172 BOARD OF AGRICULTURE. 



mineral constituents suffice for a crop of it. Upon this fact 

 deijends the rotation of crops. 



The substances necessary to the life of a plant must act 

 together within a given time, if the plant is to attain its full 

 development in that time ; the absence or deficiency or the 

 want of available form in one necessary constituent is indis- 

 pensable ; but fertility is communicated if that one substance 

 be added in due quantity and available form. 



The supply of more atmospheric food, namely, carbonic acid 

 and ammonia, by means of ammoniacal salts and humus thau 

 the air can furnish, increase in a given time the efficacy of the 

 mineral constituents in the soil. 



In a soil rich in the mineral food of plants, the produce 

 cannot be increased by adding more of the same substances. 



In a soil rich in the atmospheric food of plants, rendered so 

 by manuring, the produce cannot be increased by adding more 

 of the same substances. 



The continued fertility of a soil for all kinds of crops, de- 

 pends on the constant return to it of all the mineral constit- 

 uents removed by the different crops. 



Farm-yard manure is taken as the type of manures, because 

 it contains all the constituents removed from the land, and 

 again restored to it in a form in which they can be made 

 rapidly available. 



The carbonic acid and salts of ammonia produced by its 

 decomposition, cause water to dissolve more rapidly the min- 

 eral constituents. 



An artificial manure can be theoretically compounded to 

 take the place of farm-yard manure ; but it must contain all 

 its mineral constituents. The farmer must return to the land 

 whatever has been removed from it, for none of the constit- 

 uents of a rich soil can be removed without making compen- 

 sation, but at the cost sooner or later of impairing its fer- 

 tility. 



As bones furnish only two substances to crops, science as 

 well as experience indicate that they are more likely to be 

 useful when used as auxiliaries, for example, with farm-yard 

 manure ; if the soil is deficient in bone earth, the first appli- 

 cation will produce good results ; a constant repetition is 

 iproductive of no increased fertility, but by the addition of 



