BROAD IRRIGATION
For many years it has seemed to thoughtful persons that permitting
sewage, either from single houses or from larger communities, to be
turned into streams was a mistaken policy because of the waste of
manurial elements involved. It has long been understood that, in order
to maintain the fertility of the soil, a constant application of
fertilizers was necessary, and, while undoubtedly many farms are managed
without any such repeated applications, the more scientific and modern
farmer believes to-day that the frequent and abundant use of fertilizer
is the foundation of his success.
In ordinary sewage there exists a certain amount of fertilizing
elements. Two prominent English chemists, not many years ago, proved by
their analyses that in ordinary sewage there existed the essential
elements of a good fertilizer to the value of $2 per year for each
person contributing to that sewage. Other chemists, working at the
problem in other ways, have reached about the same result, and there can
be little doubt of their accuracy if the fertilizing elements alone are
considered. In applying these figures to the sewage of a city the
difficulty has always been that the fertilizing elements have been so
thoroughly covered up with the large volume of water present in the
sewage that it has been practically impossible to separate them from the
water. Thus, in a city of 100,000 persons, the fertilizer in the sewage
might, indeed, be worth $200,000, but to realize this amount it must be
separated from the 10,000,000 gallons of water—a task which is so
tremendous, if not impossible, as to make the value of the fertilizer of
no account. In those parts of the country where the water itself has a
value, as in the irrigated lands of the West, the fertilizing elements
of the sewage would be added to the value of the water, so that sewage
used for irrigation would be worth not merely the value of the water
alone, but also the value of the fertilizer present in that water.
Another difficulty in making use of the combined water and fertilizer is
that the large amount of water involves a large area of land and
suitable soil, on which irrigation may be practised, in the immediate
vicinity of the city. This combination of agricultural soil of suitable
texture at a suitable price for farming operations is so seldom found
that this in itself usually precludes any application of the use of
sewage for irrigation.
In the case of the sewage from a single house, however, the possibility
of making use both of the water and the fertilizer in sewage is not so
difficult. Recent writers on irrigation have pointed out that, while
irrigation of late years has made most headway in the semi-arid
districts of the West, there are many opportunities for its successful
and profitable utilization in the East, and Mr. Lute Wilcox, in a recent
book on irrigation, says: “The farmer who has a soil containing an
abundance of all the needed elements in a proper state of fineness
cannot but deem himself happy if he have always ready at hand the means
of readily and cheaply supplying all the water needed by his soil and
growing crops, just when and in just such quantities as are needed.
Happier still may he be when he realizes that he need have no ‘off
years’, and he knows that the waters he admits to his fields at will are
freighted with rich fertilizing elements usually far more valuable to
the growing crops than any that he can purchase and apply at a costly
rate—a cost that makes serious inroads upon the profits of the majority
of farmers cultivating the worn-out or deteriorated soils in the older
States year by year. Fertilizers are already needed for the most
profitable culture on many farms in Iowa, Minnesota, Eastern Kansas, and
Nebraska, in Missouri, and in all States east of those named.”
Perhaps the greatest uncertainty in the matter of farming is the
available water coming from the clouds. In one year the rainfall may
come at the proper time to moisten the seed and to insure a rapid
germination. These early rains may be followed by showers at proper
intervals to supply the little rootlets with the necessary moisture so
that the growth of the plant may be constant and vigorous. During the
ripening season the rains may be withheld so that the harvest is insured
under the most favorable conditions. In other years, however, the spring
rains may be so continuous as to cause the seeds to rot, requiring a
second sowing. Then the rains may fail so that the seeds either fail to
germinate, or at best produce scattered and imperfect growths. At the
time of harvest storm may follow storm, so that the harvesting of those
plants which have developed is made almost impossible.
Irrigation tends in part to correct these difficulties, since it
furnishes the soil with the needed water at times when the lack of rain
would cause an entire failure in future growth. Irrigation, of course,
cannot prevent rainfall, and it may be that after a copious soaking of
the ground with the irrigating water a heavy rain may follow, resulting
in an excess of moisture as bad for the ground as none at all. The
possibility of irrigation cannot prevent excessive rains at the time of
harvest, but the advantages of being able to control the soil moisture
during the period of growth are more than enough to counterbalance any
possible disadvantages. During the summer months evaporation is very
high, the dryness of the air and the high temperature combining to draw
moisture from the soil in considerable quantity. Then, too, the plants
themselves, while absorbing moisture from their roots, evaporate
moisture through their leaves, and agricultural stations have made
extensive studies on the amount of this evaporation from different
plants. The teaching of it all is that the amount of water which can be
utilized by the soil, not merely for the sake of the growth of the
plants themselves, but to make up for the demands of evaporation, is
very high.
Mr. Newell, of the United States Geological Survey, points out that
while the amount of water required for raising crops varies according to
soil and other conditions, yet a large quantity is required to maintain
the soil in such a degree of saturation as to best promote the vitality
of the plant life. He shows that for each ton of hay raised upon an
acre, from three hundred to five hundred tons of water must be furnished
either by rainfall or by artificial means. In other words, since water
covering an acre to a depth of one inch weighs about one hundred and
thirteen tons, it would be necessary to cover an acre to a depth of from
three to five inches if that acre produced one ton of hay. From actual
conditions, he shows that it has been necessary, in order to produce
five tons of barley hay per acre, to provide an amount of water which
would cover the acre to a depth of twenty inches. Although his figures
have special reference to the semi-arid regions of the West they furnish
a guide for the amount of water which may profitably be used in addition
to the rainfall, which, in the summer months, may be practically nothing
even in the East. From three to six inches in depth each month is his
estimate of the needed water for successful crop growing, the difference
depending upon the character of the soil, more being required in sandy
soils and less where the texture is finer.
The sewage from an ordinary household, on the basis of 30 gallons per
head per day, amounts to 180 gallons per day, or about 5,400 gallons per
month, or 720 cubic feet. This amount of water would cover an acre of
ground to a depth of a little less than one-fourth of an inch, and it is
plain that in order to have the sewage of a single house furnish the
necessary amount of water for successful crop growing, the area required
is only about one-twelfth of an acre, or an area about 60 feet square.
In the early days of the English experiments with the disposal of
sewage, great stress was laid on the value of the manurial elements in
sewage, and many tests were made as to the capacity of various soils for
absorbing the moisture present in sewage. One of the most enthusiastic
advocates of this method of disposing of sewage was Mr. J. Bailey
Denton, who was able to act as engineer for many installations of
various sorts. As a result of his experience he came to the conclusion
that while the area depended upon the character of the soil, and while
with the most suitable soil a very large amount of water might be taken
care of, under ordinary conditions it was safest to so design the works
that no possibility of overloading the soil with water could exist. He
places the limits of population, the sewage from whom would be cared for
on an acre, between 1,000 persons per acre and 100 persons per acre.
More recent experience, together with constant observation of farms
established in the early period of the practice, indicates that the
higher value is too great, and that where agricultural processes alone
are considered, 100 persons per acre is a suitable maximum value for
irrigation on sandy loam, and that 40 persons per acre is a suitable
number where the soil is inclined toward density and fine texture. Six
persons in a household would, according to Mr. Denton, require from
one-seventh to one-seventeenth of an acre. The amount, indicated by the
computations made earlier, indicated one-twelfth of an acre for the same
number of persons. The practical agreement of the two methods of
computing the area necessary thus makes it possible to determine in
either way the amount of land needed on a given farm for disposing of
the household sewage.
The effect of sewage irrigation has been found to be most astonishing so
far as the increased yield of the soil goes. Some years ago, in order to
determine just the effect of the addition of sewage to ordinary farm
land, a certain field of five acres was divided into four equal parts.
The four fields were treated as follows: Field No. 1 received no sewage.
Field No. 2 received six inches of sewage over its entire area on each
of five successive months. Field No. 3 received twelve inches of sewage
on each of five successive months. Field No. 4 received eighteen inches
of sewage on each of five successive months. The following table shows
the results of three successive years’ experiments at the sewage farm
referred to at Rugby, England, the figures being the number of pounds of
green grass cut from the fields.
FIVE-ACRE FIELD
═════════════════╤═════════════════════════════════════════════════════
WITHOUT SEWAGE. │ WITH SEWAGE.
─────────────────┼─────────────────┬─────────────────┬─────────────────
Lot 1. │ Lot 2. │ Lot 3. │ Lot 4.
─────────────────┼─────────────────┼─────────────────┼─────────────────
20,814│ 33,244│ 60,602│ 73,564
18,294│ 62,514│ 77,299│ 71,766
11,069│ 49,851│ 78,231│ 80,941
─────────────────┼─────────────────┼─────────────────┼─────────────────
Aver. 16,725│ 48,536│ 72,044│ 76,434
─────────────────┴─────────────────┴─────────────────┴─────────────────
It will be noticed that, whereas without sewage the amount of green
grass was about eight tons on an acre and a quarter in the field, from
lot No. 2 twenty-four tons were cut, from lot No. 3 thirty-six tons, and
from lot No. 4 thirty-eight tons. Evidently the amount of sewage applied
did not proportionately increase the yield in lots 3 and 4, and it may
be said that a depth of sewage or water of more than twelve inches per
acre has, in general, been found to be not merely unnecessary, but
undesirable. The table does not show the number of cuttings made during
the season, but the custom on the farm is to cut frequently, at
intervals of perhaps two or three weeks, no time being given for curing
the hay.
The crops suitable for growth on irrigated fields have been found by
experience to be grass and root crops, such as beets, turnips, and the
like. Mr. Wilcox, in writing of the requirements of different plants,
suggests celery as a garden crop that needs a great deal of water.
Beets, carrots, parsnips, and turnips are favorite plants for irrigated
fields. Cabbage and cauliflower are benefited by abundant irrigation
during the first part of their growth, but after the heads of the
cabbage plants are half-formed, further excessive use of water is
undesirable. The use of irrigating water in orchards has been practised
with great success not only in the recent irrigation areas of the West,
but along the Hudson River and in New England. The size of the fruit is
increased by irrigation, and it is said that the bloom is much improved.
METHODS OF APPLYING THE WATER
In distributing the water or sewage over the soil in the case of a
single house, no elaborate methods are required. In the case of large
farms supplied with sewage from a considerable population, elaborate
systems of piping or open-channel conduits are required, and the problem
of working out and adjusting the necessary sizes and grades becomes a
complicated matter for which engineering knowledge and experience are
required. But for the small flow which comes from individual houses and
from the small area involved, no such elaborate preparations are
required. The essence of the distribution consists in carrying the water
onto the field to be irrigated at such a low velocity that no surface
soil or valuable manures are washed away; and in adjusting the volume of
the flow and the requirements of the soil, there are three
characteristic conditions which require different treatments.
In the first place, the area may be practically level and the crop
raised may be either grass or grain. In such a case the sewage should be
led onto the field which may properly be enclosed on four sides with a
low, that is, six to twelve inches, earth-dike, and at each irrigation
the field may be flooded about two inches deep. The next irrigation
would probably not be required for a week, so that this method requires
a number of beds to be worked one after another and, except where the
soil is very dense, so much so that percolation is very slow, this
method is not suitable because of the slow rate at which the sewage is
delivered.
The second method of distribution, and one more suitable for the
conditions under discussion, is to lay out the field in parallel beds
from three to six feet wide and from forty to one hundred feet long.
These beds are separated by furrows into which the sewage is discharged.
If the grade of these furrows is properly adjusted to the porosity of
the soil, that is, made about six inches in one hundred feet for open,
sandy loam, and about two inches in one hundred feet for fine, clay
loam, the soil will absorb the needed moisture as the sewage flows over
it and there should be no ponding or excess of water at any point of the
field. By dividing the field into three parts, or in arranging the flow
of sewage so that it enters only two or three furrows at a time, the
flow can be so changed from day to day as to furnish all parts of the
area with the irrigating water, and at the same time not overload and
choke the soil particles. On the beds may be planted and grown whatever
vegetables are desired. A good basis for determining the area and length
of furrows required is to provide a length of thirty feet of furrow for
each person of the household. The total length thus obtained should not,
however, be made continuous, but should be arranged in three parts, or
in multiples of three, so that one-third of the total length only may be
used on any one day, the other parts serving for other days, so that a
rotation is practised.
[Illustration: FIG. 48.—Distribution of Sewage and Arrangement of Check
Levees on a Hillside.]
The third condition involves the application of the sewage to a steep
slope, and this may be treated in either one of two ways. The sewage may
be led to the top of the hill and allowed to flow, for a short distance
only, over the surface on which, presumably, grass is to be grown. If
the length of the furrow is more than about a dozen feet, the flowing
stream acquires enough velocity to wash the surface and to form gullies.
To prevent this, a secondary ditch or small bank is thrown up to arrest
the flow. The water is led out again from behind this ditch or bank at
intervals, to repeat the process further down the hill (see Fig. 48). If
the slope of the ground is moderate, so that there is no tendency of the
water to form gullies, the water may be let out of the ditch at
intervals and allowed to distribute itself over the field, as shown in
Fig. 49. The water thus overflowing should be collected in a drain at
the lower end of the slope, and will be found suitably purified for
discharge into any running stream not used for drinking purposes. The
occasional use of a shovel or hoe may be needed to change the flow of
the water over the field if it is found that any tendency exists for
definite channels to be formed.
[Illustration: FIG. 49.—Distribution of Sewage on a Hillside of Moderate
Slope.]
In order to plant vegetables on such a hill, small furrows may be made
along the hill and laid out with great care so that the flow of sewage
in the furrows shall be only at a slow velocity, so slow that the soil
can absorb the moisture as the water passes along. By zigzagging this
furrow back and forth down the hillside, vegetation on the hill will
receive the benefit of the water, and if any of the sewage succeeds in
reaching the bottom of the hill, it will be so purified that it may be
safely discharged into any depression or watercourse there found.
In the case of orchards, irrigation is practised by flooding the ground
around the tree, being careful, however, to throw up a mound of earth
around the tree so that no water comes within two feet of the tree
itself. Fig. 50 shows a Western method of forming square beds, each bed
about twenty feet on a side, with one tree at the centre. Furrows are
also used to distribute the water, a common practice followed being to
being to have the furrow always under the extreme edge of the foliage,
thus discharging the water in the vicinity of the tender rootlets of the
tree. Usually the furrow system is carried only in one direction, so
that the application of water by this method is not so complete as by
the flooding method. But for small volumes of water constantly applied,
it is probably more satisfactory. Fig. 51 shows a grain field irrigated
by the furrow method.
[Illustration: FIG. 50.—Square Beds for Orchards According to Some
Western Practice.]
In all cases where irrigation is practised, stress is laid by those
experienced in the matter on the necessity of cultivation of the soil in
connection with the irrigation. Apparently, there is a tendency for the
surface layers, with the application of water, to cake or crust over the
lower strata, thus depriving the soil of the necessary air. In order to
break up this crust, the soil must be continually worked, either by a
hoe or rake or some sort of horse cultivator. Where the ground is
shaded, as in the case of land covered with grass or grain, the tendency
to crust is not so marked, but on cultivated land where root crops are
grown the cultivator must be used regularly after each irrigation. Where
the sewage is carried onto the field in furrows, the soil in the furrow
should be hoed at frequent intervals, not only to break up the crust
which deprives the soil of the air, but in order to open the particles
of soil for the reception of the irrigating-water.
[Illustration: FIG. 51.—Grain Field in Spring, in Process of
Irrigation.]
It must also be remembered, as has been pointed out before, that the
success of any method of applying sewage to soil depends upon the
frequent change from bed to bed, the actual time interval depending on
the character of the soil. If the soil is fine, the same area may be
used for a week at a time, and then given two weeks’ rest. If the soil
is more open, this interval should be reduced, and with very coarse
particles it may be found desirable to shift the flow from one bed to
another after an interval of a few hours only. Experience and careful
observation on the moisture-carrying capacity of the bed is the best
guide to the operation of sewage irrigation.
Whether or not this method of disposing of the sewage of a single house
is to be selected depends largely upon the slope of the ground from the
house toward the garden. It is not desirable to have sewage exposed to
the air in the immediate vicinity of a dwelling-house. Rarely would any
odors be generated to such an extent as to be offensive to the occupants
of the house, since the sewage sinks into the ground before putrefaction
of the organic matter sets in and the exposed material left on the
surface of the ground is of too attenuated a type to become offensive
even if it does putrefy before drying. There is, however, the danger of
odors being formed where distribution is imperfect and where pools are
allowed to form in the furrow. There is also the danger of the
transmission of disease germs from the sewage-irrigated field to the
occupants of the house through the agency of flies. Health statistics of
English farms show this danger to be a very remote one, since the health
of the workman on those farms is as good or better than the average
throughout England. But the possibility of infection exists and must not
be overlooked.
No method of disposal requires so much and such constant care, although
the results show in the improved yield from the farm. This method of
broad irrigation is emphatically not the method to be used except where
labor is adequate for proper soil cultivation and where this labor can
be given constantly and ungrudgingly. Finally, it must be pointed out
that care should always be exercised to prevent irrigating sewage coming
in direct contact with any of the soil produce. Certainly sewage should
not be used to sprinkle over lettuce or celery or strawberries, even if
the yield is thereby increased. Undoubtedly any disease germs thus
distributed over the fruits and vegetables would, through the antiseptic
action of the sunlight and air, soon be destroyed, but the very method
of irrigation is repulsive, and the danger, while slight, is sufficient
to forbid that method of fertilizing. No statistics, however, are
available to show that cows eating sewage-irrigated grass are adversely
affected in health, and for years the practice of thus pasturing cows
has been carried on in England. For human beings, however, vegetables
grown in soil that is separated from the sewage by a foot or more is the
safer as well as more æsthetic arrangement.