ARABLE LAND. 



Silicious sand holds water in its in- 

 terstices by simple cohesive attrac- 

 tion in proportion to its fineness. It 

 lets water pass tlirouijh it rapidly, ei- 

 liier hy filtration or evaporation. Its 

 use in the soil is to keep it open, to 

 let the air and water, as well as those 

 other substances on which the growth 

 of plants depends, circulate through 

 it. Unmixed, it dries so rapidly that 

 no vegetation can continue in it, un- 

 less a constant supply of moisture be 

 given by irrigation. A small addition 

 of clay mucli improves light sands. 



Lime in its pure state is familiar 

 to every one as the basis of the mor- 

 tar used in building. It is produced 

 by burning marble, chalk, limestone, 

 or shells with a great heat. In the 

 stones which are fonned principally 

 oflime it is combined with some acid, 

 most generally carbonic acid, which 

 separates from it by the operation of 

 burning, in the form of an air or gas, 

 hence called Jixed air, from its being 

 thus fixed in a stone. These stones, 

 of various degrees of hardness, are 

 now all classed under the name of 

 carbonates oflime. 



Lime unites readily with water, 

 which it also absorbs from the atmo- 

 sphere. It then becomes slacked. By 

 uniting with carbonic acid, it returns 

 to Its former state of carbonate, with 

 this difference, that, unless much wa- 

 ter be present, it remains a fine im- 

 palpable powder. Pure lime is solu- 

 ble in water, though sparingly ; a pint 

 of water cannot dissolve more than 

 about twenty grains : the carbonate 

 is not soluble in water. Carbonate of 

 lime has a powerful effect on the fer- 

 tility of a soil, and no soil is very [iro- 

 ductive without it. It is consequently 

 used extensively as an improver of 

 the soil, otherwise called a manure ; 

 but its use in this respect, and the 

 mode in which it acts, will be given 

 in the articles M\nl-re and Li.me. 



Carbonate of lime, as an earth, is 

 neitlier so tenacious as clay nor so 

 loose as sand. In proportion to the 

 fineness of its particles, it approaches 

 the one or the other; when the parts 

 are large and hard, it takes the name 

 of Uoaestone or calcareous gravel. 



Its distinguishing feature is its sol. 

 ubility in acids, which it neutralizes, 

 depriving them of their noxious qual- 

 ities in the soil. A proper mixture 

 of these three earths, in a due state 

 of mechanical division, forms a soil 

 well fitted to the growth of every 

 species of plants, especially those 

 wliich are cultivated for food ; and 

 nothing more is required than a prop- 

 er climate as to heat, a proper degree 

 of moisture, and sufficient nourish- 

 ment, to make all the plants generally 

 cultivated thrive most luxuriantly in 

 such a mixture, which is usually call- 

 ed a loam. 



But there are some soils which, be- 

 sides a proper mechanical texture and 

 mi.xture of earths, contain a large pro- 

 portion of a natural manure, which 

 renders them extremely fertile. This 

 is a substance produced by the slow 

 decay of animal and vegetable mat- 

 ter. It can be separated from the 

 other parts of the soil, and has been 

 accurately analyzed and described by 

 many of the most experienced chem- 

 ists, particuhirly by Fourcroy, Davy, 

 Chaptal, and Theodore de Saussure. 

 (.See Rccherchcs Chtmiqucs stir la Ve- 

 getation, Paris, 1804, 8vo ) This 

 substance has been called vegetable 

 mould; but, as this is not a very dis- 

 tinct term, we shall, after Thacr and 

 other eminent writers on agriculture, 

 adopt the name of humus when speak- 

 ing of it. Humus is a dark, unctuous, 

 friable substance, nearly uniform in 

 its appearance. It is a compound of 

 oxygen, hydrogen, carbon, and nitro- 

 gen, which are the elements of all an- 

 imal and vegetable substances. It is 

 the result of the slow decomposition 

 of organic matter in the earth, and is 

 found in the greatest abundance in 

 rich garden mould, or old, neglected 

 dunghills. It varies somewhat in its 

 qualities and composition, according 

 to the sub.stances from which it has 

 been formed and the circumstances 

 attending their decay. 



Besides the four essential elements 

 in its composition, it also contains 

 other substances in smaller quanti- 

 ties, viz , phosphoric and sulphuric 

 acids combined with some base, and 



