THE

GARDENER'S

MAGAZINE,

SEPTEMBER, 1840.

ORIGINAL COMMUNICATIONS.

ART. I. Remarks and Observations suggested by the Reading of Dr. Lindley's "Theory of Horticulture." By R. LYMBURN.

THE late work of Professor Lindley, on the Theory of Horticulture, is undoubtedly the greatest help that science has hitherto furnished to practice; the scientific part is so clear and concise, so devoid of ambiguity, and so full of demonstration, that no practical man who wishes to excel in his business should be without it. Practical men have, however, a still further duty to perform; they should bring the scientific doctrines to the test of experience. They have many opportunities of detecting circumstances which do not come under the observation of professors. Every new appearance which occurs to them should be diligently weighed and examined; and if any new truth is elicited, or any corroboration or refutation of theoretical principles discovered, they should promulgate their observations for the benefit of themselves and others. Impressed with these ideas, I have ventured to send you the fruits of my observations on the different sections of the practical part of the above work; and hope the example will be followed by others more able to do justice to the subject.

On the first section, on Bottom Heat, I would remark that the colour of the soil has an effect on bottom heat; the black colour absorbs heat most readily, but it parts with it again most readily, and the brown colour is more uniform in its temperature. A great source of bottom heat is the composition of the soil. Silex, being a bad conductor, retains heat, and soils which abound in siliceous sand are much warmer than those which contain sand in very small proportion. In sandy soils, though within a short distance of clayey, vegetation commences some weeks earlier, and the various plants, according to their times of starting into growth, will be generally a month earlier; but very sandy soils part with moisture, and the vegetable food, too freely by evaporation, unless in very wet seasons. The most beneficial source of bottom heat, in the open ground, is the pulverisation of the soil. All soils have a natural tendency to consolidate ; the action 1840. SEPT.

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of the rain, and the heats and colds of the atmosphere, have a tendency to reduce the inequalities in the soil, and bind it into a solid mass, which is cold and unproductive when compared with soil which has been properly pulverised. If intended for deep-rooted crops, the soil should be broken up to a considerable depth in dry weather, in order that the particles into which the soil is divided, being thoroughly dried, may preserve its open texture as long as possible; confined air is thus generated between the particles of soil, which is the best retainer of heat, as we have all every day experience from the comfortable warmth afforded by our clothes. This confined air retains the heat imparted by the sun through the day, which accumulates and becomes very sensible in all free open soils; and keeps up a higher temperature in the night, thus showing the necessity of bottom heat to all plants wherever grown, it being natural to all good well-pulverised soils. The free admission of atmospheric air is also necessary in the decomposition of the food of plants; by parting with its oxygen, it helps to fix and retain the carbon in the state of carbonic acid, and in the act of separation always gives out heat; and it is also the great source of nitrogen, which chemists have of late more frequently discovered in plants, and which, Dr. Lindley says, is indispensable to the healthy condition of the young spongioles of the roots. To keep the soil open, it must be well drained where necessary, and must be stirred in spring when in a dry state: if broken dry into small pieces, and well dried before heavy rains occur, it keeps open till again flooded with the heavy rains of next winter; the small interstices between the particles are filled with confined air, which retain heat, and moisture is retained by the capillary attraction which water has to the sides of narrow apertures; hence the smaller these particles can be made, if worked dry and kept asunder, the more heat and moisture is retained near the surface. When soil has been some time exposed to the air by digging and turning up in the spring, if the weather is dry and frosty, the freezing of the water contained in the soil causes it to expand; and, if the ground is strong and clayey, it is beneficial in reducing coarse and lumpy soils to a finer texture on the surface: if done in summer, in dry weather, the heat, if considerable, expands the water, and the same reduction of texture takes place, though more slowly, as the expansive power of freezing is generally greater; by degrees, however, the particles, by attrition and the action of the atmosphere, are reduced into small powder, which is washed down by the heavy rains of winter; and the pressure of the soil above consolidates the earth into the same solid mass as we had the year before, requiring to be again opened up in the spring. When ground has thus been operated upon for a succession of years, it will not keep open so well, has a constant tendency to

run to powder on the surface, and get covered with moss; this is what is technically called worn out: the particles will not keep properly asunder, and the ground requires to be trenched, the old surface buried, and a fresh surface brought up; or, where this cannot be got done, where the bottom is bad, to be sown down in grass. In preparing the ground therefore for seeds, if small, we must break the soil into as small particles as possible, and do so in dry weather; if heavy rains occur before the particles are dry, the surface is diluted into a paste, which with the next drought consolidates into a crust, impervious to heat, and if the surface is left rough and lumpy, the heat and moisture are not retained at the surface, which is prejudicial to small seeds that cannot be sown deep; for larger seeds, or for rooted crops, the particles may be left larger and wider apart, as the heat and air are thus allowed to penetrate deeper, and the surface is not so easily skinned. The same remarks will apply to the soil in pots or houses: it must be kept open to retain heat, and if the temperature is properly lowered in the house in the evening, we will have all the advantage of a bottom heat superior to the colder atmosphere, and thus imitate the natural tropical climate. We also see the benefit of keeping the soil open about the roots of large trees the roots get the benefit of the heat and air, and will not incline to rise so near to the surface; but if the earth is kept solid, they will naturally incline upwards. Stirring the soil in fruit-tree borders should be beneficial, if cautiously done; if the smaller roots only are cut, it is like pruning the young wood of a tree, it stimulates to the greater production of fibres; if large roots are taken away, it is like pruning large arms of the tree, hurtful.

The soil may also be kept open by other means. Manures, and all other substances that decompose in the ground, leave interstices by their becoming of less bulk; and this is one of the great benefits to be derived from manures. The opening of the soil may be carried too far; if the manure is very strong, and not sufficiently decomposed, and the weather dry, it may have bad effects; but if so far decomposed as that it will divide like turf, it is a source of great benefit in keeping the soil open, and retaining moisture, independent of the food it contains: this keeping of the soil open is the great benefit derived from mixing turf in composts, and from mixing pieces of moss with the soil in the act of potting, which is similar; also in mixing peat, old thatch, &c., in composts. So great are the benefits to be derived from decomposing turf mixed with the soil, that I recollect in the case of a piece of old Scotch fir plantation, which surrounded one of our nursery fields, being taken down, and the ground added to the nursery; the turf was trenched down, and so great was the effects, that for some years the ends and corners of the plats

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which were added to the old soil were ready to pull a year sooner, in half the time of the rest of the ground; this could not arise from fallen leaves, as the belt was mostly all of Scotch firs; and so perceptible was the difference between the old and new ground, that we could tell the site of the plantation for several years by its effects on all the nursery plants that were grown on it. Most potted plants will grow in very different composts, but require different treatment: in light dry soils they must be more frequently watered, and kept more cool; when more loam is in the compost, they need less attention, and are more hardy, but grow more slowly. For the other great benefits derived from bottom heat, the necessity of suiting the temperature of the soil to that of the atmosphere, the disadvantages of neglecting this in the growing of grapes, the great advantages of keeping the roots warm in forcing, &c. &c., recourse must be had to the work itself.

On the analogous sections, of Temperature, and Protection from Frost, the great effort should be to retain, if possible, the heat which was accumulated near the plants through the day. If water be near, it has a tendency to assume the state of vapour, and rob the air of its heat; the sap of the plant may be more abundant also, from this cause, and increase the expansion of the fluids by frost, which may end in the bursting and laceration of the vessels, and be the cause of death. When a clear cold

night succeeds to a wet day, if the night is long, and the atmosphere does not get cloudy, the heat radiates upwards from the earth and plants into the cold air; while the evening at first is comparatively warm. The cold is also greatly accelerated by the evaporation of moisture: it is calculated that it takes above 800° of heat to convert water into steam; and though vapour does not require so much, part of the vapour being chemically attracted by the atmosphere, still the consumption is great. From these causes the earth and plants by degrees get so cold, from having parted with their heat, that their temperature descends below the freezing point. In spring and autumn the air is comparatively warm, and the nights not so long; and hence spring and autumn frosts seldom take place till near sunrise: and if a cloud happens to settle above any portion of the earth about that time, before the earth has been cooled down to the freezing point, it prevents the further radiation of the heat upwards; and hence we often find places lying contiguous and below the cloud to be saved from frost at one time, while at another they will be much hurt. Where plants partially cover one another, they help to prevent radiation; and when one plant is more covered with moisture than another, or growing more vigorously, more full of watery sap, and the bark more tender, from these and other causes one plant is often, to all

appearance, unaccountably killed, while another is left unhurt. In order to protect plants from frost, we should study to have the plants themselves, and the earth around, as dry as possible towards the evening. The situation for plants liable to be hurt by spring and autumn frosts should be as much elevated as possible, in order to have the benefit of the wind in dispersing the cold heavy air and bringing forward the warmer; in low situations, the cold air, being heavier, collects, and not being benefited by the dispersion of the wind and bringing forward of warmer air, plants are much more liable to be hurt by slight frosts in such situations. Wherever possible, when the clearness and coldness of the air indicate a tendency to frost, plants that are worth the expense should be covered with the best nonconducting substance we can fall in with. Metals are the worst; if polished and bright in the colour however they are better nonconductors than when dark-coloured and rough; wood is better; but, unless when saturated with moisture, woollen is the best of any, from the confined air retained between the hairs of the wool. Whatever covering is used, whether straw mats, bast mats, cloth, or wood, they should be elevated above the surface to be covered, so as to contain as much confined air as possible: confined air is one of the worst conductors of heat; the covering will not radiate, or give out heat, till the confined air and covering are both heated above the state of the atmosphere; and the transmission of heat will take place more slowly through the confined air than any thing else: thus, for very little trouble, by elevating our coverings, we surround our plants or plant structures with a substance which is very retentive of heat, and increases the power of the covering in an immense degree. The heat has most tendency to ascend upwards, and this should be most guarded against; but it will also escape by the sides, and to confine the air and heat completely, the plant or plant-structure must be covered all round from the external air. Wall trees should have a broad coping of wood on the wall, to prevent the ascent of heat; and woollen nets drawn before tender peaches, &c., in cold nights, and carefully removed in good weather through the day, are a great help, when not left on in all weathers. The wall is best built of porous materials, as bricks, which retain the heat from the confined air better than stone; and they should be built with hollow chambers, as advised by Mr. Loudon, for the same purpose. Where painting is needed, white is the best colour. To prevent the bad effects of cold east winds in the spring, causing the sap to descend in standard fruit trees, and destroying the blossom when expanded, by the check it gives to the ascent of the sap that should nourish it, the stems and branches should be bound with straw ropes, and the ground mulched. Various situations should be chosen, to protect tender shrubs and trees,