MANURES. 179 



as are the leaves in sustaining the plant, it is of little importance 

 in this connection, to inquire into the mode in which they 

 receive and impart nutriment, as the amount cannot be affected 

 by artificial means. The other medium through which the 

 plant receives its support, is the root fibres ; and this is of by 

 far the greatest practical importance, as this support may be 

 increased almost indefinitely by supplying to the soil substances 

 necessary for vegetation, in which it is deficient. 



Soils are composed of clay, sand, lime, mineral substances, 

 and the mould of decayed or decaying vegetation. The clay 

 and sand, excepting in locations where lime abounds, compose 

 by far the largest portion in weight ; the mineral substances 

 are in minute quantities, and the mould, although diffused 

 throughout the whole soil and apparently a considerable portion 

 of it, consists of only from 3.100 to 5.100 in weight of the 

 whole, in average soils. All these substances perform an 

 important part in the support and nourishment of the plant, 

 and for its successful growth all are requisite in proper combi- 

 nations. Yet the mould and mineral substances comprising 

 together not more than 5.100 to 6.100 of the soil, are substan- 

 tially all that afford direct nourishment to the plant. The clay 

 and sand, although by chemical action forming an ingredient of 

 some plants, are principally and almost entirely useful for the 

 purpose of giving mechanical support to the plant, and retain- 

 ing and imparting nutriment for its use. They constitute the 

 foundation to hold the plant, and the repository of its food, and 

 as such they are of the greatest consequence. The fertility and 

 permanent excellence of a soil depend very much upon the 

 proportions and combinations in which these earths are found. 

 Every farmer understands the difference between clay or heavy 

 soils, and sandy or light soils ; and that the best soils are those 

 which are composed of both in such proportions as not to be 

 too stiff and hard with an excess of clay on the one hand, or 

 too dry and porous with an excess of sand on the other hand. 

 In proper combinations the clay in the soil acts to retain the 

 moisture and the. nutriment, and prevent too great an extension 

 of the roots ; and the sand acts to loosen, render more friable 

 the soil, and to admit into it the heat and the air in proper 

 quantities. 



If the clay is in excess, its injurious effects are : — ■ 



