Chapter XIX: Part 19

The form in which nitrogen is taken up by plants is mainly, if not
wholly, that of nitrates, which are readily-soluble salts. Ammonia and
other nitrogenous bodies undergo in the soil, through the agency of
nitrifying organisms present in it (_Bacterium nitrificans_, &c.),
rapid conversion into nitrates, and as such are easily assimilable by
the plant. Similarly, they are the constituents which are most readily
removed in drainage, and hence the adequate supply of nitrogen for the
plant's use is a constant problem in agriculture. Experiments on the
rate of removal of nitrates from the soil by drainage showed that
every inch of rain passing through the drains caused a loss of 2½ lb.
of nitrogen per acre (Voelcker and Frankland). At the same time,
soils, as Way showed, have the power of absorbing, in different
degrees, ammonia from its solution in water, and when salts of ammonia
are passed through soils the ammonia alone is absorbed, the acids
passing, generally in combination with lime, into the drainage.

Other experiments at Rothamsted on drainage showed that, though large
quantities of ammonia salts were applied to the land, the drainage
water contained merely traces of ammonia, but, on the other hand,
nitrates in quantity, thus proving that it is as nitrates, and not as
ammonia, that plants mainly, if not entirely, take up their
nitrogenous food.

From these investigations it follows that much more nitrogen must be
added to the land than would be needed to produce a given increase in
the crop. Nitrogen, then, being so all-important, the question is,
where is it to come from? We have seen that the leaves take up only
minute quantities of ammonia, comparatively small amounts are supplied
in the rain, dew, snow, &c.,[1] and in the case of Leguminosae alone
have we any evidence of plants being able to provide themselves with
nitrogen from atmospheric sources. Some few organisms present in
fertile soils, e.g. _Azotobacter chroococcum_, have also the power,
under certain conditions, of fixing the free nitrogen of the
atmosphere without the intervention of a "host," but all these sources
would be very inadequate to meet the demands of an intensive
cultivation. An ordinary fertile arable soil will not show, on
analysis, much more than .15% of nitrogen, and it is evident that the
great source of supply of the needed nitrogen must be the direct
manuring of the soil with materials containing nitrogen. These
materials will be considered in detail later.

2. _Phosphorus._--This is the most important mineral element which has
to be supplied to the soil by the agency of manuring. It occurs in
ordinary fertile soils to the extent of only about .15%, reckoned as
phosphoric acid, and though its absence in sufficiency is not so
marked or so soon shown under prolonged cultivation as is that of
nitrogen, yet the fact that it is needed by all classes of crops, and
that its application in manurial form is attended with great benefits,
makes its supply one of great importance. From the time that Liebig,
in 1840, suggested the treatment of bones with sulphuric acid in order
to make them more readily available for the use of crops, and that the
late Sir John Lawes (in 1843) began the dissolving of mineral
phosphates for the purpose of manufacturing superphosphate, the
"artificial manure" trade took its rise, and ever since then the whole
globe has been exploited for the purpose of obtaining the raw
phosphatic materials which form the base of the artificial manures of
the past and of the present day. The functions which phosphoric acid
fulfils in plant life would appear to be connected rather with the
maturing of the plant than with the actual growth of the structure.
Phosphates are found concentrated in those parts of the plant where
cell growth and reproduction are most active. More especially is this
the case with the seed in which phosphates are present in greatest
quantity. While nitrogen delays maturity, phosphoric acid has just the
opposite effect, and cereal crops not sufficiently supplied with it
ripen much more tardily than do others. Moreover, the grain is formed
more early when phosphatic manures have been given than when they are
withheld. Phosphates increase the proportion of corn to straw, and, as
regards the grain itself, they render it less nitrogenous, richer in
phosphates, and altogether improve its quality.

While these are the principal functions of phosphates, they also
exercise an influence on the young plant in its early stages. This is
well seen in the almost universal practice of applying superphosphate
to the young turnip or swede crop in order to push it beyond the
attack of "fly." Undoubtedly phosphates in readily available form
stimulate the young seedling, enabling it to develop root growth, and,
later on, causing the plant to "tiller out" well. Phosphoric acid
occurs in the soil bound up with the oxides of iron and alumina, or,
it may be, with lime, and the extent to which it may become useful to
plants will depend largely upon the readiness with which it becomes
available. For the purpose of ascertaining this different analytical
methods have been suggested, the best known one being that of B. Dyer,
in which a 1% solution of citric acid is used as a solvent. As a
result of experimenting with Rothamsted soils of known capability it
has been put forward that if a soil shows, by this treatment, less
than .01% of phosphoric acid it is in need of phosphatic manuring.

Experiments carried on for many years at Rothamsted and Woburn have
clearly established the beneficial effects of phosphatic manuring on
corn crops, for though no material increase marks the application of
mineral manures in the absence of nitrogen, yet the results when
phosphates and nitrogen are used together are very much greater than
when nitrogen alone has been applied; and this is true as regards not
only the better ripening and quality of the grain, but also as regards
the actual crop increase.

With root crops phosphates are almost indispensable; and, owing to the
limited power which these crops have of utilizing the phosphoric acid
in the soil, the supply of a readily available phosphatic manure like
superphosphate is of the highest importance.

The assimilation of phosphoric acid goes on in a cereal crop after the
time of flowering and to a later date than does that of nitrogen and
potash, and it is ultimately stored in the seed. Soils possess a
retentive power for phosphoric acid which enables the latter to be
conserved and not removed to any extent by drainage. This function is
exercised mainly by the presence of oxide of iron. Alumina acts in a
similar way. In the case of soils that contain clay only traces of
phosphoric acid are found in the drainage water.

3. _Potassium._--The element third in importance, which requires to be
supplied by manuring, is potassium, or, as it is generally expressed,
potash. This in its functions resembles phosphoric acid somewhat,
being concerned rather with the mature development of the plant than
with its actual increase of growth. Like phosphoric acid, potash is
found concentrated throughout the plant in the early stages of its
growth, but, unlike it, is in the case of a cereal crop all taken up
by the time of full bloom, whereas with phosphoric acid the
assimilation continues later. Potash would appear to have an intimate
connexion with the quality of crops, and to be favourable to the
production of seed and fruit rather than to stem and leaf development.
Certain crops, such as vegetables, fruit, hops, as well as root crops
generally, make special demands upon potash supply, and, as checking
the tendency to over-development of leaf, &c., induced by nitrogenous
manures when used alone, potash has great practical importance. Potash
appears to be bound up in a special way with the process of
assimilation, for it has been clearly shown that whenever potash is
deficient the formation of the carbohydrates, such as sugar, starch
and cellulose, does not go on properly. Hellriegel and Wilfarth showed
by experiment the dependence of starch formation on an adequate supply
of potash. Cereal grains remained small and undeveloped when potash
was withheld, because the formation of starch did not go on. The same
effect has been strikingly shown in the Rothamsted experiments with
mangels, a plot receiving potash salts as manure giving a crop of
roots nearly 2½ times as heavy as that grown on a plot which has
received no potash. In this case the increase is due almost entirely
to the sugar and other carbohydrates elaborated in the leaves, and not
to any increase of mineral constituents.

The effect of potash on maturity is somewhat uncertain, inasmuch as in
the case of grain crops it would appear to delay maturity and to
hasten it in that of root crops.

The influence of potash on particular crops is very marked. On clovers
and other leguminous crops it is highly beneficial, while on grass
land it is of particular importance as inducing the spread of clovers
and other leguminous herbage. This is well seen in the Rothamsted
grass experiments, where with a mineral manure containing potash
one-half of the herbage is leguminous in nature, whereas the same
manure without potash gives only 15% of leguminous plants. Similarly,
where nitrogen is used by itself and no potash given there are no
leguminous plants at all to be found. Potash occurs in an ordinary
fertile soil to the extent of about .20%; a sandy soil will have less,
a clay soil may have considerably more. Potash, however, is mostly
bound up in the soil in the form of insoluble silicates, and these are
often in a far from available form, but require cultivation, the use
of lime and other means for getting them acted on by the air and
moisture, and so liberating the potash. According to B. Dyer's method
of ascertaining the availability of potash in soils, the amount of
potash soluble in a 1% citric acid solution should be about .005%,
otherwise the addition of potash manures will be a requisite. In the
case of soils containing much lime a larger quantity would, no doubt,
be needed.

Potash, like phosphoric acid, is readily retained by soils, and so is
not subject to any considerable losses by drainage. This retention is
exercised by the ferric-oxide and alumina in soils, but still more so
by the double silicates, and to some extent also by the humus of the
soil. Potash will be liberated from its salts by the action of lime in
the soil, the lime taking the place of the potash. Lime is, therefore,
of much importance in setting free fresh stores of potash. Soda salts
also, when in considerable excess, are able to liberate potash from
its compounds, and to this is probably due, in many cases, the
beneficial action attending the use of common salt.

4. _Calcium._--Though calcium, or lime, is found in sufficiency in
most cultivated soils, there are, nevertheless, soils in which lime is
clearly deficient and where that deficiency has shown itself in
practice. Moreover, so comparatively easy is the removal of lime from
the soil by drainage, and so important is the part which lime plays in
liberating potash from its compounds, and in helping to retain bases
in the soil so that they are not lost in drainage, that the
significance of lime cannot be ignored. Further, the availability of
both potash and phosphoric acid in the soil has been found to be much
increased by the presence of lime. Lime, as carbonate of calcium, is
also necessary for the process of nitrification to go on in the soil.
Some sandy soils, and even some clays, contain so little lime as to
call for the direct supply of lime as an addition to the soil. When
this is the case nothing can adequately take the place of lime, and in
this sense lime may be called a "manure." In the majority of cases,
however, the practice of liming or chalking, which was a common one in
former times, was resorted to mainly because of the ameliorating
effects it produced on the land, both in a mechanical and in a
physical direction. Thus, on clay soil it flocculates the particles,
rendering the soil less tenacious of moisture, improving the drainage
and making the soil warmer. Nor must the directly chemical results be
overlooked, for in addition to those already mentioned, of liberating
plant food (chiefly potash and phosphoric acid), retaining bases, and
aiding nitrification, lime acts in a special way as regards the
sourness or "acidity" which is sometimes produced in land when lime is
deficient. In soils that are acid through the accumulation of humic
acid nitrification does not go on, and bacterial life is repressed.
The addition of lime has the effect of "sweetening" the land, and of
restoring its bacterial activity. This acidity is also seen in the
occurrence of the disease known as "finger and toe" in turnips, the
fungus producing this being one that thrives in an acid soil. It is
only found in soils poor in lime, and the only remedy for it is
liming. The growth of weeds like spurry, marigold, sorrel, &c., is
also a sign of land being wanting in lime. The most striking instance
of this "soil acidity" is that afforded by the Woburn experiments,
where, on a soil originally poor in lime, the soil has, through the
continuous use of ammonia salts, been impoverished of its lime to such
an extent that it has become quite sterile and is distinctly acid in
character. The application of lime, however, to such a soil has had
the effect of quite restoring its fertility.

The amount of lime which soils contain is a very variable one, chalk
soils being very rich in lime, whereas sandy and peaty soils are
generally very poor in it. If the amount of lime in a soil falls below
1% of carbonate of lime on the dried soil, the soil will sooner or
later require liming.

5. _Magnesium._--This is not known to be deficient in soils, although
an essential element in them, and it is seldom directly applied as a
manurial ingredient. Some natural potash salts, such as kainit,
contain magnesia salts in considerable quantity; but their influence
is not known to be of beneficial nature, though, like common salt,
magnesia salts will, doubtless, render some of the potash in the soil
available. At the same time magnesia salts are not without their
influence on crops, and experiments have been undertaken at the Woburn
experimental farm and elsewhere to determine the nature of this
influence. Carbonate of magnesia has been tried in connexion with
potato-growing, and, it is said, with good results.

6. _Iron._--Iron is another essential ingredient of soil that is found
in abundance and does not call for special application in manurial
form. Iron is essential for the formation of chlorophyll in the
leaves, and its presence is believed also to be beneficial for the
development of colour in flowers, and for producing flavour in fruits
and in vines especially. Ferrous sulphate has, partly with this view,
and partly for its fungus-resisting properties, been suggested as a
desirable constituent of manures. The function performed by ferric
oxide in the soil of retaining phosphoric acid, potash and ammonia has
been already alluded to.

7. _Sulphur._--This, the last of the "essential" elements, is seldom
specially employed in manurial form. There would appear to be no lack
of it for the plant's supply, and it is little required except for the
building-up, with carbon, hydrogen, oxygen and nitrogen, of the
albuminoids. There are few artificial manures which do not contain
considerable amounts of sulphur, notably superphosphate. Sulphate of
lime (gypsum) is sometimes applied to the land direct as a way of
giving lime; this is employed in the case of clover and hops
principally.

Having thus dealt with the essential ingredients which plants must have, and which may require to be supplied to them in the form of additional manures, we may briefly pass over the other constituents found in plants, which may, or may not, be given as manures.

8. _Sodium._--This is a widely distributed element. The influence of
common salt (chloride of sodium) in liberating, when used in large
excess, potash from the silicates in which it is combined in the soil
has been already referred to, and in this way common salt and also
nitrate of soda (the two forms in which soda salts are used as
manures) may have some benefit. The principal purpose for which common
salt, however, is used, is that of retaining moisture in the land. It
is specially useful in a dry season, or for succulent crops such as
cabbage, kale, &c., or again for plants of maritime origin (such as
mangels), which thrive near the sea shore.

9. _Silicon._--All soils contain silica in abundance. Though silica
forms so large a part of the ash of plants and is especially abundant
in the straw of cereals, there is no evidence that it is required in
plant life. Popularly, it is believed to "stiffen" the stems of
cereals and grasses, but plants grown without it will do perfectly
well. It would, however, appear that soluble silica does play some
part in enabling phosphoric acid to be better assimilated by the
plant. Silicates, however, have not justified their use as direct
fertilizers.

10. _Chlorine._--A certain amount of chlorine is brought down in the
rain, and chlorides are also used in the form of common salt, with the
effect, as aforesaid, of liberating potash from silicates, when given
in excess, but there is no evidence as to any particular part which
the chlorine itself plays.

11. _Manganese_, &c.--Manganese occurs in minute quantities in most
plants, and it, along with lithium (found largely in the
tobacco-plant), caesium, titanium, uranium and other rare elements,
may be found in soils. Experiments at the Woburn pot-culture station
and elsewhere, point to stimulating effects on vegetation produced by
the action of minute doses of salts of these elements, but, so far,
their use as manurial ingredients need not be considered in practice.

12. _Humus._--Though not an element, or itself essential, this body,
which may be described as decayed vegetable matter, is not without
importance in plant life. Of it, farm-yard manure is to a large extent
composed, and many "organic manures," as they are termed, contain it
in quantity. Dead leaves, decayed vegetation, the stubble of cereal
crops and many waste materials add humus to the land, and this humus,
by exposure to the air, is always undergoing further changes in the
soil, opening it out, distributing carbonic acid through it, and
supplying it, in its further decomposition, with nitrogen. The
principal effects of humus on the soil are of a physical character,
and it exercises particular benefit through its power of retaining
moisture. Humus, however, has a distinct chemical action, in that it
forms combinations with iron, calcium and ammonia. It thus becomes one
of the principal sources of supply of the nitrogenous food of plants,
and a soil rich in humus is one rich in nitrogen. The nitrogen in
humus is not directly available as a food for plants, but many kinds
of fungi and bacteria are capable of converting it into ammonia, from
which, by the agency of nitrifying organisms, it is turned into
nitrates and made available for the use of plants. Humus is able to
retain phosphoric acid, potash, ammonia and other bases. So important
were the functions of humus considered at one time that on this Thaer
built his "humus theory," which was, in effect, that, if humus was
supplied to the soil, plants required nothing more. This was based,
however, on the erroneous belief that the carbon, of which the bulk of
the plant consists, was derived from the humus of the soil, and not,
as we now know it to be, from the carbonic acid of the atmosphere.
This theory was in turn replaced by the "mineral theory" of Liebig,
and then both of them by the "nitrogen theory" of Lawes and Gilbert.

We pass next to review, in the light of the foregoing, the manures in common use at the present day.

Manures, as already stated, may be variously classified according to the materials they are made from, the constituents which they chiefly supply, or the uses to which they are put. But, except with certain few manures, such as nitrate of soda, sulphate of ammonia and potash salts, which are used purely for one particular purpose, it is impossible to make any definite classification of manures, owing to the fact that the majority of them serve more than one purpose, and contain more than one fertilizing constituent of value. It is only on broad lines, therefore, that any division can be framed. Between so-called "natural" manures like farm-yard manure, seaweed, wool waste, shoddy, bones, &c., which undergo no particular artificial preparation, and manufactured manures like superphosphate, dissolved bones, and other artificially prepared materials, there may, however, be a distinction drawn, as also between these and such materials as are imported and used without further preparation, e.g. nitrate of soda, kainit, &c. On the whole, the best classification to attempt is that according to the fertilizing constituents which each principally supplies, and this will be adopted here, with the necessary qualifications.

I.--NITROGENOUS (WHOLLY OR MAINLY) MANURES

These divided themselves into: (a) Natural nitrogenous manures; (b) imported or manufactured manures.

a. NATURAL NITROGENOUS MANURES

Under this heading come--farm-yard manure; seaweed; refuse cakes and
meals; wool dust and shoddy; hoofs and horns; blood; soot; sewage
sludge.

_Farm-yard Manure._--This is the most important, as well as the most
generally used, of all natural manures. It consists of the solid and
liquid excreta of animals that are fed at the homestead, together with
the material used as litter. The composition of farm-yard manure will
vary greatly according to the conditions under which it is produced.
The principal determining factors are (1) the nature and age of the
animals producing it, (2) the food that is given them, (3) the kind
and quantity of litter used, (4) whether it be made in feeding-boxes,
covered yards or open yards, (5) the length of time and the way in
which it has been stored. The following analysis represents the
general composition of well-made farm-yard manure, in which the litter
used is straw:--

Water 75.42
*Organic matter 16.52
Oxide of iron and alumina .36
Lime 2.28
Magnesia .14
Potash .48
Soda .08
**Phosphoric acid .44
Sulphuric acid .12
Chlorine .02
Carbonic acid, &c. 1.38
Silica 2.76
------
100.00
------

* Containing nitrogen = .59%,
which is equal to ammonia .72%

** Equal to phosphate of lime .96

Put broadly, farm-yard manure will contain from 65 to 80% of water,
from .45 to .65% of nitrogen, from .4 to .8% of potash, and from .2 to
.5% of phosphoric acid.

This analysis shows that farm-yard manure contains all the
constituents, without exception, which are required by cultivated
crops in order to bring them to perfection, and hence it may be called
a "perfect" manure. Dung, it may be observed, contains a great variety
of organic and inorganic compounds of various degrees of solubility,
and this complexity of composition--difficult, if not impossible, to
imitate by art--is one of the circumstances which render farm-yard
manure a perfect as well as a universal manure.

The excrements of different kinds of animals vary in composition, and
those of the same animal will vary according to the nature and
quantity of the food given, the age of the animal, and the way it is
generally treated. Thus, a young animal which is growing, needs food
to produce bone and muscle, and voids poorer dung than one which is
fully grown and only has to keep up its condition. Similarly, a
milking-cow will produce poorer dung than a fattening bullock. Again,
cake-feeding will produce a richer manure than feeding without cake.
Straw is the most general litter used, but peat-moss litter, sawdust,
&c., may be used, and they will affect the quality of the manure to
some extent. Peat-moss is the best absorbent and has a higher manurial
value than straw. Box-fed manure, and that made in covered yards will
suffer much less loss than that made in an open yard. Lastly, manure
kept in a heap covered with earth will be much richer than that left
in an uncovered heap. The solid and liquid excrements differ much in
composition, for, while the former contain principally phosphoric
acid, lime, magnesia, and silica and comparatively little nitrogen,
the urine is almost destitute of phosphoric acid, and abounds in
alkaline salts (including salts of potash) and in nitrogenous organic
matters, among which are urea and uric acid, and which on
decomposition yield ammonia. Unless, therefore, the two kinds of
excrements are mixed, a perfect manure supplying all the needs of the
plant is not obtained; care must accordingly be taken to absorb all
the urine by the litter. Farm-yard manure, it is well known, is much
affected by the length of time and the way in which it has been kept.
Fresh dung is soluble in water only to a limited extent, and, in
consequence, it acts more slowly on vegetation, and the action lasts
longer than when dung is used which has been kept some time; fresh
dung is therefore generally used in autumn or winter, and thoroughly
rotten dung in spring, when an immediate forcing effect is required.

The changes which farm-yard manure undergoes on keeping, have been
made the subject of much inquiry. In Germany, Maercker and
Schneidewind; in France, Muntz and Girard; and in England, Voelcker,
Wood, Russell and others, have investigated these losses, coming to
very similar conclusions concerning them. Perhaps the most complete
set of experiments is one conducted at the Woburn experimental station
and extending over three years (1899-1901). The dung was cake-fed
manure made in feeding-boxes from which no drainage issued, and, after
removal, it was kept in a heap, covered with earth. Hence it was made
under as good conditions as possible; but, even then, the
losses--after deduction for live-weight increase of the animals--were
found to be 15% of the total nitrogen of the food, during the making,
and 34% (or a further 19%) during storing and by the time the manure
came to be put on the land. Accordingly, under ordinary farm
conditions it is quite clear that only about 50% of the nitrogen of
the food given is recovered in the dung that goes on the land. This is
the figure which Lawes and Gilbert suggested in the practical
application of their Tables of Compensation for Unexhausted Manure
Value.

During the fermentation of dung a large proportion of the
non-nitrogenous organic matters disappear in the forms of carbonic
acid and water, while another portion is converted into humic acids
which fix the ammonia gradually produced from the nitrogenous
constituents of the solid and liquid excreta. The mineral matters
remain behind entirely in the rotten dung, if care be taken to prevent
loss by drainage. For proper decomposition, both air and moisture are
requisite, while extreme dryness or too much water will arrest the due
fermentation of the mass.

Well-fermented dung is more concentrated and consequently more
efficacious than fresh farm-yard manure. Neither fresh nor rotten dung
contains any appreciable quantity of volatile ammonia, and there is no
advantage from applying gypsum, dilute acid, superphosphate, kainit,
or other substances recommended as fixers of ammonia. If dung is
carted into the field and spread out at once in thin layers it will
suffer comparatively little loss. But if dung be kept for a length of
time in shallow heaps, or in open straw-yards and exposed to rain, it
loses by drainage a considerable proportion of its most valuable
soluble fertilizing constituents. Experiments with farm-yard manure
kept in an open yard showed that, after twelve months' exposure to the
weather, nearly all the soluble nitrogen and 78.2% of the soluble
mineral matters were lost by drainage (A. Voelcker). To prevent this
loss, farm-yard manure, as had been pointed out, should, whenever
possible, be carted into the field, spread out at once, and ploughed
in at the convenience of the farmer. It is, however, not always
practicable to apply farm-yard manure just at the time it is made,
and, as the manure heap cannot be altogether dispensed with, it is
necessary to see how the manure may best be kept. The best dung is
that made in regular pits or feeding-boxes. In them the urine is
thoroughly absorbed, and, the manure being more compact through the
constant treading, air enters less freely and the decomposition goes
on less rapidly, the volatile matters, in consequence, not being so
readily lost. External agents, such as rain, wind, sun, &c., do not
affect the manure as they would in the case of open yards. Next best
to box-fed manure is that made in covered yards, then that in sheds,
and lastly that in open yards. When removed from the box or yard, the
manure should be put in a heap upon a floor of clay or
well-beaten-down earth, and then be covered with earth. When kept in
an open yard, care should be taken not to let spoutings of buildings
lead on to it, and if there be a liquid-manure tank, this might be
pumped out over the manure again when the latter is too dry.

The advantages of farm-yard manure consist, not only in its supplying
all the constituents of plant food, but also in the improved physical
condition of the soil which results from its application, inasmuch as
the land is thereby kept porous, and air is allowed free access.
While, however, farm-yard manure has these advantages, experience has
shown that artificial manures, properly selected so as to meet the
requirements of the crops intended to be grown on the particular land,
may be employed to greater advantage. In farm-yard manure about
two-thirds of the weight is water and one-third dry matter; a large
bulk thus contains only a small proportion of fertilizing substances,
and expense is incurred for carriage of much useless matter when dung
has to be carted to distant fields. When a plentiful supply of good
farm-yard manure can be produced on the farm or bought at a moderate
price in the immediate neighbourhood, it is economy to use it either
alone or in conjunction with artificial manures; but when food is dear
and fattening does not pay, or farm-yard manure is expensive to buy,
it will be found more economical to use artificial manures. This has
obtained confirmation from the experience of Mr Prout, at
Sawbridgeworth, Herts, where since 1866, successive crops of corn have
been grown, and entirely with the use of artificial manures.

The real difficulty with farm-yard manure is to get enough of it, and,
if it were available in sufficiency, it would be safe to say that
farmers generally would not require to go farther in regard to the
manuring of any of the crops of the farm. Moreover, experiments at
Rothamsted and Woburn have shown of how "lasting" a character
farm-yard manure is, its influence having told for some 15 to 20 years
after its application had ceased.

Light land is benefited by farm-yard manure through its supplying to
the soil organic matter, and imparting to it "substance" whereby it
becomes more consolidated and is better able to retain the manurial
ingredients given to it. By improving the soil's moisture-holding
capacity, moreover, "burning" of the land is prevented.

With heavy clay soils the advantages are that these are kept more open
in texture, drainage is improved, and the soil rendered easier of
working. On light land, well-rotted manure is best to apply; and in
spring, whereas on heavy land freshly-made, "long," manure is best,
and should be put on in autumn or winter.

Farm-yard manure, where the supply is limited, is mostly saved for the
root-crop, which, however, generally needs a little superphosphate to
start it, as farm-yard manure is not sufficiently rich in this
constituent. It serves a great purpose in retaining the needed
moisture in the soil for the root crop.

For potato-growing, for vegetables, and in market-gardening, farm-yard
manure is almost indispensable. On grass-land and on clover-ley it is
also very useful, and in the neighbourhood of large towns is employed
greatly for the production of hay.

For corn crops also, and especially for wheat on heavy land, farm-yard
manure is much used, and, in a dry season in particular, shows
excellent results, though experiments at Rothamsted and Woburn have
shown that, on heavy and light land alike, heavier crops of wheat and
barley can be produced in average seasons by artificial manures.

_Seaweed._--Along the sea-coast seaweed is collected, put in heaps and
allowed to rot, being subsequently used on the land, just as farm-yard
manure is. According to the nature of the weed and its water-contents,
it may have from .3 to 1% of nitrogen, or more, with potash in some
quantity.

_Green-manuring._--Though properly belonging to cultivation rather
than to manuring, and acting chiefly as a means of improving the
condition of the soil, the practice of green-manuring carries with it
manurial benefits also, in that it supplies humus and nitrogen to the
soil, and provides a substitute for farm-yard manure. The ploughing-in
of a leguminous green-crop which has collected nitrogen from the
atmosphere should result in a greater accumulation of nitrogen for a
succeeding corn-crop, and thus supply the cheapest form of manuring.
Green-manuring is most beneficial on light land, poor in vegetable
matter.

_Manure Cakes, Malt Dust, Spent Hops, &c._--Many waste materials of
this kind are used because of their supplying, in the form of
nitrogenous organic matter, nitrogen for crop uses. The nitrogen in
these is of somewhat slow-acting, but lasting, nature. In addition to
nitrogen, some of these materials, e.g. rape cake, cotton cake and
castor cake, contain appreciable amounts of phosphoric acid and
potash. Rape cake, or "land cake," as it is called in Norfolk, is used
considerably for wheat. It is also believed to be a preventive of
wireworm, and so is often employed for potatoes and root-crops.
Rape-seed from which the oil has been extracted by chemical means, and
which is called "rape refuse," is made use of in hop-gardens as a
slowly acting supplier of nitrogen. It will contain 4 to 5% of
nitrogen with 3 to 4% of phosphates. Damaged cotton and other
feeding-cakes, no longer fit for feeding, are ground into meal and put
on the land. Castor cake is directly imported for manurial purposes,
and will have up to 5% of nitrogen with 4 to 5% of phosphates. Spent
hops, malt dust and other waste materials are similarly used. The
principal use of these materials is on light land, and to give bulk to
the soil while supplying nitrogen in suitable form.

_Wool-dust, Shoddy, &c._--The clippings from wool, the refuse from
cloth factories, silk, fur and hair waste, carpet clippings and
similar waste materials are comprised in this category. They are
valuable purely for their nitrogen, and should be purchased according
to their nitrogen-contents. They are favourite materials with
hop-growers and fruit-farmers, whose experience leads them to prefer a
manure which supplies its nitrogen in organic form, and which acts
continuously, if not too readily. It is the custom in hop-lands to
manure the soil annually with large quantities of these waste
materials till it has much fertility stored up in it for succeeding
crops. According to its nature, wool-dust or shoddy may contain
anything from 3% of nitrogen up to 14%.

Leather is another waste material of the same class, but the process
of tanning it has undergone makes its nitrogen but very slowly
available and it is avoided, in consequence, as a manure. There have
been several processes started with the object of rendering leather
more useful as a manure.

_Hoofs and Horns._--The clippings and shavings from horn factories are
largely used by some hop-growers, and, though very slow in their
action, they will contain 14 to 15% of nitrogen. They are sometimes
very finely ground and sold as "keronikon," chiefly for use in
compound artificial manures.

_Dried Blood_ is another purely nitrogenous material, which however
seldom finds its way to the farmer, being used up eagerly by the
artificial manure maker. It will contain from 12 to 14% of nitrogen.
It is obtained by simply evaporating down the blood obtained from
slaughter-houses. It is the most rapidly acting of the organic
nitrogenous materials enumerated, and, when obtainable, is a favourite
manure with fruit-growers, being also used for root and vegetable
growing.

_Soot_ is an article of very variable nature. It owes its manurial
value mainly to the ammonia salts it contains, and a good sample will
have about 4% of ammonia. It is frequently adulterated, being mixed
with ashes, earth, &c. Flue sweepings of factory chimneys are
sometimes sold as soot, but possess little value. Besides the ammonia
that soot contains, there would undoubtedly seem to be a value
attaching to the carbonaceous matter. Soot is a favourite top-dressing
for wheat on heavy land, and is efficacious in keeping off slugs, &c.
Speaking generally, the lighter a sample of soot is the more likely is
it to be genuine.

_Sewage Manure._--Where methods of dealing with the solid matters of
sewage are in operation, it frequently happens that these matters are
dried, generally with the aid of lime, and sold locally. Occasionally
they are prepared with the addition of other fertilizing materials and
made up as special manures. It may be taken for granted that sewage
refuse by itself is not worth transporting to any distance. When made
up with lime, the "sludge," as it is generally termed, is often useful
because of the lime it contains. But, on the whole, the value of such
preparations has been greatly exaggerated. Where land is in need of
organic matter, or where it is desirable to consolidate light land by
the addition of material of this class, sludge may, however, have
decided value on mechanical and physical grounds, but such land
requires to be near at hand.

b. _Imported or Manufactured Nitrogenous Manures._

These are nitrate of soda; sulphate of ammonia; calcium cyanamide;
calcium nitrate.

_Nitrate of Soda._--This is the best known and most generally used of
purely nitrogenous manures. It comes from the rainless districts of
Chile and Peru, from which it was first shipped about the year 1830.
By 1899 the export had reached to 1,344,550 tons. It is uncertain what
its origin is, but it is generally believed to be the deposit from an
ancient sea which was raised by volcanic eruption and its waters
evaporated. Another theory puts it as the deposit from the saline
residues of fresh-water streams. The crude deposit is termed
_caliche_, and from this (which contains common salt and sulphates of
soda, potash and lime) the nitrate is crystallized out and obtained as
a salt containing 95 to 96% pure nitrate of soda. It is sold on a
basis of 95% pure, and is but little subject to adulteration.

As a quickly acting nitrogenous manure nitrate of soda has no equal,
and it is in great demand as a top-dressing for corn crops, also for
roots. On grass-land, if used alone, it tends to produce grass but to
exterminate leguminous herbage. Its tendency with corn crops is to
produce, if used in quantity, inferiority of quality in grain. It can
be employed in conjunction with superphosphate and other artificial
manures, though it should not be mixed with them long before the
mixture is to be put on. It is a very soluble salt, and the nitrogen
being in the form of nitrates, it can be readily taken up by plants.
On the other hand, it is readily removed from the soil by drainage,
and its effects last only for a single season. Owing to its
solubility, it requires to be used in much larger amount than the crop
actually will take up. On a heavy soil it has a bad influence if used
repeatedly and in quantity, causing the land to "run," and making the
tilth bad. Though, doubtless, exhaustive to the soil, when used alone,
there is no evidence yet of nitrate of soda causing land to "run out,"
as has been shown to be the case with sulphate of ammonia. One cwt. to
the acre is a common dressing for corn crops, but for mangels it has
been used to advantage up to 4 cwt. per acre. As a top-dressing for
corn crops it differs little in its crop-results from its rival
sulphate of ammonia, but in a dry season it answers better, owing to
its more ready solubility and quicker action, whereas in a wet season
sulphate of ammonia does better.

_Sulphate of Ammonia._--This is the great competitor with nitrate of
soda, and, like the latter, is useful purely as a nitrogenous manure.
It is obtained in the manufacture of gas and as a by-product in the
distillation of shale, &c., as also from coke ovens. By adding
sulphuric acid to the ammoniacal liquor distilled over from the coal,
&c., the salt is crystallized out. It is seldom adulterated, and, as
sold in commerce, generally contains 24 to 25% of ammonia. It is not
quite so readily soluble as nitrate of soda; it does not act quite so
quickly on crops, but is less easily removed from the soil by
drainage, leaving also a slight amount of residue for a second crop.
It is nearly as efficacious as a top-dressing for corn crops as is
nitrate of soda, and for some crops, e.g. potatoes, it is considered
superior. It may also be used like nitrate of soda for root crops. On
grass-land its effect in increasing gramineous but reducing leguminous
herbage is similar to that of nitrate of soda, but with corn crops it
has not the same deteriorating influence on the quality of grain. It
can be mixed quite well with superphosphate and other artificial
manures, and is therefore a common form in which nitrogen is supplied
in compound manures. It does not produce the bad effect on the tilth
of certain soils that nitrate of soda does, but it is open to the
objection that, if used continually on soil poor in lime, it will
gradually exhaust the soil and leave it in an acid condition, so that
the soil is unable to bear crops again until fertility is restored by
the addition of lime. A usual dressing of sulphate of ammonia is 1
cwt. per acre.

_Calcium Cyanamide._--This is a new product which represents the
earliest result of the utilization, in a commercial form, of
atmospheric nitrogen as a manurial substance. It is obtained by
passing nitrogen gas over the heated calcium carbide obtained in the
electric furnace, the nitrogen then uniting with the carbide to form
calcium cyanamide. The product contains from 19 to 20% of nitrogen,
and, though still under trial as a nitrogenous manure, it bids fair to
form a valuable source of supply, especially should the natural
deposits of nitrate of soda become exhausted. The cost of production
limits its manufacture to places where electrical power can be cheaply
generated. In its action it would seem to resemble most closely
sulphate of ammonia.

_Calcium Nitrate._--This is another product of the utilization of
atmospheric nitrogen as a manurial agent. Nitrogen and oxygen are made
to combine within the electric arc and the nitric acid produced is
then combined with lime, forming nitrate of lime. Nitrate of lime
contains, as put on the market, about 13% of nitrogen. In its action
it should be very similar to nitrate of soda, with, possibly, some
added benefit to certain soils by reason of the lime it contains. Like
cyanamide, it is still in the experimental stage as regards its
agricultural use, and can only be produced where electric power is
cheaply obtainable.

Neither material is altogether free from objection, the cyanamide
heating when mixed with other manures and even with soil, and being
liable to give off acetylene gas owing to the presence of calcium
carbide, whereas the calcium nitrate is a salt which on exposure to a
moist atmosphere readily deliquesces.

II.--PHOSPHATIC MANURES

Under the heading of manures that are used purely for their phosphatic benefit to the soil are superphosphate and basic slag.

_Superphosphate._--This is the typical phosphatic manure, and is the
base of the numerous artificial manures used on the farm.
Superphosphate is made by dissolving raw phosphatic minerals in
sulphuric acid (oil of vitriol), the tribasic phosphate of lime which
these contain being converted into the so-called "soluble phosphate,"
sulphate of lime being formed at the same time. The first impetus to
the manufacture of superphosphate was given by Liebig, when he
suggested, in 1840, the treatment of bones with oil of vitriol in
order to make them act more quickly in the soil. Lawes subsequently,
in 1843, applied this to mineral phosphates, using phosphorite, first
of all, and the great manufacture of mineral superphosphate then
began. Coprolites, as found in Cambridgeshire, Suffolk, Bedfordshire
and elsewhere were the raw materials at first employed in the United
Kingdom. But gradually the demand for the new manure became so great
that distant parts of the world were searched to bring in the raw
material for conversion into superphosphate. Many new sources of
supply have been worked, and many worked out or abandoned in favour of
better and richer phosphates. Among these were the crystalline
apatites of Canada and Norway, French, Spanish and German (Lahn)
phosphates, and, at a later period, Carolina (land and river),
Florida, Tennessee, Somme, Belgian, Algerian and Tunisian phosphates.
In addition to these came other materials which, in their origin, were
really of the nature of guano, being bird deposits the ammoniacal
matters of which were gradually washed out. The mineral matters
remained and altered the composition of the original rock on which the
guano was deposited, thus forming rich deposits of phosphate of lime.
Such were the phosphates obtained from many of the islands of the West
Indies and South Pacific, and known under such various names as
Sombrero, Curaçao, Aruba, Maiden Island, Megillones, Baker Island,
Fanning Islands, Lacepedes Islands, &c. guanos. Few of these are now
worked, but their place has been largely taken by the rich deposits of
Ocean Island and Christmas Island, which are of similar origin. The
principal supplies of phosphatic minerals at the present time come
from Florida, Algeria, Tunis, Ocean Island and Christmas Island. Other
phosphates imported are Redonda and Alta Vela phosphates, but these
consist mainly of phosphate of alumina, and are not used for
superphosphate manufacture but for phosphorus production.

Coprolites, as formerly used, contained from 50 to 60% of phosphate of
lime, but they are not worked now, the richer sources, which are also
better adapted for superphosphate manufacture, having taken their
place. The amount of oxide of iron and alumina in raw phosphates is of
great importance, as phosphates containing these bodies are liable to
cause superphosphate to "go back" or form what is called "reverted"
phosphate, the percentage of "soluble phosphate" being reduced
thereby. For this reason many of the older supplies have been replaced
by newer and better ones. Florida rock phosphate of high grade
contains 75 to 78% of phosphate of lime, and Florida land pebble
phosphate about 70%. Algerian and Tunisian phosphates have from 55 to
65% of phosphate of lime, and are very free from iron and alumina,
this fitting them especially for superphosphate making. Tennessee
phosphate has about 70% of phosphate, Somme and Belgian phosphates 40
to 50%, while Ocean Island and Christmas Island phosphates are of very
high grade and yield over 80 and up to 86% of phosphate of lime.
Superphosphate is made by finely grinding the raw phosphate and mixing
it with oil of vitriol (chamber acid); what actual product is formed
is a matter of some uncertainty, but it is a phosphate soluble in
water, and believed to be mono-calcic phosphate. This is the true
"soluble phosphate," but in commercial transactions it is universal to
express the amount in terms of the original tribasic phosphate which
has been rendered soluble. Ordinary grades of mineral superphosphate
give from 25 to 27% of soluble phosphate and higher grades 30 to 35%.
On reaching the soil, the soluble phosphate becomes precipitated by
the calcium and iron compounds in the soil. But it is precipitated in
a very fine form of division, in which it is readily attacked by the
plant roots. Superphosphate is used practically for all crops,
including cereals, clover and other leguminous crops. Its use tends to
early maturity in a crop. Its value for giving a start to root crops
is particularly recognized, and root crops generally are dependent on
it, as they have little power of utilizing the phosphoric acid in the
soil itself. On land poor in lime superphosphate must be used with
caution owing to its acid nature, and in such cases an undissolved
phosphate is preferable. The quantity in which it is applied ranges
from 2 and 3 cwt. per acre to 5 cwt. It suffers but little loss
through drainage, and will exercise an influence on crops beyond the
year of application.

_Basic Slag._--This other principal phosphatic manure is of more
recent origin, and is an undissolved phosphate. It is the waste
product of steel-making where the Thomas-Gilchrist or "basic" process
of manufacture has been employed. This process is used with ores
containing much phosphorus, the removal of which is necessary in
steel-manufacture. The "converters" which hold the molten iron are
lined with lime and magnesia, and the impurities of the iron form a
"slag" with these materials. For a long time the slag was regarded as
a waste product, but ultimately it was found that, by grinding it very
finely, it had distinct agricultural value, and now its use is
universal. Basic slag is of various grades, containing 12 to 20% of
phosphoric acid, which is believed to exist in the form of a
tetracalcic phosphate. This phosphate is found to be readily attacked
by a weak solution of citric acid, and this probably accounts for the
comparative ease with which plants can utilize the phosphate. With it
is also a good deal of lime, and the presence of this undoubtedly, in
many cases, accounts partly for the benefits that follow the use of
basic slag. It should be very finely ground; a common standard is that
80 to 90% should pass through a sieve having 10,000 meshes to the
square inch.

The principal use of basic slag is on grass-land, especially where the
soil is heavy or clayey. Its effect on such land in causing white
clover to appear is in many cases most remarkable, and without doubt,
much poor, cold grass-land has been immensely benefited by its use. It
is also employed for root crops; but its effect on these, as on
cereals, is not so marked as on grass-land. On light land its benefit
is not nearly so great or universal as on heavier land.

III.--MANURES CONTAINING NITROGEN AND PHOSPHATES

These may be classified as follows: (a) Natural manures--bones, fish and meat guanos, Peruvian guano, bats' guano; (b) Manufactured manures--dissolved bones, compound manures.

a. _Natural Manures_

_Bones.._--The value and use of these in agriculture has long been
known, as also the comparative slowness of their action, which latter
induced Liebig to suggest their treatment with sulphuric acid. Natural
bones will contain from 45 to 50% of phosphate of lime with 4 to 4½%
of nitrogen. It is usual to boil bones lightly after collection, in
order to remove the adhering particles of flesh and the fat. If
steamed under pressure the nitrogenous matter is to a great extent
extracted, yielding glue, size, gelatine, &c., and the bones--known
then in agriculture as "steamed bones"--will contain from 55 to 60% of
phosphate of lime with 1 to 1½% of nitrogen. Bones are also imported
from India, and these are of a very hard and dry nature. Bones are
principally used for root crops, and to some extent on grass-land. The
more finely they are ground the quicker is their action, but they are
a slow-acting manure, which remains some years in the land. Mixed with
superphosphate, bone meal forms an excellent manure for roots, and
obviates the difficulty of using superphosphate on land poor in lime.
Steamed bones, sometimes ground into flour, are much used in dairy
pastures.

_Fish and Meat Guanos._--The term "guano," though generally applied to
these manures, is wrongly so used, for they are in no sense guano
(meaning thereby the droppings of sea birds). They are really fish or
meat refuse, being generally the dried fish-offal or the residue from
meat-extract manufacture. They vary much in composition, according to
their origin, some being highly nitrogenous (11 to 12% nitrogen) and
comparatively low in phosphate of lime, and others being more highly
phosphatic (30 to 40% phosphate of lime) with lower nitrogen. These
materials are to some extent used for root and vegetable crops, and
chiefly for hop-growing, but they go largely also to the artificial
manure maker.