LIMING. Among the various constituents of soil essential to plant growth are some which are apt to be deficient, and where that is the case the supply must be increased to ensure successful cultivation. They are nitrogen, phosphoric acid, potash and lime. Lime is most commonly deficient in sandy soils and those which, owing to excess of organic matter, are "sour," or, in other words, acid. The use of lime for correcting acidity was known very early in the history of agriculture. Immense tracts of land in England were originally marsh, and when these were brought into cultivation it was no doubt soon discovered that only by the application of lime in some form could they be made continuously productive. On the wide stretches of sandy heathland lime also was needed but in this case marl would be even more effective. (See MARLING.) Limestone is a carbonate of lime and consists of carbonic acid chemically united with lime. Lime may be obtained by burning limestone in a kiln and raising sufficient heat to drive off the carbonic acid. The substance left is quick-lime, which is pure lime, or, in chemical terminology, oxide of calcium. If water is poured on quick-lime it is rapidly absorbed and heat is engendered, the resulting product being hydrated or "slaked" lime.
Lime acts on the soil both chemically and mechanically. When the carbonic acid and water are expelled by heat it becomes a strong alkaline earth and in this caustic state combines with the oxygen and carbonic acid set free by the decomposition of organic matter present in the soil. The decay of organic matter is thereby quickened. It also breaks up the elements of inorganic matter which have become dormant or insoluble and renders them active and thus available for plant food. It neutralizes injurious acids pre sent in the soil which render it sour, thereby making it sweet and mellow. Lime acts mechanically in loosening and rendering more
friable heavy clay land and giving cohesion and firmness to light sandy soils.
The wide variation in the amount of lime found naturally is shown in the following analyses of three different classes of soil— sandy, peaty and chalky.
Some agricultural writers in the 19th century warned farmers against over-liming but this is not a practice from which the land has suffered generally in recent years. On the contrary it is commonly asserted by competent observers that in many dis tricts the productivity of the land is much reduced by the need for lime. Suggestions have indeed been made in England that measures should be taken by the State to encourage, by some direct stimulus, a revival of the practice of liming. Land which needs liming requires it at intervals of ten to fifteen years.
(R. H. R.) United States.—In the United States two forms of liming material are in general use, calcium carbonate represented by ground limestone, ground oyster shells, or marl, and calcium oxide represented by burnt lime made from limestone or oyster shells. Burnt lime is supplied either in the form of lump lime (quick lime) in which case it is slaked by the farmer before using, or in the form of the hydrated lime used by builders. The latter form is usually of higher grade, and is in a finely divided dry form con venient for immediate use. In the use of ground limestone, a specification that all material shall pass a ten mesh sieve ( meshes per sq.in.) seems to be most generally acceptable. Chalk as found and used extensively for liming in England is not found in the United States in quantity sufficient to furnish any material for this purpose. (E. C. SH.)