damp soils are colder than dry soils because of the evaporation going on. Buchan finds as the result of drainage of the soil, that (1) the mean temperature of arable land is raised 0·8° Fahr.; (2) cold is propagated more quickly through undrained land; (3) drained land loses less heat by evaporation; (4) the temperature of drained land is more equable, and (5) in summer is often 1·5° to 3° above that of undrained land.

DISEASES ARISING FROM THE SOIL.—The soil may be a cause of disease: (_a_) indirectly and (_b_) directly.

_Indirectly_ a damp soil may cause disease by acting as a means of lowering the vitality of man and diminishing his resistance to disease. It is in this way that it has been credited with causing such diseases as neuralgia, catarrhs, and rheumatism. It is one of the elements in producing a climate unfavourable to health. As to rheumatism, see page 225.

_Directly_ the soil may transmit the actual contagia (micro-organisms) of disease either by means of the subsoil water or its air. In the former case the disease-causing material gains access to the drinking water of wells, springs, or rivers; in the latter case it may be borne to the surface of the soil by currents of the ground-air or by insects, and then inhaled as dust, or gain access to food.

Certain disease-producing micro-organisms have been proved to be capable of living for some time in the soil. The chief of these found in the soil are the bacilli of tetanus (lockjaw), of anthrax, of malignant oedema, and of enteric (typhoid) fever. There are reasons for thinking also that the micro-organisms causing diphtheria, rheumatic fever, and epidemic diarrhœa, and possibly some other diseases, may occasionally live in the soil. In some diseases as enteric fever, cholera, dysentery and anthrax, the contamination of the soil can be shown to be derived from a patient suffering from the same diseases. In others, and particularly in tetanus, the same chain of evidence is obtainable.

(1) The conditions favourable to the production of =malarial diseases= have been generally considered to be the presence of a certain proportion of dead organic matter, the exposure of the soil to alternations of heat and moisture, with a limited access of air, and a temperature of at least 65°F. Though most common in marshy districts, and in recent alluvial soils, malaria may develop in connection with any geological formation. That it may be removed by drainage of the subsoil, is well known. The true nature of the connection between soil and malaria is stated on page 220.

(2) According to observations made by Pettenkofer in Munich, attacks of =enteric (typhoid) fever= are connected with fluctuations of the subsoil-water. He states his conclusions as follows:—

“Between the fluctuations of subsoil water and the amount and severity of enteric fever there is an unmistakable connection in this wise, that the total number of cases of and deaths from enteric fever falls with a rise of the subsoil water, and rises with fall of it; that the level reached by the disease is not in proportion, however, to the then level of the subsoil water, but only to the variation in it on each occasion; or in other words, that it is not the high or low level of the subsoil water that is decisive, but only the range of fluctuation.”

His observations have not been confirmed in this country; and the coincidence between excess of enteric fever and lowness of ground-water has been explained by the fact that under these circumstances the water in wells is low, and the area of drainage and the consequent risk of contamination are proportionately increased. There can be no doubt that the most common origin of enteric fever is from the infection of water or milk by infective matter from a recent case of the disease. This does not exclude the fact that enteric fever in this country is more prevalent in hot dry autumns, in which the ground-water is low. Probably under such conditions the contagium of the disease multiples more rapidly in the soil, in privies and other polluted places, and consequently the risks of infection of water and food as well of infection by dust carried from the contaminated spot are greatly increased.

(3) In regard to =cholera=, Pettenkofer holds similar views. He believes that the contagium of cholera can only be developed when there is a damp porous subsoil to receive the infected stools from a cholera patient; the damp porous subsoil forming a second host in which the poison of cholera must pass through one stage of its existence, before it is again capable of producing the disease. Such an essential relationship of the soil is not borne out by observations in India; and in England cholera has been repeatedly shown to be due to contamination of food (_e.g._ oysters) or water by the stools of preceding cholera patients, without the intervention of any agency of the soil.

(4) It has been repeatedly stated that a damp soil favours the prevalence of =diphtheria=. I have shown elsewhere, however, that this is not true, and that the greatest epidemics of diphtheria have occurred in exceptionally dry years, especially when several years of exceptionally small rainfall have succeeded each other; and have suggested that this may be associated with an intermediate stage in the life-history of the diphtheria-bacillus in the soil. A low ground-water and a comparatively high temperature of the soil go along with deficient rainfall, and would probably favour the multiplication of this bacillus in the soil.

(5) In =rheumatic fever= I have similarly shown that the supposed connection between damp soil and this disease is erroneous, the disease being most prevalent, both in this and other countries, in years of exceptional drought.

(6) =Epidemic or Summer Diarrhœa= has been supposed to have a special relationship with soil-temperature, Ballard having found that the summer rise in the mortality from this disease does not commence until the mean temperature recorded by the four-foot earth thermometer has attained somewhere about 56°F. The soil-temperature may be accepted as a convenient index of the conditions causing this disease. The disease I have elsewhere shown occurs most severely with a high temperature of the air and a deficient rainfall, and its fundamental cause is an unclean soil, the particulate poison from which infects the air, and is swallowed most commonly with food, especially milk.

(7) The close connection of =consumption= (phthisis) with a damp soil has been independently stated by Drs. Buchanan and Bowditch. Buchanan found that in the districts where improved sanitary arrangements had led to a drying of the soil, the death-rate from phthisis diminished; but where with sanitary improvements the soil was not dried, the death-rate from phthisis remained in one or two instances almost stationary. In Salisbury, Ely, Rugby, and Banbury, the death-rate from phthisis fell from 141 to 49 per cent. The amount of reduction in the death-rate from phthisis did not appear to be consistently proportional to the amount of drying of the subsoil. In a later investigation into the incidence of deaths from phthisis in the south-east of England, Buchanan came to the further conclusions that (_a_) there was less phthisis among populations living on pervious soils than among populations living in impervious soils; (_b_) less phthisis among populations living on high-lying pervious soils than among populations living on low-lying pervious soils; and (_c_) less phthisis among populations living on sloping impervious soils than among populations living on flat impervious soils. He, therefore, concluded that _wetness of soil is a cause of phthisis to the population living upon it_. (See also page 313).

=Drainage of the Soil.=—There are two chief plans for rendering the soil drier—deep drainage and opening the outflow.

=Subsoil Drainage= should always be carried out by drains, separate from those for sewage. If the sewers are utilised for this purpose, their contents when full contaminate the surrounding soil. The subsoil drains should be composed of agricultural, _i.e._ unglazed, drain-pipes laid in towns in the same trench, but above the sewers, and they should discharge into the nearest water-course. If it is necessary to join them with a sewer, they should not pass directly into it, but into a disconnecting man-hole.

=Opening the Outflow=, in order that water may not remain stagnant in the soil, is occasionally required. This may be done by clearing water-courses, removing obstructions, and forming fresh channels.

The provision of sufficient =surface-drains= to carry off ordinary water and storm-water helps in drying the soil of urban districts.

=Vegetation= tends to diminish dampness of soil by causing rapid evaporation, and at the same time uses up the organic matter in the soil. Certain plants are more active in producing these effects than others: the _Eucalyptus_ genus, including many species, and represented by the well-known _blue-gum tree_ of Australia, is noted for its power in this respect; and the common sun-flower, which is very easy of cultivation, has a powerful influence in the same direction.