THE SOIL CONTAINS THE FOOD OF PLANTS. 63 



tion — Conversion in practice of the chemiciilly fixed food into an available 

 form — Eft'ect of a manure depends upon the property of the soil — 

 Improper relative proportions of the dilierent elements of food in the 

 soil : effect of this upon the different cultivated plants : means for restor- 

 ing the proper relative proportions. 



FROM the soil plants receive the food necessary for 

 their developement ; hence an acquaintance with its 

 chemical and physical properties is important in helping 

 us to understand the nutritive processes of plants, and the 

 operations of agricultiu-e. As a matter of course, a soil 

 to be fertile for cultivated plants, must, as a primary 

 condition, contain in sufficient quantity the nutritive 

 substances required by those plants. But chemical 

 analysis which determines this relation gives but rarely 

 a correct standard by which to measure the fertility of 

 different soils, because the nutritive substances therein con- 

 tained, to be really available and effective, must have a 

 certain form and condition, which analysis reveals but 

 imperfectly. 



Eouo;h uncultivated ground, and soil formed from the 

 dust and diied mud of the highroads, are speedily 

 overgrown with weeds, and though often still unfit for 

 the cultivation of cereal and kitchen plants, may yet 

 prove not unfruitful for other plants, requiring, like 

 clover, sanfoin, and lucerne, a large amount of food, 

 and which are often seen thriving luxuriantly on the 

 slopes of railway embankments formed of eailh that has 

 never been under cultivation. A similar relation is 

 shown by the subsoil of many fields. In many of them 

 the earth from the deeper layers improves the surface 

 soil, and increases its fertility ; in others, the subsoil mixed 

 with the surface soil destroys the fertihty of the latter. 



It is a remarkable fact that rough uncultivated soil, 

 unsuited for cereal and kitclien plants, may by dihgent 

 cultivation during several years, and by the influence 



