III.THE SUBSOIL.General remarks on subsoil and its properties.Before dealing more in detail with the processes taking place in the pores of the subsoil of sewage farms, it may not be out of place to make here a few general observations on the mechanical structure of soil, its permeability, water capacity, retentive power, the capillary movements in the same, its temperature, the subsoil air, the movement of water in and through the same, the micro-organic life in soil, and its absorbing powers.1. Mechanical Structure of Soil.Size of grain and pores.Here is of interest the size of the grains or particles composing the soil, the size of the pores and their collective capacity.According to the character of the soil, its grains or particles will vary from very large in coarse gravel to very fine in fine sand and clay.Variable size of pores.Surface attraction.The size of the pores will vary as the size of its grains from large to small, but frequently a certain kind of soil will contain a mixture of large and small pores. The finer the pores the more energetic will, as a rule, be the surface attraction of the grains composing the soil.Pore-volume.With particles of equal size pore-volume amounts to about 38 per cent. of the total space, and sinks down to 10 or 15 per cent. with particles of unequal size.With equally sized particles the pore-volume is the same whether the particles are small or large.The collective capacity of the pores or the pore-volume mainly depends on the equal or unequal sizes of the particles. When the same are of equal size the pore-volume amounts to about 38 per cent. of the total space occupied by the soil, but when this is not the case it may sink to as low as from 10 to 15 per cent. of this space. With equally sized particles the pore-volume is the same whether the individual particles are large or small. In nature it will be the exception to find all the particles of equal size, such a condition of things prevails only when careful sorting by sifting or riddling has taken place, and in the majority of cases the larger pores will be partly filled up by the smaller particles of the soil.2. Permeability of Soil.Permeability depends first on the size of the pores, and secondly on the pore-volume.The permeability of a soil for the passage of air and water depends, in the first instance, on the size of the pores, and is further to some extent influenced by the pore-volume.Effect of large and small pores.Soil with large pores will offer but little resistance to the passage of air and water, but when the pores are small these movements will be greatly impeded.Permeability is proportional to the fourth power of the pore-diameter.It has been ascertained that the permeability of soils is proportional to the fourth power of the diameter of the pores, so that it decreases very rapidly with the diminishing size of the pores.In frozen soil permeability decreases rapidly.In subsoil with small pores all movements of air practically cease when it is half full of water, and in frozen soil the decrease of the permeability is still more marked.3. Water Capacity of Soil.Water capacity is equal to the pore-volume.Air can never be wholly driven out of the pores.The water capacity of a soil is that quantity of water which can be stored in its pores; it is therefore equal tothe pore-volume. For very accurate measurements allowance must be made for a small amount of air, which even after filling remains in the pores and cannot be dislodged, but for practical purposes this can be overlooked.1 cubic yard of soil with particles of equal size will hold about 85 gallons of water.As has already been stated, the pore-volume of a soil consisting of equal particles throughout, amounts to about 38 per cent. of the space occupied by it, and 1 cubic yard of such a soil—whether we have to deal with coarse gravel or fine sand—will hold about 85 gallons of water.4. Water-retentive Power of Soil.The water-retentive power of soil is a percentage of its water capacity.The water-retentive power of a soil is expressed by that quantity of water which can be retained by it; it will always be a percentage or portion of the water capacity of this soil.Soil with a large pore-volume and a large percentage of fine pores retains more water than soil with a small pore-volume and large pores.Clean gravel retains about 10gal.and clean sand about 70gal.Soil with a large pore-volume and with a large percentage of fine pores will retain more water than soil with a small pore-volume and few fine pores. Clean gravel will retain about 12 per cent. of its water capacity, i.e. 10 gallons per cubic yard, whereas fine sand may retain as much as 84 per cent. of its water capacity, i.e. about 70 gallons per cubic yard.Organically polluted soil retains more water than clean soil.This will explain why a polluted subsoil containing a large amount of organic substances will retain more water than the same soil in a clean condition.The retentive power of a soil is due to its surface attractions.The retentive power of a soil is due to the surface attraction of its particles, and when the space between them is small, or when, in other words, the pores are small, this attractive power will be all the greater.When, after the limit of the retentive power has been reached, of water are poured upon the soil, a portion of the previously stored water is driven out, and its place in the pores taken up by the fresh supply.It is further of interest to observe here, that if after the limit of the retentive power has been reached further quantities of water are poured upon the soil, the waterretained in the lower layers will commence to drain away. This means that the water freshly poured upon the soil will drive out a portion of the water previously stored in the pores. It is important to bear this in mind when dealing with polluted water, as owing to this action the water penetrating into deeper layers will to some extent at least have become purified in the upper layers.5. Capillary Movements of Water in Soil.Capillary attraction causes an upward movement of the water.Through capillary attraction an ascending movement of the water is caused in direct opposition to the laws of gravity, and the height to which water will thus ascend depends mainly on the smallness of the pores; large pores do not assist in this movement. As the same, however, extends over the whole pore-volume the quantity of water thus raised may exceed the water-retentive power of soil.Capillary attraction also causes lateral and downward movements.In addition to the upward movement brought about by capillary attraction, this power is also continually at work in a lateral and downward direction; but for the present purposes only the upward movement will be noticed.Time occupied by upward movement. Height reached by upward movement.In observing the upward movement, it is interesting to notice the time occupied by it and the total height reached. As to the time occupied, it has been established that the upward movement in gravel and coarse sand is much quicker than in fine and loamy sand, but the heights attained are reversed. For whereas the height in a material consisting of coarse or large pores amounts to from 2 inches to 4 inches; a height of about 4 feet after thirty to thirty-five days has been recorded in fine or loamy sand; in peaty soil one observer states that the upward movement of the water may reach a height of 20 feet.6. Temperature of Soil.Three principal sources of heat.The earth’s crust receives its supply of heat from three principal sources,viz.:1. From the sun through its rays;2. From the interior of the earth through conduction; and3. From various physical and chemical processes which take place in it and create heat.Heat through sun’s rays.Dark soils absorb more heat than light-coloured soils.Capacity for heat is greater in damp and fine-grained soils.Evaporation and condensation of aqueous vapour produce the greatest effect in fine-grained soils.Dealing with the upper layers of the crust, it may be said that, besides the intensity of the sun’s rays, the temperature also depends on a variety of properties possessed by various kinds of soil, amongst which latter may be mentioned the absorption of heat, which is much greater in dark than in light-coloured soils; the heat conductivity and the capacity for heat, which lead to higher temperatures in damp and fine-grained soils; and finally the evaporation and condensation of aqueous vapour, which tend to prevent extremes of heat and cold and which likewise produce the greatest effects in fine-grained soils.A fine-grained damp soil does not get so hot, but retains the heat better.It follows from these observations that a coarse-grained, dark coloured and dry soil will show the highest and lowest temperatures, whereas a fine-grained damp soil does not get so hot but retains the heat better.The temperature of the surface of the soil may exceed that of the air.It ought to be pointed out in this place that a variety of circumstances may bring about very high temperatures on the surface of the ground which considerably exceed the average temperatures of the air at the same time.Laws regulating the subsoil temperatures.Concerning the laws that have been deduced from careful and long continued observations of subsoil temperatures, it will not be necessary at this point to dealminutely with them; it must on the contrary suffice to summarise only the more important ones.With the distance from the surface of the ground,1. The differences of temperature become less,2. The temperatures are retarded, and3. The variations of short durations gradually disappear.Subsoil temperatures 18in.below surface.Subsoil temperatures at depths of 4ft.6in.and 9ft.At a depth of 18 inches below the surface the daily fluctuations are hardly observable, the temperature differences of various days become obscured, the differences between the monthly mean temperatures are less by several degrees, and the yearly fluctuation amounts only to about 10° C. At a depth of 4 feet 6 inches the latter is only 4° C., and at a depth of 9 feet it is only 1°C.Subsoil temperatures at depths from 9ft.to 33ft.Between 9 and 33 feet, according to the yearly mean of the surface, the yearly fluctuation ceases and the temperature remains the same throughout the year.Below this point an increase of temperature is observable towards the earth’s centre, which amounts to about 1° C. for every 40 feet.Retardation of temperatures with increase in depth.Concerning the retardation of the temperatures with an increase in depth below the surface, it is interesting to point out that this, according to Fodor, amounts to about three weeks for every yard, so that the yearly maximum at a depth of 1 yard will take place in August, at a depth of 2 yards in the beginning of September, and at a depth of 4 yards in October. This is on the assumption that the maximum temperature of the atmospheric air is reached in July.Frost depth about 3ft.The depth to which frost under ordinary conditions penetrates is about 3 feet, but there are cases on record where water pipes at depths of from 4 to 5 feet have been frozen up during long continued severe frost.7. Subsoil Air.Subsoil air is saturated with aqueous vapour and contains large quantities of carbonic acid.The pores of soil are either partly or wholly filled with air, which as a rule is saturated with aqueous vapour. This air consists very largely of carbonic acid (from 0·2 to 14 per cent., on an average from 2 to 3 per cent.) and to a small extent of oxygen, which has been used up for the formation of carbonic acid. It also contains traces of ammonia and gases of decomposition.The movements of subsoil air need not be considered here, and beyond these few general observations it will not be necessary to deal with the subject.8. Movements of Water in Soil.Strata above level of subsoil water.Two main strata may here be distinguished in subsoil, one above the level of the subsoil water and one below this level. The latter strata do not interest us, and those above the level of the subsoil[7]water may again be subdivided into three zones, which in descending order are as follows:--The evaporation zone;The passage zone; andThe capillary zone.One-third of the rain-water evaporates. One-third flows off the surface. One-third percolates.All these three zones must be passed by the water in its descent from the surface of the ground to the subsoil water level, and the quantity of water retained by them will depend on their state of dryness. Speaking quite generally and within wide limits, one-third of the rain-water flows off the surface, one-third evaporates, and one-third percolates into the subsoil.Evaporation zone.The evaporation zone reaches from the surface of the soil to that point below, which marks the extent of the drying influence of the atmospheric air. In the same the quantity of water stored in the pores may at times sink below the retentive power of the soil, i.e. below that quantity which can be retained in the pores owing to the mechanical powers of adhesion, etc. When it has become very dry through evaporation and other causes the zone, especially when it extends some way down, may retain large quantities of water. In a depth of 10 inches, 1 square yard of soil, with fine pores, may retain about 10 gallons of water, and as a rainfall of ½ inch produces only 2·3 gallons per square yard, it is clear that subsoil of this nature may retain a number of successive showers. During the height of summer fine porous soil may become so dry that practically no water finds its way into deeper zones; in this state the evaporation zone can be compared to a large sponge.Passage zone.The next zone traversed by the water in its downward movement is the passage zone, which lies beyond the drying influence of atmospheric air. When too far removed from the level of the subsoil water, its pores will not be completely filled with water, but will only contain that amount which is due to the retentive powers of the soil. By direct measurement it has been found that on an average a cubic yard of fine porous soil will retain from 30 to 80 gallons of water, and it can easily be calculated that in a layer from 1 to 2 yards in thickness the rainfall of a whole year may be retained. The passage zone, especially if it is of considerable thickness, represents a very large storage reservoir.Capillary zone.The last zone before the level of the subsoil water is reached is the capillary zone, in which the pores are partially or wholly filled by the upward movement—dueto capillary attraction—from the subsoil water. The extent of this filling will depend on the size of the pores.Springs.When the descending water has finally reached the subsoil water it either comes to a standstill altogether on the impervious layer or moves along the same, if the latter is not horizontal, until it may eventually leave the subsoil again by issuing therefrom in the form of visible or invisible springs.Rate of downward movement governed by pores.The rate of movement of any liquid—rain-water, sewage or other polluting liquid—is largely governed by the size of the pores. Where these are large, as for instance in coarse gravel, the descent of the water will be comparatively rapid, but when they are small it may take a very long time before the water reaches the level of the subsoil water, and in that case it will have undergone material changes as regards its chemical or bacterial composition.With a high level of subsoil water zones become indistinguishable.With a high level of subsoil water the zones may become indistinguishable, one zone reaching into the other, with the result that the whole of the soil becomes very wet.When subsoil has been artificially drained the amount of water reaching the subsoil water below the general level of the drains will depend on the size of the latter and the distance between them. In such a case the downward movement of the water through undrained soil, previously described, may be further interfered with through the ventilation of the subsoil by drains, and the drying up action caused thereby.[7]The term subsoil water is here used to denote that portion of the water in the pores of the soil, which is either at rest on or moves along the inclined plane of an impervious layer.9. The Micro-Organic Life in Soil.Soil probably original home of micro-organisms.Distribution of micro-organisms in soil.The soil is probably the original home of all micro-organisms, from which they have emigrated into other media. It contains vast numbers, and, according to some observers, 1ccm.may hold 100,000 germs. By far the greater number is found on or near the surface, and in lower layers the numbers gradually diminish, until at last a depth is reached, which depends on local conditions, where the soil is perfectly sterile. The aerobes live near the surface and carry on their work in this region, whereas the anaerobes are at work lower down in the soil.Cycle of micro-organic activity during the year.The picture of the cycle of micro-organic activity in the upper layers of the soil during the various seasons of the year is probably the following. In winter, especially during that period when frost and ice bind the earth, micro-organic life is apparently at its lowest ebb, and may in some very cold climates come to a standstill altogether, when micro-organisms may be said to hold their vegetative winter sleep. With the return of life and the awakening of nature in spring—especially with the approach of higher temperatures and the formation of moisture—micro-organic activity once more makes itself felt all round. During the summer months it is exposed to some injurious influences such as the heating and drying up of the upper layers of the soil, but, still gradually increasing, micro-organisms reach the climax of their activity during the autumnal rains, to remain in this state until with the advent of the cold season their activity gradually declines again.Micro-organic life in layers from 3ft.to 6ft.in depth.In the lower layers of the soil, down to 3 feet and 6 feet, micro-organisms are more protected against the injurious influences of the atmosphere, sunlight anddrying up, but the want of oxygen, together with the greater difficulty of removing such products as carbonic acid, has an injurious influence. As the temperature in these layers is considerably more uniform, it may be inferred that the micro-organic activity is there of a more uniform kind, less influenced by sudden changes, probably also less intense, but without pronounced periods of rest.Micro-organisms probably quickly perish in depths greater than 6ft.In depths greater than 6 feet micro-organisms probably perish very quickly owing to unfavourable conditions, and if found their presence must be explained by emigration from higher layers, not by actual growth at these depths.On sewage farms the micro-organic activity is without doubt greatly modified, and proceeds all the year round at a more uniform rate than on ordinary land, as the sewage always contains the necessary warmth and moisture so beneficial for it.10. The Absorbing Powers of Soil.Absorbing powers due to surface attraction of the particles of the soil.The finer the pores the greater the absorption.The absorbing powers of soil are due to the surface attraction of its particles or grains, and these, as has already been pointed out, will be all the greater the finer the pores are; they extend on the one hand to aqueous and other vapours and gases, and on the other to matters in solution.1cub. yd.of coarse gravel may contain 50sq. yds.of surface and 1cub. yd.of fine sand 9200sq. yds.That the attractive force of the surface of the particles is pretty considerable will be at once apparent when it is stated that 1 cubic yard of coarse gravel may contain about 140,000 grains with a combined surface of 50 square yards, and 1 cubic yard of fine sand 40 million grains with a combined surface of 9200 square yards, which is a little under 2 acres.Deodorising action of soil absorption of gases.Concerning the absorption by soil of aqueous vapour and gases (apart from condensation through a fall in temperature), dry soil with fine pores acts most energetically. The almost instantaneous deodorisation of foul-smelling gases, such as are formed by decomposing fæcal matters (earth closet) or coal gas, through a thin layer of fine dry soil is well known, and is to be explained in this way.Absorption of dissolved substances by soil.More interesting still, and also more important, is the absorption of dissolved substances by soil. In this way is to be explained the decolorising effect and the retention of dissolved polluting substances such as are contained in sewage. In the same way soil has the power of destroying such poisons as strychnine, nicotine, coniine, etc., and the experiments of Falk and others go to show that ptomaines and toxines are likewise retained and rendered harmless by it. This absorbing power of soil is of the utmost importance in agriculture, and without it soil could not possess purifying powers for polluting liquids. It is quite true that in this process of purification other factors play an important part, but they could not come into play if this absorption did not exist.The absorbing powers of soil are in some way dependent on the presence of micro-organisms and air, and in the absence of these they will soon come to a standstill.
General remarks on subsoil and its properties.
Before dealing more in detail with the processes taking place in the pores of the subsoil of sewage farms, it may not be out of place to make here a few general observations on the mechanical structure of soil, its permeability, water capacity, retentive power, the capillary movements in the same, its temperature, the subsoil air, the movement of water in and through the same, the micro-organic life in soil, and its absorbing powers.
1. Mechanical Structure of Soil.
Size of grain and pores.
Here is of interest the size of the grains or particles composing the soil, the size of the pores and their collective capacity.
According to the character of the soil, its grains or particles will vary from very large in coarse gravel to very fine in fine sand and clay.
Variable size of pores.Surface attraction.
The size of the pores will vary as the size of its grains from large to small, but frequently a certain kind of soil will contain a mixture of large and small pores. The finer the pores the more energetic will, as a rule, be the surface attraction of the grains composing the soil.
Pore-volume.With particles of equal size pore-volume amounts to about 38 per cent. of the total space, and sinks down to 10 or 15 per cent. with particles of unequal size.With equally sized particles the pore-volume is the same whether the particles are small or large.
The collective capacity of the pores or the pore-volume mainly depends on the equal or unequal sizes of the particles. When the same are of equal size the pore-volume amounts to about 38 per cent. of the total space occupied by the soil, but when this is not the case it may sink to as low as from 10 to 15 per cent. of this space. With equally sized particles the pore-volume is the same whether the individual particles are large or small. In nature it will be the exception to find all the particles of equal size, such a condition of things prevails only when careful sorting by sifting or riddling has taken place, and in the majority of cases the larger pores will be partly filled up by the smaller particles of the soil.
2. Permeability of Soil.
Permeability depends first on the size of the pores, and secondly on the pore-volume.
The permeability of a soil for the passage of air and water depends, in the first instance, on the size of the pores, and is further to some extent influenced by the pore-volume.
Effect of large and small pores.
Soil with large pores will offer but little resistance to the passage of air and water, but when the pores are small these movements will be greatly impeded.
Permeability is proportional to the fourth power of the pore-diameter.
It has been ascertained that the permeability of soils is proportional to the fourth power of the diameter of the pores, so that it decreases very rapidly with the diminishing size of the pores.
In frozen soil permeability decreases rapidly.
In subsoil with small pores all movements of air practically cease when it is half full of water, and in frozen soil the decrease of the permeability is still more marked.
3. Water Capacity of Soil.
Water capacity is equal to the pore-volume.Air can never be wholly driven out of the pores.
The water capacity of a soil is that quantity of water which can be stored in its pores; it is therefore equal tothe pore-volume. For very accurate measurements allowance must be made for a small amount of air, which even after filling remains in the pores and cannot be dislodged, but for practical purposes this can be overlooked.
1 cubic yard of soil with particles of equal size will hold about 85 gallons of water.
As has already been stated, the pore-volume of a soil consisting of equal particles throughout, amounts to about 38 per cent. of the space occupied by it, and 1 cubic yard of such a soil—whether we have to deal with coarse gravel or fine sand—will hold about 85 gallons of water.
4. Water-retentive Power of Soil.
The water-retentive power of soil is a percentage of its water capacity.
The water-retentive power of a soil is expressed by that quantity of water which can be retained by it; it will always be a percentage or portion of the water capacity of this soil.
Soil with a large pore-volume and a large percentage of fine pores retains more water than soil with a small pore-volume and large pores.Clean gravel retains about 10gal.and clean sand about 70gal.
Soil with a large pore-volume and with a large percentage of fine pores will retain more water than soil with a small pore-volume and few fine pores. Clean gravel will retain about 12 per cent. of its water capacity, i.e. 10 gallons per cubic yard, whereas fine sand may retain as much as 84 per cent. of its water capacity, i.e. about 70 gallons per cubic yard.
Organically polluted soil retains more water than clean soil.
This will explain why a polluted subsoil containing a large amount of organic substances will retain more water than the same soil in a clean condition.
The retentive power of a soil is due to its surface attractions.
The retentive power of a soil is due to the surface attraction of its particles, and when the space between them is small, or when, in other words, the pores are small, this attractive power will be all the greater.
When, after the limit of the retentive power has been reached, of water are poured upon the soil, a portion of the previously stored water is driven out, and its place in the pores taken up by the fresh supply.
It is further of interest to observe here, that if after the limit of the retentive power has been reached further quantities of water are poured upon the soil, the waterretained in the lower layers will commence to drain away. This means that the water freshly poured upon the soil will drive out a portion of the water previously stored in the pores. It is important to bear this in mind when dealing with polluted water, as owing to this action the water penetrating into deeper layers will to some extent at least have become purified in the upper layers.
5. Capillary Movements of Water in Soil.
Capillary attraction causes an upward movement of the water.
Through capillary attraction an ascending movement of the water is caused in direct opposition to the laws of gravity, and the height to which water will thus ascend depends mainly on the smallness of the pores; large pores do not assist in this movement. As the same, however, extends over the whole pore-volume the quantity of water thus raised may exceed the water-retentive power of soil.
Capillary attraction also causes lateral and downward movements.
In addition to the upward movement brought about by capillary attraction, this power is also continually at work in a lateral and downward direction; but for the present purposes only the upward movement will be noticed.
Time occupied by upward movement. Height reached by upward movement.
In observing the upward movement, it is interesting to notice the time occupied by it and the total height reached. As to the time occupied, it has been established that the upward movement in gravel and coarse sand is much quicker than in fine and loamy sand, but the heights attained are reversed. For whereas the height in a material consisting of coarse or large pores amounts to from 2 inches to 4 inches; a height of about 4 feet after thirty to thirty-five days has been recorded in fine or loamy sand; in peaty soil one observer states that the upward movement of the water may reach a height of 20 feet.
6. Temperature of Soil.
Three principal sources of heat.
The earth’s crust receives its supply of heat from three principal sources,viz.:
1. From the sun through its rays;
2. From the interior of the earth through conduction; and
3. From various physical and chemical processes which take place in it and create heat.
Heat through sun’s rays.Dark soils absorb more heat than light-coloured soils.Capacity for heat is greater in damp and fine-grained soils.Evaporation and condensation of aqueous vapour produce the greatest effect in fine-grained soils.
Dealing with the upper layers of the crust, it may be said that, besides the intensity of the sun’s rays, the temperature also depends on a variety of properties possessed by various kinds of soil, amongst which latter may be mentioned the absorption of heat, which is much greater in dark than in light-coloured soils; the heat conductivity and the capacity for heat, which lead to higher temperatures in damp and fine-grained soils; and finally the evaporation and condensation of aqueous vapour, which tend to prevent extremes of heat and cold and which likewise produce the greatest effects in fine-grained soils.
A fine-grained damp soil does not get so hot, but retains the heat better.
It follows from these observations that a coarse-grained, dark coloured and dry soil will show the highest and lowest temperatures, whereas a fine-grained damp soil does not get so hot but retains the heat better.
The temperature of the surface of the soil may exceed that of the air.
It ought to be pointed out in this place that a variety of circumstances may bring about very high temperatures on the surface of the ground which considerably exceed the average temperatures of the air at the same time.
Laws regulating the subsoil temperatures.
Concerning the laws that have been deduced from careful and long continued observations of subsoil temperatures, it will not be necessary at this point to dealminutely with them; it must on the contrary suffice to summarise only the more important ones.
With the distance from the surface of the ground,
1. The differences of temperature become less,
2. The temperatures are retarded, and
3. The variations of short durations gradually disappear.
Subsoil temperatures 18in.below surface.Subsoil temperatures at depths of 4ft.6in.and 9ft.
At a depth of 18 inches below the surface the daily fluctuations are hardly observable, the temperature differences of various days become obscured, the differences between the monthly mean temperatures are less by several degrees, and the yearly fluctuation amounts only to about 10° C. At a depth of 4 feet 6 inches the latter is only 4° C., and at a depth of 9 feet it is only 1°C.
Subsoil temperatures at depths from 9ft.to 33ft.
Between 9 and 33 feet, according to the yearly mean of the surface, the yearly fluctuation ceases and the temperature remains the same throughout the year.
Below this point an increase of temperature is observable towards the earth’s centre, which amounts to about 1° C. for every 40 feet.
Retardation of temperatures with increase in depth.
Concerning the retardation of the temperatures with an increase in depth below the surface, it is interesting to point out that this, according to Fodor, amounts to about three weeks for every yard, so that the yearly maximum at a depth of 1 yard will take place in August, at a depth of 2 yards in the beginning of September, and at a depth of 4 yards in October. This is on the assumption that the maximum temperature of the atmospheric air is reached in July.
Frost depth about 3ft.
The depth to which frost under ordinary conditions penetrates is about 3 feet, but there are cases on record where water pipes at depths of from 4 to 5 feet have been frozen up during long continued severe frost.
7. Subsoil Air.
Subsoil air is saturated with aqueous vapour and contains large quantities of carbonic acid.
The pores of soil are either partly or wholly filled with air, which as a rule is saturated with aqueous vapour. This air consists very largely of carbonic acid (from 0·2 to 14 per cent., on an average from 2 to 3 per cent.) and to a small extent of oxygen, which has been used up for the formation of carbonic acid. It also contains traces of ammonia and gases of decomposition.
The movements of subsoil air need not be considered here, and beyond these few general observations it will not be necessary to deal with the subject.
8. Movements of Water in Soil.
Strata above level of subsoil water.
Two main strata may here be distinguished in subsoil, one above the level of the subsoil water and one below this level. The latter strata do not interest us, and those above the level of the subsoil[7]water may again be subdivided into three zones, which in descending order are as follows:--
The evaporation zone;The passage zone; andThe capillary zone.
One-third of the rain-water evaporates. One-third flows off the surface. One-third percolates.
All these three zones must be passed by the water in its descent from the surface of the ground to the subsoil water level, and the quantity of water retained by them will depend on their state of dryness. Speaking quite generally and within wide limits, one-third of the rain-water flows off the surface, one-third evaporates, and one-third percolates into the subsoil.
Evaporation zone.
The evaporation zone reaches from the surface of the soil to that point below, which marks the extent of the drying influence of the atmospheric air. In the same the quantity of water stored in the pores may at times sink below the retentive power of the soil, i.e. below that quantity which can be retained in the pores owing to the mechanical powers of adhesion, etc. When it has become very dry through evaporation and other causes the zone, especially when it extends some way down, may retain large quantities of water. In a depth of 10 inches, 1 square yard of soil, with fine pores, may retain about 10 gallons of water, and as a rainfall of ½ inch produces only 2·3 gallons per square yard, it is clear that subsoil of this nature may retain a number of successive showers. During the height of summer fine porous soil may become so dry that practically no water finds its way into deeper zones; in this state the evaporation zone can be compared to a large sponge.
Passage zone.
The next zone traversed by the water in its downward movement is the passage zone, which lies beyond the drying influence of atmospheric air. When too far removed from the level of the subsoil water, its pores will not be completely filled with water, but will only contain that amount which is due to the retentive powers of the soil. By direct measurement it has been found that on an average a cubic yard of fine porous soil will retain from 30 to 80 gallons of water, and it can easily be calculated that in a layer from 1 to 2 yards in thickness the rainfall of a whole year may be retained. The passage zone, especially if it is of considerable thickness, represents a very large storage reservoir.
Capillary zone.
The last zone before the level of the subsoil water is reached is the capillary zone, in which the pores are partially or wholly filled by the upward movement—dueto capillary attraction—from the subsoil water. The extent of this filling will depend on the size of the pores.
Springs.
When the descending water has finally reached the subsoil water it either comes to a standstill altogether on the impervious layer or moves along the same, if the latter is not horizontal, until it may eventually leave the subsoil again by issuing therefrom in the form of visible or invisible springs.
Rate of downward movement governed by pores.
The rate of movement of any liquid—rain-water, sewage or other polluting liquid—is largely governed by the size of the pores. Where these are large, as for instance in coarse gravel, the descent of the water will be comparatively rapid, but when they are small it may take a very long time before the water reaches the level of the subsoil water, and in that case it will have undergone material changes as regards its chemical or bacterial composition.
With a high level of subsoil water zones become indistinguishable.
With a high level of subsoil water the zones may become indistinguishable, one zone reaching into the other, with the result that the whole of the soil becomes very wet.
When subsoil has been artificially drained the amount of water reaching the subsoil water below the general level of the drains will depend on the size of the latter and the distance between them. In such a case the downward movement of the water through undrained soil, previously described, may be further interfered with through the ventilation of the subsoil by drains, and the drying up action caused thereby.
[7]The term subsoil water is here used to denote that portion of the water in the pores of the soil, which is either at rest on or moves along the inclined plane of an impervious layer.
9. The Micro-Organic Life in Soil.
Soil probably original home of micro-organisms.Distribution of micro-organisms in soil.
The soil is probably the original home of all micro-organisms, from which they have emigrated into other media. It contains vast numbers, and, according to some observers, 1ccm.may hold 100,000 germs. By far the greater number is found on or near the surface, and in lower layers the numbers gradually diminish, until at last a depth is reached, which depends on local conditions, where the soil is perfectly sterile. The aerobes live near the surface and carry on their work in this region, whereas the anaerobes are at work lower down in the soil.
Cycle of micro-organic activity during the year.
The picture of the cycle of micro-organic activity in the upper layers of the soil during the various seasons of the year is probably the following. In winter, especially during that period when frost and ice bind the earth, micro-organic life is apparently at its lowest ebb, and may in some very cold climates come to a standstill altogether, when micro-organisms may be said to hold their vegetative winter sleep. With the return of life and the awakening of nature in spring—especially with the approach of higher temperatures and the formation of moisture—micro-organic activity once more makes itself felt all round. During the summer months it is exposed to some injurious influences such as the heating and drying up of the upper layers of the soil, but, still gradually increasing, micro-organisms reach the climax of their activity during the autumnal rains, to remain in this state until with the advent of the cold season their activity gradually declines again.
Micro-organic life in layers from 3ft.to 6ft.in depth.
In the lower layers of the soil, down to 3 feet and 6 feet, micro-organisms are more protected against the injurious influences of the atmosphere, sunlight anddrying up, but the want of oxygen, together with the greater difficulty of removing such products as carbonic acid, has an injurious influence. As the temperature in these layers is considerably more uniform, it may be inferred that the micro-organic activity is there of a more uniform kind, less influenced by sudden changes, probably also less intense, but without pronounced periods of rest.
Micro-organisms probably quickly perish in depths greater than 6ft.
In depths greater than 6 feet micro-organisms probably perish very quickly owing to unfavourable conditions, and if found their presence must be explained by emigration from higher layers, not by actual growth at these depths.
On sewage farms the micro-organic activity is without doubt greatly modified, and proceeds all the year round at a more uniform rate than on ordinary land, as the sewage always contains the necessary warmth and moisture so beneficial for it.
10. The Absorbing Powers of Soil.
Absorbing powers due to surface attraction of the particles of the soil.The finer the pores the greater the absorption.
The absorbing powers of soil are due to the surface attraction of its particles or grains, and these, as has already been pointed out, will be all the greater the finer the pores are; they extend on the one hand to aqueous and other vapours and gases, and on the other to matters in solution.
1cub. yd.of coarse gravel may contain 50sq. yds.of surface and 1cub. yd.of fine sand 9200sq. yds.
That the attractive force of the surface of the particles is pretty considerable will be at once apparent when it is stated that 1 cubic yard of coarse gravel may contain about 140,000 grains with a combined surface of 50 square yards, and 1 cubic yard of fine sand 40 million grains with a combined surface of 9200 square yards, which is a little under 2 acres.
Deodorising action of soil absorption of gases.
Concerning the absorption by soil of aqueous vapour and gases (apart from condensation through a fall in temperature), dry soil with fine pores acts most energetically. The almost instantaneous deodorisation of foul-smelling gases, such as are formed by decomposing fæcal matters (earth closet) or coal gas, through a thin layer of fine dry soil is well known, and is to be explained in this way.
Absorption of dissolved substances by soil.
More interesting still, and also more important, is the absorption of dissolved substances by soil. In this way is to be explained the decolorising effect and the retention of dissolved polluting substances such as are contained in sewage. In the same way soil has the power of destroying such poisons as strychnine, nicotine, coniine, etc., and the experiments of Falk and others go to show that ptomaines and toxines are likewise retained and rendered harmless by it. This absorbing power of soil is of the utmost importance in agriculture, and without it soil could not possess purifying powers for polluting liquids. It is quite true that in this process of purification other factors play an important part, but they could not come into play if this absorption did not exist.
The absorbing powers of soil are in some way dependent on the presence of micro-organisms and air, and in the absence of these they will soon come to a standstill.