30 CHEMICAL MANURES. 



It was then necessary, you see, to establish a distinction between 

 the two states of the assimilable elements. 



The clay has the property of absorbing and retaining much water 

 an important function, since it maintains a proper degree of humidity 

 in the soil, without which vegetation would become impossible. But 

 you know that at last the clay becomes dry and hardened when ex- 

 posed to the action of the sun, and then it becomes so compact the 

 roots of the plant cannot penetrate it. 



Here the sand, which alone would be improper for vegetation, 

 because it would. form a changeable soil and one incapable of retain- 

 ing water, opportunely intervenes. Formed of isolated grains always 

 independent of each other, the sand by mingling with clay acquires 

 its compactness, and communicates to it its more porous and movable 

 character, making it as permeable to air as to water qualities abso- 

 lutely necessary to the exercise of vegetable life. 



Clay possesses another quality, which deserves to be noticed that 

 of fixing in the soil the azotic and mineral compositions which essen- 

 tially determine fertility. This fixity is not complete arid definitive ; 

 it is in a measure exterior and transitory, for the clay ends by giving 

 to vegetation the principles of which it seems to be possessed. 



To make you better comprehend the character of this function, I 

 will cite an example. 



Dilute a piece of clay in the liquor of manure ; the liquid is dis- 

 colored, and analysis shows that at the end of a certain time it has 

 lost a part of the ammonia as well as the salts it contains, and which 

 we will find again in the clay. 



Make an inverse experiment: dilute the same clay in distilled 

 water ; by degrees it will give out the products it has extracted from 

 the liquor of manure. 



Finally, if the active principles of the soil are not washed away by 

 rain, it is due to the clay, which has the property of retaining the 

 fertilizing principles of the soil and of regulating their more tardy 

 dissolution. 



Here is the process : 



The absorbent power of the clay is greater in proportion as the 

 solutions upon which it acts are more concentrated. In a solution 

 containing four per 'cent, of potash or ammonia the clay absorbs more 

 of these two alkalies than in a solution containing one to two per 

 cent, of them. It follows, therefore, from this, that if a drought 

 occurs there is no fear that the soluble part of the soil will acquire a 

 degree of concentration dangerous to the plants. The clay prevents 

 it. If the rain is continued, the clay returns to the water the pro- 

 ducts it has fixed. 



It results from this acting and reacting that the clay performs the 

 office of regulator to the assimilable elements of the soil, holding or 

 giving them out according as the earth passes from a state of drought 

 to an excess of moisture. 



You see, then, gentlemen, that although clay and sand do not take 

 a part in vegetable life, they fill an office of the highest importance. 



