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.
[Illustration: FIG. 42.]
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.