THE APPLICATION OF MANURES.
The conditions which regulate the application of manures are many and
varied, and the subject, it must be admitted, despite the large amount
of investigation already carried out, is most imperfectly understood.
For these reasons it is impossible to do little more than lay down
certain general principles which may be of service to the agriculturist
in guiding him in carrying out the manuring of his crops.
_Influence of Manures in increasing Soil-fertility._
In the first place it may be asked, How far can what we may call the
permanent fertility of a field be influenced by the application of
manures? And to this question the answer must be made, that the
influence of manuring in increasing soil-fertility is very slight and
only very gradually felt. This is illustrated by the difficulty
experienced in attempting to restore to a fertile condition a soil which
has long been treated by an exhaustive system of cultivation. In such a
case it will be found impossible to restore the fertility of the soil,
except very gradually. Farmers who farm in new countries, and in rich
virgin soils, little realise sometimes how quickly they may impoverish
the fertility of their soils by exhaustive treatment, and how slow the
process of restoration is. Nor is this strange when we reflect on the
relatively small quantities of fertilising ingredients we are in the
habit of adding to the soil by the application of manures, and the
nature of their action. The small rate at which they are applied, and
the impossibility of distributing them equally in the soil, explain how
comparatively limited their action must necessarily be. Some manures, it
is true--viz., those which are soluble--are more equally distributed;
but then such manures, from their very nature, are little likely to
affect the permanent fertility of the soil.
_Influence of Farmyard Manure on the Soil._
Of manures which have the best effect in improving a soil's permanent
fertility, farmyard manure is undoubtedly the most important. This is
owing partly to the fact that it is applied in such large quantities,
and partly on account of its composition. Liberal manuring with farmyard
manure, systematically carried out, will in time do much to build up a
soil's fertility. But liberal manuring with artificial manures will also
effect the same end. This it does in an indirect manner by means of the
increased crop residues obtained under such treatment. Indeed one of the
speediest methods of bringing a soil into good condition is by heavily
manuring certain green crops, and then ploughing them in.
_Farmyard Manure v. Artificials._
The question how far farmyard manure may be supplanted by artificials is
one often discussed. We have already referred to this question in the
chapter on Farmyard Manure. It is possible that, with our increasing
knowledge of agricultural science, we may in the future be able to
dispense with farmyard manure, and make shift to do with artificials
alone. At present, however, all our experience points to the fact that
the most satisfactory results are obtained from manures by using
artificials in conjunction with farmyard manure. It is better both for
farmyard manure and artificial manures to be applied together,[241] so
that they may mutually act as supplementary the one to the other. While
this is so, there may be circumstances in which it will be best to use
artificials alone. Where, for example, fields, owing to their situation,
are inaccessible, and where the expense of conveying the bulky farmyard
manure would be very considerable, it may be found more economical to
apply the more concentrated artificial manures. With few exceptions,
however, it will be found most desirable to use artificial manures as
supplementary to farmyard manure, and not as substitutes for it.
_Farmyard Manure not favourable to certain Crops._
While the above is true, it may be well to point out one or two facts
regarding the nature of the influence of farmyard manure on certain
crops. For instance, it has long been recognised as inadvisable in
strong rich soils to apply it directly to certain grain crops, such as
barley and wheat, since such a practice is apt to encourage rankness of
growth--an undue development of straw at the expense of the grain. It is
consequently customary to apply farmyard manure to the preceding crop.
The direct application of farmyard manure to wheat, however, according
to Sir J. B. Lawes, is not fraught with unfavourable results where the
soil is a light one; it is only when the soil is of a heavy nature that
it is best to apply it to the preceding crop. Potatoes are another crop
to which it is best not to apply it directly. On the other hand, many
are of the opinion that mangels seem to be able to benefit from large
applications of farmyard manure.
_Conditions determining the Application of Artificial Manures._
In the application of artificial manures a large number of
considerations have to be taken into account. Among these may be
mentioned the nature of the manure itself, and its mechanical and
chemical condition; the nature of the soil and its previous treatment
with manures, as well as the nature of the climate, the nature of the
crop, and the previous cropping. It may be well, therefore, to examine
somewhat in detail some of these considerations.
_Nature of the Manure._
Nitrogen, phosphoric acid, and potash exist in the common manures, as
has already been pointed out, in different states of availability.
Nitrogen, for example, may exist in a soluble or insoluble condition, as
nitrates, as ammonia, or in various organic forms. Phosphoric acid,
similarly, may exist in a soluble form, as it does in superphosphate of
lime, or in an insoluble form, as it does in bones or basic slag.
Potash, on the other hand, exists--or should exist--in artificial
manures only in a soluble form. Now a correct knowledge of the behaviour
of these different forms of the common manurial ingredients when applied
to the soil is, in the first place, necessary for their successful and
economical use.
_Nitrogenous Manures._
Thus our knowledge of the inability of the soil-particles to retain
nitrogen in the form of nitric acid, as well as our knowledge of the
fact that nitrogen is in this form immediately available for the
plant's needs, teaches us that nitrate of soda should never be applied
before the plant is ready to utilise it--in short, that it should only
be applied as a top-dressing; and further, that the use of such a
fertiliser in a damp season is less likely to be economical than in a
dry one. Again, with regard to nitrogen in the form of ammonia salts,
our knowledge of the fact that ammonia is retained by the
soil-particles, and that before it becomes available for the plant's
needs it has to undergo the process of nitrification, teaches us the
desirability of applying it a short time before it is likely to be used.
While, lastly, with regard to the nitrogen in the various organic forms
in which it occurs, our knowledge of the rate at which these are
converted into an available form in the soil will determine when they
are best applied. Some forms of organic nitrogen are in a soluble
condition, and are quite as speedy in their action as sulphate of
ammonia. This is the case with a considerable proportion of the
different organic forms of nitrogen present in guano. Other forms of
organic nitrogen are only slightly less so--as, for example, dried
blood, which ferments very speedily. With regard, therefore, to nitrates
and ammonia salts, as well as the more quickly available organic forms
of nitrogen, they should either be applied as a top-dressing after the
plant has started growth, or only shortly before seed-time. Bones,
shoddy, and the various so-called native guanos, should be applied a
considerable period before they are likely to be required--not later
than the previous autumn.
_Phosphatic Manures._
With regard to phosphatic manures the same considerations hold good.
Inasmuch as phosphoric acid, whether applied in the soluble condition,
as in superphosphate, or the insoluble form, as in bones, basic slag,
&c., is not liable to be washed out of the soil, the risk of loss is
very slight, and need not be taken into account. As we have pointed out
in considering the action of superphosphate, phosphoric acid in this
latter form is more speedily available to the crop, and the necessity of
applying it much before it is likely to be used does not exist. Hence
superphosphate and manures which contain any appreciable amount of
soluble phosphoric acid, such as guano, should only be applied shortly
before seed-time. Bones, basic slag, or mineral phosphate ought to be
applied, on the other hand, a long time before they are likely to be
used. Hence an autumn application is to be recommended in the case of
such manures.
_Potash Manures._
Lastly, with regard to potash manures, as these are soluble, there is no
necessity for applying them much before they are likely to be absorbed
by the plant. Some are of the opinion that potash is, except in the case
of sandy soils, best applied some little time before it is likely to be
used, so as to permit of its being washed down into the soil--a process
which takes place only comparatively slowly. As potash manures have
often been found to give a better result on pastures during the second
year than during the first, they are best applied in the autumn.
The above statement as to the behaviour of the different fertilisers
when applied to the soil, has a not unimportant bearing on the
quantities in which they may safely be respectively applied. The rate at
which manures may be applied depends, as we shall immediately see, on
other conditions; but what it is here desirable to point out is, that it
is not safe to apply such manures as nitrate of soda, or, for that
matter, sulphate of ammonia, in large quantities at a time. In fact
these manures, especially the former, will best be applied in very small
quantities, and rather in several doses. With regard to other manures,
more especially phosphatic manures, the same reasons for small
application do not exist.
The truth of the above statements is so obvious that it may be regarded
as superfluous to make them. As, however, their clear apprehension is
essential to understanding the conditions of successful manuring, no
apology need be made for making them.
_Nature of Soil._
Another condition which has to be taken into account in considering the
application of manures is the nature of the soil, as well as its
previous treatment. Soils poor in organic matter are those which are
most likely to be benefited by the application of nitrogenous manures.
Soils of a dry light character require less phosphoric acid than they do
of nitrogen and potash; while on a damp and heavy soil phosphatic
manures are more likely to be beneficial than nitrogenous or potassic
manures. Lastly, a soil rich in organic matter generally requires
phosphates, and possibly potash. A point of considerable importance to
notice is, that a soil rich in lime can stand a larger application of
phosphoric acid than one poor in lime. As a rule, it will be found that
the best results with potash will be obtained when applied to a sandy
soil. The nature of the soil is an important consideration in
determining how far it is advisable to apply readily soluble manures. To
a very light and non-retentive soil the risk of loss in applying an
easily soluble manure is considerably increased. The nature of the
climate is also of importance. Thus, in a dry climate, manures of a
soluble nature will have a better effect than in a wet climate, while
the opposite will be the case with the more slowly acting manures.
_Nature of previous Manuring._
A consideration of equal importance is the previous treatment of the
soil with manure. For example, where a soil has been liberally treated
with farmyard manure, it has been found that mineral manures have a
very inferior effect to that obtained by nitrogenous manure. Lawes and
Gilbert have found this to be strikingly the case in their experiments
on the growth of wheat. In these experiments it was found that the
application of mineral manures was accompanied with little or no benefit
to the crop, whereas very striking results followed the application of
nitrogen. This they attributed to the fact that the supply of mineral
fertilisers in the straw of the farmyard manure is largely in excess of
the supply of nitrogen. The nature of the action of the manure
previously applied is also to be taken into account in determining how
long its influence may probably last. Where, for example, the manure has
been nitrate of soda or sulphate of ammonia, it may be safely concluded
that its direct influence is no longer felt a year after application.
The influence of superphosphate of lime, while scarcely so temporary,
may be said to last only for a comparatively short time.[242] On the
other hand, when the manure applied is of a slow-acting nature, such as
bones or basic slag, its influence will probably be felt for a number of
years.
_Nature of the Crop._
But more important than any of the above-mentioned conditions is the
nature of the crop itself. Our knowledge of the requirements of the
different farm crops is still very imperfect. A very wide experience,
however, of the effect of different manures on different crops, has
conclusively proved that their manurial requirements differ very
considerably. The subject is complicated by other considerations, such
as the nature of the soil, &c.; but notwithstanding this fact, certain
points seem to be pretty well established.
In seeking to understand the respective requirements of the different
crops for different fertilisers, two important considerations must be
borne in mind. These are--(1) _the quantities of the three fertilising
ingredients--nitrogen, phosphoric acid, and potash--which different
crops remove from the soil;_ and (2) _the different power crops possess
of assimilating these ingredients._
_Amounts of Fertilising Ingredients removed from the Soil by different
Crops._
The most convenient way of instituting a comparison between the
requirements of the different crops in this respect is by calculating
the amount, in pounds, of nitrogen, phosphoric acid, and potash, which
average amounts of the different crops remove per acre. The following
table shows this for the common crops:--
FERTILISING INGREDIENTS REMOVED FROM SOIL.
----------------------------+-----------+------------+----------
| Nitrogen. | Phosphoric | Potash.
| | Acid. |
----------------------------+-----------+------------+----------
Mangels {Root, 22 tons | 87 | 36.4 | 222.8
{Leaf | 51 | 16.5 | 77.9
|-----------|------------|----------
Total crop | 138 | 52.9 | 300.7
|-----------+------------+----------
Turnips {Root, 17 tons | 63 | 22.4 | 108.6
{Leaf | 49 | 10.7 | 40.2
|-----------+------------+----------
Total crop | 112 | 33.1 | 148.8
|-----------+------------+----------
Beans {Grain, 30 bushels | 77 | 22.8 | 24.3
{Straw | 29 | 6.3 | 42.8
|-----------+------------+----------
Total crop | 106 | 29.1 | 67.1
|-----------+------------+----------
Red clover hay, 2 tons | 102 | 24.9 | 83.4
|-----------+------------+----------
Swedes {Root, 14 tons | 70 | 16.9 | 63.3
{Leaf | 28 | 4.8 | 16.4
|-----------+------------+----------
Total crop | 98 | 21.7 | 79.7
|-----------+------------+----------
Oats {Grain, 45 bushels | 38 | 13.0 | 9.1
{Straw | 17 | 6.4 | 37.0
|-----------+------------+----------
Total crop | 55 | 19.4 | 46.1
|-----------+------------+----------
Meadow hay, 1-1/2 ton, | 49 | 12.3 | 50.9
|-----------+------------+----------
Wheat {Grain, 30 bushels | 33 | 16.0 | 9.8
{Straw | 15 | 4.7 | 25.9
|-----------+------------+----------
Total crop | 48 | 20.7 | 35.7
|-----------+------------+----------
Barley {Grain, 30 bushels | 35 | 16.0 | 9.8
{Straw | 13 | 4.7 | 25.9
|-----------+------------+----------
Total crop | 48 | 20.7 | 35.7
|-----------+------------+----------
Potatoes, 6 tons | 47 | 21.5 | 76.5
|-----------+------------+----------
Maize {Grain, 30 bushels | 28 | 10.0 | 6.5
{Stalks, &c. | 15 | 8.0 | 29.8
|-----------+------------+----------
Total crop | 43 | 18.0 | 36.3
----------------------------+-----------+------------+----------
From the table it will be seen that the crops which remove the largest
quantities of all three fertilising ingredients are the root
crops--mangels and turnips; that beans remove twice as much nitrogen as
the cereals--oats, barley, and wheat--which, in this respect,
practically differ very little from one another; while potatoes remove
about the same quantity of nitrogen as the cereals. It will further be
noticed that the amounts of phosphoric acid removed by the different
crops differ very much less than those of nitrogen and potash. Mangels
remove slightly more, and turnips slightly less, than double the amount
removed by cereals. Meadow-hay, it will be seen, of all crops removes
the least phosphoric acid.
In looking at the amounts of potash, we are at once struck by their
great discrepancy. Such a crop as mangels removes more than six times as
much potash from the soil as the cereals. Turnips also make large
demands on this ingredient, removing over four times as much as the
cereals. Leguminous crops, such as red clover and beans, remove about
twice as much.
_Capacity of Crops for assimilating Manures._
Instructive though these figures undoubtedly are, _they must not be
regarded, as often erroneously they are, as furnishing by themselves
sufficient data upon which to base the practice of manuring_. A
consideration which is of much greater importance is the capacity that
different crops possess for assimilating the various manurial
ingredients from the soil. Considered from the point of view of absolute
amount, there is in most soils an abundant supply of plant-food; but of
this amount only a small proportion is available. Further, the amount of
this available plant-food will vary with different crops--one crop being
able to grow where another crop would starve. As illustrative of this,
in the Norfolk experiments it was found that the turnip was able to
assimilate potash from a soil on which the swede was practically
starved. It is on this fact more than any other that the principles of
manuring are based. Several explanations of the different capacities
crops possess of assimilating their food may be put forward. And we may
here point out that crops belonging to the same class exhibit, on the
whole, a certain amount of similarity in their manurial requirements.
Thus, for example, we may say that _gramineous crops_ so far resemble
one another in possessing _small capacity for assimilating nitrogen_,
_root crops for assimilating phosphoric acid_, and _leguminous crops for
assimilating potash_, and that, consequently, these crops are generally
most benefited by the application, respectively, of nitrogen, phosphoric
acid, and potash. But while a certain general resemblance exists, crops
belonging to the same class differ in many cases very considerably, as
we shall immediately see.
_Difference in Root Systems of different Crops._
One explanation of the different capacity possessed by different crops
for absorbing plant-food from the soil is to be found in the difference
of their root systems. Every agriculturist knows that crops in this
respect differ very widely. Crops having deep roots will naturally have
a larger surface of soil from which to draw their food-supplies than
crops having shallower roots. Such crops as red clover, wheat, and
mangels are able to draw their food-supplies from the subsoil to an
extent not possessed by shallower-rooted crops, such as barley, turnips,
and grass. Crops having surface-roots, on the other hand, have often
greater capacity for assimilating nitrogen,--this ingredient, as has
already been pointed out, being chiefly located in the surface-soil. The
tendency of growing shallow-rooted crops will therefore be towards
impoverishing the surface-soil; whereas the occasional growth of a
deep-rooted crop brings the plant-food in the subsoil into requisition.
In this connection it may be well to draw attention to the singular
capacity possessed by certain crops for absorbing nitrogen. Of these the
case of clover is the most striking, and has long puzzled
agriculturists. The discovery, which has been repeatedly referred to in
these pages, that the leguminous order of crops, to which clover
belongs, have the power of absorbing the free nitrogen of the air
through the agency of micro-organic life in the plant and in the soil,
has furnished an explanation of this long-debated problem.
_Period of Growth._
A further reason is the difference in the period of a crop's growth. A
crop which grows quickly, and consequently occupies the ground during a
comparatively short period, will naturally require a richer soil, and
therefore a more liberal treatment with manure, than one whose growth is
more gradual.
Another consideration is the season of the year during which active
growth of the crops takes place. For example, in the case of the wheat
crop, active growth takes place in spring and ceases early in the
summer. Since, however, nitrification goes on right through the summer,
and nitrates are most abundant in the soil in late summer and autumn,
such a crop as wheat is ill suited to obtain any benefit from this
bountiful provision of nature, and is consequently particularly
benefited by the application of nitrogenous manures. Root crops, on the
other hand, sown in summer, continue their active growth into autumn,
and are thus enabled to utilise the nitrates formed in the process of
nitrification. The custom of sowing a quickly growing green crop, such
as rye, mustard, rape, &c., after a wheat crop, is a practice which aims
at conserving the nitrates and preventing their loss by autumn and
winter rains. The name "catch crop" has been applied to such a crop. By
ploughing under the green crop, the nitrogen removed from the soil in
the form of easily soluble nitrates is restored in an insoluble organic
form, and the soil is at the same time enriched by the addition of much
valuable organic matter.[243]
It is chiefly the above facts that form the scientific basis of the
long-pursued practice of the rotation of crops.
_Variation in Composition of Crops._
A point of considerable interest is the influence exerted by manures on
the composition of crops. It has been assumed in the preceding pages
that the composition of crops of the same plant is uniform; but this is
not strictly the case, as it has been proved that not merely the manure
and soil have an appreciable influence on the crop's composition, but so
also has the climate.
_Absorption of Plant-food._
The laws regulating the absorption of plant-food are most interesting,
although, unfortunately, very imperfectly understood as yet. The
fertilising ingredients are capable of considerable movement in the
plant, and are only absorbed up to a certain period of growth. This in
many plants is reached when they flower. After this period they are no
longer capable of absorbing any more food. The popular belief that
plants in ripening exhaust the soil of its fertilising matters is
consequently a fallacy.
_Fertilising Ingredients lodge in the Seed._
The tendency of fertilising matters is to move upward in the plant as it
matures, and finally to become lodged in the seed. It is for this reason
that the cereals prove such an exhaustive crop. That nature, however,
can in certain cases be very economical of her food-supplies, is
strikingly illustrated by the fact that much of the fertilising matter
contained in the mature leaves in autumn passes back into the tree
before the leaves fall from it.
_Forms in which Nitrogen exists in Plants._
The form in which nitrogen is present in the plant is chiefly as
albuminoids. As, however, albuminoids belong to that class of bodies
known as colloids, which cannot easily pass through porous membranes
like those forming the walls of plant-cells, they are changed during
certain periods of the plant's growth into amides, which are
crystalloids, and consequently able to move freely about in the plant.
Amides are most abundant in young plants during the period of their most
active growth, and as the plant ripens the amides seem to be largely
converted into albuminoids.
While the subject is not very clearly understood, it would seem to be
pretty conclusively proved that there is a direct relation between the
amount of the phosphoric acid and of the nitrogen absorbed.
_Bearing of above Facts on Agricultural Practice._
The bearing of these facts upon practice is obvious. In the first place,
they show how important it is that plants should be well fed when they
are young, and that in the practice of green manuring it is best to
plough in the crop when it is in flower, as no additional benefit is
gained by allowing it to ripen, seeing that no further absorption of
fertilising ingredients takes place after the period of flowering.
_Influence of excessive Manuring of Crops._
The influence of large quantities of manures is seen in the case of
certain root crops. It is found, in such a case, that while the roots
are larger, they are more watery in composition and of less nutritive
value. Again, it seems to be a fact pretty generally known to practical
men, that nitrate of soda seems to have a bad effect on the quality of
hay. It would seem, further, that the influence of nitrogenous
fertilisers on cereals is to increase the percentage of nitrogen in the
grain, but that they have no such influence in the case of leguminous
crops. Phosphatic manures, on the other hand, in the case of leguminous
crops, seem to have the effect of diminishing the amount of nitrogen in
the seed.
FOOTNOTES:
[241] Though not necessarily at the same time or to each succeeding
crop. There may be comparatively long intervals between the applications
of farmyard manure in many cases.
[242] Of course what is meant here is the direct influence of such
manures. Their indirect value may be shown in the soil by the increased
crop residues they give rise to.
[243] This is very concisely and clearly put in Mr Warington's admirable
'Chemistry of the Farm.'