THE CHEMISTRY OF GARDEN CROPS

A Consideration of the chemistry of the crops that engage attention in

this country will afford an explanation of one great difference between

farming and gardening. And this difference should be kept in mind by all

classes of cultivators as the basis of operations in tillage, cropping,

and the order and character of rotations. The first thing to discover in

the cropping of a farm is the kind of vegetation for which the land is

best adapted to insure, in a run of seasons, fairly profitable results.

If the soil is unfit for cereals, then it is sheer folly to sow any more

corn than may be needful for convenience, as, for example, to supply

straw for thatching and litter, and oats for horses, to save cost of

carriage, &c. On large farms that are far removed from markets it is

often necessary to risk a few crops that the land is ill fitted for, in

order to satisfy the requirements of the homestead, and to save the

outlay of money and the inconvenience of hauling from distant markets.

But everywhere the cropping must be adapted to the soil and the climate

as nearly as possible, both to simplify operations and enlarge to the

utmost the chances of success. In the cropping of a garden this plain

procedure cannot be followed. We are compelled certainly to consider

what the soil and climate will especially favour amongst garden crops,

but, notwithstanding this, the gardener must grow whatever the household

requires. He may have to grow Peas on a hot shallow sand; and Potatoes

and Carrots on a cold clay; and Asparagus on a shallow bed of pebbles

and potsherds. To the gardener the chemistry of crops is a matter of

great importance, because he cannot restrict his operations to such

crops as the land is particularly adapted for, but must endeavour to

make the land capable of carrying more or less of all the vegetables

and fruits that find a place in the catalogue of domestic wants. That he

must fail at certain points is inevitable; nevertheless his aim will be,

and must be, of a somewhat universal kind, and a clear idea of the

relations of plants to the soil in which they grow will be of constant

and incalculable value to him.



We are bound to say at the outset that a complete essay on the chemistry

of vegetation is not our purpose. We are anxious to convey some useful

information, and to kindle sufficient interest to induce those who have

hitherto given but slight attention to this question to inquire further,

with a view to get far beyond the point at which we shall have to quit

the subject.



Plants consist of two classes of constituents--the Inorganic, which may

be called the foundation; and the Organic, which may be considered the

superstructure. With the former of these we are principally concerned

here. A plant must derive from the soil certain proportions of silica,

lime, sulphur, phosphates, alkalies, and other mineral constituents, or

it cannot exist at all; but, given these, the manufacture of fibre,

starch, gum, sugar, and other organic products depends on the action of

light, heat, atmospheric air, and moisture, for the organic products

have to be created by chemical (or vital) action within the structure,

or, as we sometimes say, the tissues of the plant itself. To a very

great extent the agencies that conduce to the elaboration of organic

products are beyond our control (though not entirely so), whereas we can

directly, and to a considerable degree, provide the plant with the

minerals it more particularly requires; first, by choosing the ground

for it, and next by tilling and manuring in a suitable manner. A clay

soil, in which, in addition to the predominating alumina, there is a

fair proportion of lime, may be regarded as the most fertile for all

purposes; but we have few such in Britain, our clays being mostly of an

obdurate texture, retentive of moisture, and requiring much cultivation,

and containing, moreover, salts of iron in proportions and forms almost

poisonous to plants. But there are profound resources in most clays, so

that if it is difficult to tame them, it is also difficult to exhaust

them. Hence a clay that has been well cultivated through several

generations will generally produce a fair return for whatever crop may

be put upon it. Limestone soils are usually very porous and deficient of

clay, and therefore have no sustaining power. Many of our great tracts

of mountain limestone are mere sheep-walks, and would be comparatively

worthless except for the lime that may be obtained by burning. On the

other hand, chalk, which is a more recent form of carbonate of lime, is

often highly productive, more especially where, through long

cultivation, it has been much broken up, and has become loamy through

accumulation of humus. Between the oldest limestone and the latest chalk

there are many intermediate kinds of calcareous soils, and they are

mostly good, owing to their richness in phosphates, the products of the

marine organisms of which these rocks in great part, and in some cases

wholly, consist. For the growth of cereals these calcareous soils need a

certain proportion of silica, and where they have this we see some of

the finest crops of Wheat, Trifolium, Peas and Beans in these islands.

If we could mix some of our obdurate clays with our barren limestones,

the two comparatively worthless staples would probably prove remarkably

fertile. Although this is impossible, a consideration of the chemistry

of the imaginary mixture may be useful, more especially to the gardener,

who can in a small way accomplish many things that are impracticable on

a great scale. Sandy soils are characterised by excess of silica, and

deficiency of alumina, phosphates and potash. Here the mechanical

texture is as serious a matter as it is in the case of clay. The sand is

too loose as the clay is too pasty, and it may be that we have to

prevent the estate from being blown away. It is especially worthy of

observation, however, that sandy soils are the most readily amenable of

any to the operation of tillage. If we cannot take much out of them, we

can put any amount into them, and it is always necessary to calculate

where the process of enrichment is to stop. It is not less worthy of

observation that sandy soils can be rendered capable of producing almost

every kind of crop, save cereals and pulse, and even these can be

secured where there is some basis of peat or loam or clay with the sand.

The parks and gardens of Paris, Versailles, and Haarlem are on deep

sands that drift before the wind when left exposed for any length of

time with no crop upon them; and not only do we see the finest of

Potatoes and the most nutritious of herbage produced on these soils, but

good Cauliflowers, Peas, Beans, Onions, fruits, and big trees of sound

timber.



Garden soils usually consist of loam of some kind, the consequence of

long cultivation. Natural loams are the result of the decay and

admixture of various earths, and they are mostly of a mellow texture,

easily worked and highly productive. They are, as a rule, the best of

all soils, and their goodness is in part due to the fact that they

contain a little of everything, with no great predominance of any one

particular earth. Cultivation also produces loam. On a clay land we

find a top crust of clayey loam, and on a lime or chalk land a top crust

of calcareous loam. Where cultivation has been long pursued the staple

is broken and manures are put on, and the roots of plants assist in

disintegration and decomposition. Thus there is accumulation of humus

and a decomposition of the rock proceeding together, and a loam of some

sort is the result. Hence the necessity of caution in respect of deep

trenching, for if we bury the top soil and put in its place a crude

material that has not before seen daylight, we may lose ten years in

profitable cropping, because we must now begin to tame a savage soil

that we have been at great pains to bring up, to cover a stratum of a

good material prepared for us by the combined operations of Nature and

Art during, perhaps, several centuries. But deep and good garden soils

may be safely trenched and freely knocked about, because not only does

the process favour the deep rooting of the plants, but it favours also

that disintegration which is one of the causes of fertility. Every

pebble is capable of imparting to the soil a solution--infinitesimal,

perhaps, but not the less real--of silica, or lime, or potash, or

phosphates, or perhaps of all these; but it must be exposed to light and

air and moisture to enable it to part with a portion of its substance,

and thus it is that mechanical tillage is of the first importance in all

agricultural and horticultural operations.



The principal inorganic or mineral constituents of plants are potash,

soda, lime, iron, phosphorus, sulphur, chlorine, and silica. Clays and

loams are generally rich in potash, sulphur, and phosphates, but

deficient in soluble silica and lime. Limestone and chalk are usually

rich in lime and phosphates, but deficient in humus, silica, sulphur,

and alkalies. Sandy soils are rich in silica, but are generally poor in

respect of phosphates and alkalies. Therefore, on a clay or loam,

farmyard manure is invaluable, because it contains ingredients that all

crops appreciate, and also because it is helpful in breaking up the

texture of the soil. The occasional application of lime also is

important for its almost magical effect on garden soil that has been

liberally manured and heavily cropped for a long term of years.

Calcareous soils are greatly benefited by a free application to them of

manure from the stable and cow-byre; but as a rule it would be like

carrying coals to Newcastle to dress these soils with lime. Clay may be

put on with advantage; and nothing benefits a hot chalky soil more than

a good dose of mud from ponds and ditches, which supplies at once humus,

alumina, and silicates, and gives 'staple' to the soil, while

preventing it also from 'burning.' In the manuring of sandy soils great

care is requisite, because of their absorbing power. In the bulb-growing

districts of Holland, manure from cowsheds is worth an enormous price

for digging into loose sand for a crop of Potatoes, to be followed by

bulbs. Sandy soils are generally deficient in phosphates and alkalies;

hence it will on such soils be frequently found that kainit (a crude

form of potash) and superphosphate of lime will conjointly produce the

best results, more especially in raising Potatoes, Onions, and Carrots,

which are particularly well adapted for sandy soils. Probably one of the

best fertilisers is genuine farmyard manure from stall-fed cattle, for

it contains phosphates, alkalies, and silicates in available forms. For

similar reasons Peruvian Guano is often useful on such soils. Artificial

manure should be selected with a view to correct the deficiencies of the

soil, and to satisfy the requirements of the crops to be grown on it.



While we have thus dealt principally with the Inorganic or mineral

constituents of plants, and the way in which the deficiencies of the

soil in respect of any of them may be supplied by artificial

applications, we must not ignore the other class of constituents, the

Organic. These are supplied almost entirely from the atmosphere itself,

though, to a limited extent, the presence in the soil of humus or

vegetable matter contributes also. Yet this latter, as seen in the case

of land heavily dressed with farmyard or stable manure, vegetable

refuse, &c, exercises important functions in other directions. Not only

are mineral constituents, in forms available for assimilation, supplied,

but soils so treated derive peculiar advantages as regards their

mechanical state and improved physical conditions, chiefly in respect of

retention of moisture, warmth, &c. Thus, sandy soils, which are very

apt, through poverty in humus, to lose their moisture readily and to

'burn,' are rendered more retentive of moisture and fertilising

constituents by the use of farmyard manure, &c., and have more 'staple'

or substance given to them, while heavy, tenacious clays are opened out,

lightened, and rendered more amenable to the influences of drainage,

aeration, &c., and so become less cold and inactive.



For the present purpose the principal garden crops may be grouped in two

classes, in accordance with their main characteristics and the

predominance of certain of their mineral elements. The figures given on

the following page show the average percentage proportions of the

several minerals in the ashes of the different plants.



In Class I. Phosphates and Potash predominate. This class consists of

the less succulent plants, and includes the following: The Pea:

containing, in 100 parts of the ashes, phosphates, thirty-six; potash,

forty. Bean: phosphates, thirty; potash, forty-four. Potato (tubers

only): phosphates, nineteen; potash, fifty-nine; soda, two; lime, two;

sulphuric acid, six. Parsnip: phosphates, eighteen; potash, thirty-six;

lime, eleven; salt, five. Carrot: phosphates, twelve; potash,

thirty-six; soda, thirteen; sulphuric acid, six. Jerusalem Artichoke:

phosphates, sixteen; potash, sixty-five.



In Class II. Sulphur, Lime and Soda Salts are predominant. This class

consists of the more succulent plants, and includes the following:

Cabbage: containing, in 100 parts of the ashes, phosphates, sixteen;

potash, forty-eight; soda, four; lime, fifteen; sulphuric acid, eight.

Turnip: phosphates, thirteen; potash, thirty-nine; soda, five; lime,

ten; sulphuric acid, fourteen. Beet: phosphates, fourteen; potash,

forty-nine; soda, nineteen; lime, six; sulphuric acid, five.



As a matter of course, Lentils and other kinds of pulse agree more or

less with Peas and Beans in the predominance of phosphates and potash.

So, again, all the Brassicas, whether Kales, Cauliflower, or whatever

else, agree nearly with the Cabbage in the prominent presence of lime

and sulphur; ingredients which fully account for the offensive odour of

these vegetables when in a state of decay. Fruits as a rule are highly

charged with alkalies, and are rarely deficient in phosphates; moreover,

stone-fruits require lime, for they have to make bone as well as flesh

when they produce a crop. As regards the alkalies, plants appear capable

of substituting soda for potash under some circumstances, but it would

not be prudent for the cultivator to assume that the cheaper alkali

might take the place of the more costly one as a mineral agent, for

Nature is stern and constant in her ways, and it can hardly be supposed

that a plant in which potash normally predominates can attain to

perfection in a soil deficient in potash, however well supplied it may

be with soda. The cheaper alkali in combination as salt (chloride of

sodium) may, however, be usually employed in aid of quick-growing green

crops; and more or less with tap-roots and Brassicas. Salt, too, is very

useful in a dry season by reason of its power of attracting and

retaining moisture. As regards Potatoes, it is worthy of observation

that they contain but a trace of silica, and yet they generally thrive

on sand, and in many instances crops grown on sand are free from disease

and of high quality, although the weight may not be great. The

mechanical texture of the soil has much to do with this; and when that

is aided by a supply of potash and phosphates, whether from farmyard

manure or artificials, sandy soils become highly productive of Potatoes

of the very finest quality. On the other hand, Potatoes also grow well

on limestone and chalk, and yet there is but little lime in them. Here,

again, mechanical texture explains the case in part, and it is further

explained by the sufficiency of potash and phosphates, as also of

magnesia, which enters in a special manner into the mineral constitution

of this root.



Thus far we have not even mentioned nitrogen, or its common form of

salts of ammonia; nor have we mentioned carbon, or its very familiar

form of carbonic acid. These are important elements of plant growth; and

they account for the efficacy of manures derived directly from the

animal kingdom, as, for example, the droppings of animals, including

guano, which consisted originally of the droppings of sea-birds. Some of

the nitrogen in these substances, however, is of an evanescent

character, and rapidly flies away in the form of carbonate of ammonia;

hence, a heap of farmyard manure, left for several years, loses much of

its value as manure, and guano should be kept in bulk as long as

possible, and protected from the atmosphere, or its ammonia will largely

disappear. One difficulty experienced by chemists and others in

preparing artificial manures is that of 'fixing' the needful ammonia, so

that it may be kept from being dissipated in the atmosphere, and at the

same time be always in a state in which it can be appropriated by the

plant. In all good manures, however, there is a certain proportion of it

in combination, and in many instances the percentage of nitrogen is made

the test of the value of a manure.



The importance of humus--the black earthy substance resulting from the

decay of vegetation--in a soil is that it contains in an assimilable

form many of the ingredients essential to plant life. Humus when it

decomposes gives off carbonic acid, which breaks up the mineral

substances in the soil and renders them available as plant food. When

vegetable refuse is burned, the nitrogen--one of the costliest

constituents--is dissipated and lost. But by burying the refuse the soil

gets back a proportion of the organic nitrogen it surrendered and

something over in the way of soluble phosphatic and potassic salts; and

as this organic nitrogen assumes ultimately the form of nitric acid, it

can be assimilated by the growing plant, to the great benefit of

whatever crop may occupy the ground.



The practical conclusion is, that in the treatment of the soil a skilful

gardener will endeavour to promote its fertility by affording the

natural influences of rain, frost and sun full opportunity of liberating

the constituents that are locked up in the staple; by restoring in the

form of refuse as much as possible of what the soil has parted with in

vegetation; and by the addition of such fertilising agents as are

adapted to rectify the natural deficiencies of the soil. Thus, instead

of following a process of exhaustion, the resources of the garden may be

annually augmented.