CHAPTER IX._Elements of Agricultural Chemistry

COMPOSITION AND PROPERTIES OF VEGETABLE MANURES.

Many vegetable substances have been employed as manures, either alone or

as auxiliaries to farm-yard manure. Like that substance, they are

general manures, and contain all the constituents of ordinary crops;

but, owing to the absence of animal matter, they in general undergo

decomposition and fermentation much more slowly, although some of them

contain a so largely preponderating proportion of nitrogen, that they

may in some respects be compared to the strictly nitrogenous manures.

_Rape-dust, Mustard, Cotton and Castor Cake._--Rape-dust has long been

employed as a manure, and the success which has attended its use has led

to the introduction of the refuse cake from some other oil seeds, such

as those of mustard and castor-oil, which cannot be employed for

feeding. Like the seeds of all plants, these substances are rich in

nitrogen, and their ash, containing of course all the constituents of

the plant, supplies the necessary inorganic elements. The following are

analyses of these substances, which, in addition to the amount of

nitrogen and phosphates, shew also that of water and oil, to which

reference will be made in a future chapter, in relation to the feeding

value of some of them. The detailed composition of their ash may be

judged of from that of the seeds from which they are made, and which

have been given under that head.

+--------------------+------------+-------------+-------------+---------+

| +------------+-------------+-------------+---------+

| Water | 10.68 | 11.63 | 11.19 | 12.31 |

| Oil | 11.10 | 5.95 | 9.08 | 24.32 |

| Albuminious } | 29.53 | 31.16 | 25.16 | 21.91 |

| compounds } | | | | |

| Ash | 7.79 | 12.98 | 5.64 | 6.08 |

| Other constituents | 40.90 | 38.18 | 48.93 | 35.38 |

| +------------+-------------+-------------+---------+

| | 100.00 | 100.00 | 100.00 | 100.00 |

| Nitrogen | 4.38 | 4.94 | 3.95 | 3.20 |

| Silica | 1.18 | 3.36 | 1.32 | 1.96 |

| Phosphates | 3.87 | 69.3 | 2.19 | 2.81 |

| Phosphoric acid } | | | | |

| in combination } | 0.39 | 3.27 | 0.15 | 0.64 |

| with alkalies } | | | | |

+--------------------+------------+-------------+-------------+---------+

A general similarity may be observed in the composition of all these

substances; they are rich in nitrogen, and contain as much of that

element as is found in six or seven times their weight of farm-yard

manure, and a somewhat similar proportion exists in the amount of

phosphates, and probably of their other constituents. They have all been

employed with success, but the most accurate observations have been made

with rape-dust, which has been longer and more extensively used than any

of the others. It has been employed alone for turnips, or mixed with

farm-yard manure, and also as a top-dressing to cereals. But the most

marked advantage is derived from it when applied in the latter way on

land which has been much exhausted, and its effects are then very

striking. An adequate supply of moisture is essential to the production

of its full effects, and hence it often proves a failure in very dry

seasons, and on dry soils. It must not be applied in too great

abundance, experience having shewn that after a certain point has been

reached, an increase in the quantity produces no benefit, and even

sometimes positively diminishes the crop. The other substances of the

same class, in all probability, act in the same way, but as their

introduction is recent, and their use limited, less is known regarding

their effects.

_Malt-Dust, Bran, Chaff, etc._--The value of these substances as manures

is chiefly dependent on the nitrogen they contain, though to some extent

also on their inorganic constituents. Malt-dust contains about 4.5 per

cent, and bran 3.2 per cent of nitrogen. But they are little used as

manures, as they can generally be more advantageously employed for

feeding. The value of chaff more nearly resembles that of straw.

_Straw_ is occasionally employed as a manure, and sometimes even as a

top-dressing for grass land. It is generally admitted, however, that its

application in the dry state, and especially as a top-dressing, is a

practice not to be recommended, as it decomposes too slowly in the soil;

and it is always desirable to ferment it in the manure heap, so as to

facilitate the production of ammonia from its nitrogen. Still

circumstances may occur in which it becomes necessary to employ it in

the dry state, and it will generally prove most valuable on heavy soils,

which it serves to keep open, and so promotes the access of air, and

enables it to act on the soil. On light sandy soils it generally proves

less advantageous, as its tendency of course is to increase the openness

of the soil, and render it less able to retain the essential

constituents of the plant.

The quantity of nitrogen in straw does not exceed 0.2 per cent, and its

value is mainly due to its inorganic constituents and to its mechanical

effect on the soil.

_Saw-dust_ has little value as a manure, as it undergoes decomposition

with extreme slowness. It is a good _mechanical_ addition to heavy

soils, and diminishes their tenacity; and though its manurial effects

are small, it sooner or later undergoes decomposition, and yields what

valuable matters it contains. The saw-dust of hard wood is to be

preferred, both because it contains more valuable matters than that of

soft wood, and because the absence of resinous matters permits its more

rapid decomposition. It is a useful absorbent of liquid manure, and may

be advantageously added to the dung-heap for that purpose.

_Manuring with Fresh Vegetable Matter--Green Manuring._--The term green

manuring is applied to the system of sowing some rapidly growing plant,

and ploughing it in when it has attained a certain size, and the success

attending it, especially on soils poor in organic matters, is very

marked. It is obvious that this mode of manuring can add nothing to the

mineral matters contained in the soil, and its utility must therefore be

due to the plant gathering organic matters from the air, which, by their

decomposition, yield nitrogen and carbonic acid--the former to be

directly made use of by subsequent crops, the latter, in all

probability, acting also on the soil, and setting free its useful

constituents. Hence those plants which obtain the largest quantity of

their organic elements from the air ought to be most advantageous for

green manuring. The plants used for this purpose act also as a means of

bringing up from the lower parts of the soil the valuable matters which

exist in it out of reach of ordinary crops, and mixing them again with

the surface part. Many of the plants found most useful for green

manuring send down their roots to a considerable depth; and when they

are ploughed in, all the substances which they have brought up are of

course deposited in the upper few inches of the soil. Vegetable matter

when ploughed in in the fresh state, also decomposes rapidly, and is

therefore able immediately to improve the subsequent crop; and as this

decomposition takes place in the soil without the loss of ammonia and

other valuable matters, which is liable to occur to a greater or less

extent when they are fermented on the dung-heap, it will be obvious that

in no other mode can equally good results be obtained by its use.

Many plants have been employed as green manure, and different opinions

have been expressed as to their relative values. In the selection of any

one for the purpose, that should of course be taken which grows most

rapidly, and produces within a given time the largest quantity of

valuable matters, but no general rule can be given for the selection, as

the plant which fulfils those conditions best will differ in different

soils and climates. The plants most commonly employed in this country

are spurry, white mustard, and turnips. Rye, clover, buckwheat, white

lupins, rape, borage, and some others, have been largely employed

abroad. Some of these are obviously unfitted for the climate of the

British Islands; and the others, although they have been tried

occasionally, do not appear to have been very extensively employed. The

turnip is sown broadcast at the end of harvest, and ploughed in after

two months. White mustard and spurry are employed in the same way as a

preparation for winter wheat, and with the best results. The latter is

sometimes sown as a spring crop in March, ploughed in in May, and

another crop sown which is ploughed in in June, and immediately

followed by a third. The effect of this treatment is such that the worst

sands may be made to bear a remunerative crop of rye.

It is not easy to estimate the addition made by green manuring to the

valuable matters contained in the soil, but it is probably far from

inconsiderable. A crop of turnips, cultivated on the ordinary

agricultural system, after two months' growth, weighs between five and

seven tons per acre, and contains nitrogen equivalent to about 48 lbs.

of ammonia, and half a ton of organic matters; but nothing is known as

to the quantity produced when it is sown broadcast, and is not thinned,

although it must materially exceed this. Neither is it possible to

determine the relative proportions derived from the soil and the air,

although it is, in all probability, dependent on the resources of the

soil itself,--plants grown on a rich soil obtaining their chief supplies

from it, while, on poorer soils, a larger proportion is drawn from the

atmosphere. Hence light and sandy soils are most benefited by green

manuring, partly on this account, and partly also, no doubt, because the

valuable inorganic matters, which are so liable to be washed out of

these soils, are accumulated by the plants and retained in them in a

state in which they are readily available for the subsequent crop.

_Sea-Weed._--Sea-weeds have been employed from time immemorial as a

manure on the coasts of Scotland and England, in quantities varying from

10 to 20 tons per acre. Their action is necessarily similar to that of

green manure ploughed in, as they contain all the ordinary constituents

of land plants.

The subjoined analyses of three of the most abundant species will

sufficiently indicate their general composition.

+----------------+--------+----------+----------------------------+-----------+

| |Fucus | Fucus +----------------+-----------+in the |

+----------------+--------+----------+----------------+-----------+-----------+

|Water | 74.31 | 70.57 | 88.69 | 77.31 | 80.44 |

| compounds | 1.76 | 2.01 | 0.93 | 3.32 | 2.85 |

|Fibre, etc. | 19.04 | 22.05 | 4.92 | 10.39 | 6.40 |

|Ash | 4.89 | 5.37 | 5.46 | 8.98 | 10.31 |

| +--------+----------+----------------+-----------+-----------+

| |100.00 | 100.00 | 100.00 | 100.00 | 100.00 |

|Nitrogen | 0.28 | 0.32 | 0.15 | 0.53 | 0.45 |

| | | | | | |

|Peroxide of iron| 0.25 | 0.35 | 0.20 0.50 | 0.45 | 2.35 |

|Lime | 9.60 | 8.92 | 7.21 7.29 | 4.62 | 18.15 |

|Magnesia | 6.65 | 5.83 | 2.73 5.91 | 10.94 | 6.48 |

|Potash | 20.03 | 20.75 | 5.55 11.91 | 12.16 | 12.77 |

| potassium | ... | ... | 58.42 26.59 | 25.83 | 9.10 |

| potassium | 0.44 | 0.23 | 1.51 2.09 | 1.22 | 1.68 |

|Soda | 4.58 | 6.09 | ... ... | ... | ... |

| of sodium[M]| 3.66 | ... | ... ... | ... | ... |

| of sodium | 24.33 | 24.81 | 15.29 30.77 | 19.34 | 22.08 |

|Phosphoric acid | 1.71 | 2.14 | 2.42 2.66 | 1.75 | 4.59 |

|Sulphuric acid | 21.97 | 28.01 | 2.23 8.80 | 7.26 | 6.22 |

|Carbonic acid | 6.39 | 2.20 | 4.11 2.49 | 15.23 | 13.58 |

|Silicic acid | 0.38 | 0.67 | 0.33 0.99 | 1.20 | 3.00 |

| | ----- | ------ | ------ ------ | ------ | ------ |

| |100.00 | 100.00 | 100.00 100.00 | 100.00 | 100.00 |

+----------------+--------+----------+----------------+-----------+-----------+

The first four analyses give the composition of the weeds after they

have been separated from all foreign substances; the last, that of the

mixture taken from the heap just as it is used in Orkney; and its value

is then enhanced by small shells and marine animals adhering to the

plants, which increase the amount of phosphoric acid and nitrogen.

The ease with which all sea-weeds pass into a state of putrefaction,

adapts them in a peculiar manner to the manurial requirements of a cold

and damp climate. The rapidity of their decomposition is such, that when

spread on the land they are seen to soften and disappear in a short

time. They form therefore a rapid manure, and their effects are said to

be confined to the crop to which they are applied; but this is probably

due to the fact, that they are chiefly used in inferior sandy soils, in

which any manure is rapidly exhausted. In good soils there is no reason

why their effect should not be as lasting as that of farm-yard manure,

which, in many particulars, they considerably resemble. The method of

applying sea-weeds most generally in use, is to spread them on the soil,

and plough them in after putrefaction has commenced, and it is on the

whole the most advantageous. But they are sometimes composted with lime

and earth, or mixed with farm-yard manure, and occasionally, also, they

are used as a top-dressing to grass land.

On some parts of the western coast of Scotland and in the Hebrides,

sea-weed is the chief manure. It gives excellent crops of potatoes, but

they are said to be of inferior quality, unless marl or shell-sand is

employed at the same time.

_Leaves_ may be used as a manure, simply by ploughing them in, by

composting them with lime, or by adding them to the manure heap.

_Peat._--As a source of organic matter, peat may be used with advantage,

especially on soils in which it is naturally deficient. Dry peat of good

quality contains about one per cent of nitrogen, and a quantity of ash

varying from five to twenty per cent. These substances, however, become

available very slowly, owing to the tardy decay of peat in its natural

state; and in order to make it useful, it is necessary to compost it

with lime, or to mix it with farm-yard manure, or some readily

putrescible substance, so that its decomposition may be accelerated. It

may be most advantageously used as an absorbent of liquid manure, and on

this account, forms a useful addition to the manure heap.

The observations which have been made regarding the use of these

substances, lead directly to the inference that all vegetable matters

possess a certain manurial value, and that they ought to be carefully

collected and preserved. In fact, the careful farmer adds everything of

the sort to his manure heap, where, by undergoing fermentation along

with the manure, their nitrogen becomes immediately available to the

plant; while the seeds of weeds are destroyed during the fermentation,

and the risk of the land being rendered dirty by their springing up when

the manure comes to be used is prevented.

FOOTNOTES:

[Footnote M: The presence of sulphuret of sodium in this case is due to

the difficulty of completely burning the ash. It exists in the plant as

sulphate of soda.]