26 CHEMICAL MANURES. 



acre for wheat ; for the colza and the beet you may go as high as 88 

 to 177 pounds without injury. 



The sulphate of ammonia contains in round numbers 20 per cent, 

 of azote, and the nitrate of soda 15 per cent. 



As these compositions are very powerful, too much care cannot be 

 taken in spreading them equally. This is easily done by mixing 

 them with four or five times their weight of dry earth. They should 

 be used after the last working, and then harrowed, to mix them well 

 with the surface soil. 



From the ideas presented to you, gentlemen, we gather that, in 

 an agricultural point of view, there is a great difference between car- 

 bon, oxygen and hydrogen on the one side, and azote on the other; 

 and it is this : that Nature furnishes the first three in superabundance, 

 and we need not occupy ourselves with them, while she gives azote 

 .only exceptionally and under certain conditions. 



The secret of successful culture consists in alternating those plants 

 which draw azote from the air with those which find ' it in the soil, 

 and in reserving for these last all the azotic compositions we can 

 procure. 



The nitrates and the salts of ammonia are not the only azotic com- 

 pounds to which we have recourse. We may use animal matter. 

 During putrefaction it acts as salts of ammonia. But I prefer the 

 -ibrnier, for they admit of direct assimilation, and because of 100 parts 

 f azote which organic matter contains, at least 30 are lost to vege- 

 ^ation. This loss proceeds from the decomposition to which this 

 matter is subject ; 30 per cent, of all its azote escapes in the form of 

 elementary azote, under which form the atmosphere already contains 

 more than vegetation can make use of. 



I cannot too frequently repeat one of the great secrets of remuner- 

 ative culture viz., to draw as much azote as possible from the air by 

 iin alternation of crops. 



The efforts of all agriculturists should tend to this end, and the 

 most useful aid Science has given them has been to show this truth as 

 clearly as possible. 



If Science is a guide which we must sometimes follow with caution 

 for moneyed questions are involved in agricultural operations we 

 must not forget that all our useful facts are conformable to her laws, 

 and if we would accomplish a progress superior to all the conquests 

 of the past, it is still to Science we must turn. 



In our next lecture we will treat of the office of minerals in the 

 economy of vegetable productions. 



