This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
These are of a most important nature, and consist of lime in some form, such as quicklime, slaked lime, chalk, marl, gas lime, and lime shells. Lime is not only an essential plant food (see p. 108), but it plays an important part in the generation and activity of bacteria in the soil, and must be present to ensure fertility. Some soils are naturally of a calcareous nature, while others may be deficient. In a fertile soil it has been estimated that there are about 120,000 lb. of lime to the acre, but the quantity is very much less in others. In the Broadbalk Field at Rothamsted, which had not been manured for fifty years, as much as 62,250 lb. of lime is given for an acre of ground.
Since the advent of so many chemical manures the ancient practice of "liming" the soil has largely gone out of fashion. Market gardeners and farmers, however, appear to be again waking up to the importance of lime, not only as a cheap and excellent manure, but also as a powerful check to "clubroot" in Cabbage crops, to "eelworm" in Cucumbers and Tomatoes and other crops, and by keeping many other pests at bay. The old saying that "Lime and lime without manure Makes both farm and farmer poor ", is perfectly true, and it illustrates the wisdom of our forefathers. Owing to chemical actions set up in the soil by the presence of lime, organic matter like stable manure is rapidly converted into such a state that its nitrates, potash, phosphoric acid, etc, are soon liberated and absorbed. Consequently, unless manure is added regularly to the soil, it would soon be brought into an impoverished state by the continual application of lime alone.
In a state of nature lime does not occur in a free state. It is usually combined with carbonic acid, and in many parts is found in abundance as carbonate of lime - the commonest forms of which are limestone and chalk. Pure carbonate of lime is composed of 536 per cent of lime and 437 per cent of carbonic acid. When burnt in a kiln the carbonic acid gas is driven off into the atmosphere, and the residue - quicklime - is formed. This quicklime absorbs water greedily, and in coming in contact with it becomes "slaked". This is then called hydrate of lime, or, more properly, slaked lime. If left exposed to the air the slaked lime gradually loses its water, and absorbs carbonic acid gas instead. It thus becomes carbonate of lime once more.
In wet, heavy, clayey soils the application of "quicklime" or caustic lime to the surface is of the utmost benefit after the soil has been turned up with the spade, fork, or plough. The quicklime readily absorbs the surrounding moisture, generates great heat, and brings the soil into a drier and better condition for working. For heavy land there is nothing better than a good dressing of quicklime to bring it into a state of cultivation. From 30 to 200 bus. per acre is applied according to circumstances.
Chalk, or carbonate of lime, is also an excellent dressing for most farms and gardens that receive liberal dressings of manure. The latter is apt to generate acidity if the soil has not been deeply dug or trenched, and this acidity in turn is apt to produce clubbing of Cabbage crops, eelworm, and other plant diseases, owing to the lack of oxygen in the soil. Lime in any form helps to check this state of affairs.
Marl, which is a mixture of clay and chalk in varying proportions, is a useful adjunct to light or gravelly soils, because it makes the particles more tenacious, and this enables the soil to hold manures better. There are several kinds of marl, such as clay marl, sandy marl, chalk marl, slaty or stony marl, shell marl, and peaty marl - all containing a certain quantity of calcareous matter.
A SHOWER BOUQUET.
A BASKET OF FLOWERS. FLORAL ART.
Gas Lime obtained from the gasworks is often used for garden purposes. In a fresh state it contains many compounds fatal or poisonous to plant life; but in this state it is a valuable dressing for soil infested with clubroot (Plasmodiophora). It must not, however, be applied in a fresh state to land already carrying a crop. After exposure to the weather for about three months gas lime loses its poisonous properties and then becomes a very useful manure. In composition it may contain as much as 40 per cent of chalk (calcium carbonate) and 15 per cent of slaked lime, but the amount of these varies considerably. About 5 tons to the acre is a fair dressing.
Other lime manures are shells of various descriptions when ground and obtainable in sufficient quantity. They are valuable for their carbonate of lime and a certain amount of organic matter.
Since basic slag (see p. 157) has become prominent it is often used instead of lime, and an excellent substitute it is, as it contains large quantities of lime in a mild and useful form.
It has been found by experiment at Rothamsted that the application of sulphate of ammonia to the soil causes a loss of carbonate of lime (chalk), and growers would do well to bear this fact in mind. About 800 lb. of lime per annum is naturally dissipated from the top 9 in. of the soil by the action of the weather and cultivation, but the application of 400 lb. of ammonium salts raised the loss to 1045 lb. The loss of lime was still further increased to 1429 lb. per acre by the application of 400 lb. of ammonium salts and superphosphate. It is therefore a simple matter to rob a soil of lime simply by the careless or injudicious application of sulphate of ammonia, superphosphate, and other manures.