400 THE HOME, FARM AND BUSINESS CYCLOPAEDIA. 



well stocked with available plant food ; and so situated with reference to 

 subsoil and climate as to insure the realization of the above good qualities. 

 POROSITY. A fertile soil must be porous, i.e., the particles which com- 

 pose it must not be too near together, but allow room for an interstitial 

 atmosphere, the free percolation and retention of water, and for the con- 

 densation of valuable fertilising matters upon the interstitial surfaces. 



The porosity of soils is an exceedingly interesting subject. It occupied 

 the attention of Jethro Tull early in the last century, and it afforded a 

 fertile theme for investigation to the late Sir H. Thompson, of Kirby, and 

 subsequently to the late Baron Liebig, Professors Way and Voelcker, Sir 

 J. B. Lawes, and other chemists. The porosity of a soil may be measured 

 by the fineness of its particles. A coarse-grained sand, although more 

 open in its texture, is in reality less porous than a finely grained clay. It 

 has fewer pores. Every time we break a fragment of any substance we 

 increase the extent of its superficies, and this is practically true ad infini- 

 tum; so that an impalpable powder presents the largest possible surface, 

 and is in a condition of maximum porosity. Our most porous soils are 

 therefore our clays, a statement that is capable of satisfactory demonstra- 

 tion, although the use of the word in this connection would scarcely be 

 accepted by agriculturists, who speak of sands as porous, in opposition to 

 clays, which are spoken of as retentive soils. 



The porosity of soils explains some of their most interesting physical 

 functions. It is owing to this property that they are able to retain suffi- 

 cient moisture for the use of growing vegetables. It is also owing to the 

 same property, assisted by others, that soils are able to appropriate and 

 hold certain valuable fertilizing matters with sufficient strength to over- 

 come the tendency of the rainfall to wash them beyond the reach of 

 plants. That these important functions are possessed in the highest de- 

 gree by clay soils, is sufficient proof that these soils possess the highest 

 degree of porosity. 



CAPILLARITY. This property also depends upon porosity. It is observ- 

 able that when a fluid is admitted between very closely contiguous sur- 

 faces, such as two plates of glass held almost touching, and dipped into 

 water, the fluid will be seen to rise between the plates to a considerably 

 higher level than its own. Lump-sugar and blotting-paper dipped into 

 water are familiar examples of the same force ; and a lump of clay, if im- 

 mersed in a saucer of water, will become wet to its summit from the same 

 cause. The finer the interstitial spaces, the higher will the fluid ascend ; 

 and hence we find that a column of finely-powdered clay will become wet 

 thirty-six inches above the surface of the water into which its base is 



