

PLANT FOOD AND MANURES 127 



especial care and thought owing to the fact that 

 nitrogenous substances are capable of many changes, 

 and that, if carelessly dealt with, there are many ways 

 in which nitrogen may be wasted and lost. 



Mineral Matter. 



Of the mineral constituents of plant food, lime has 

 already been mentioned ; others are potash, magnesia, 

 iron, phosphates, chlorides and sulphates. Of these, 

 potash and phosphates are often present in the soil in 

 such small proportions, and are so constantly needed by 

 plants, that beneficial results follow their addition to 

 the soil. It is estimated that about the following 

 amounts of potash and phosphoric acid are removed by 

 the crops mentioned : 



Potash. Phosphoric Acid. 



Potato . . . . 75 Ib. 20 Ib. 



Corn (grain only, 30 bushels) . 7 10 



Manuring. 



Every crop taken off the land represents so much 

 actual weight of nitrogen, phosphates and potash 

 removed from the soil. This fact is too often lost sight 

 of in practice, and crops are removed, year after year, 

 without any attempt being made to keep up the supply 

 of food-stuffs in the soil. The plants draw upon the 

 supply of food material present in the soil, and thrive 

 until it is no longer able to satisfy their wants. The 

 object of manuring is to maintain this supply, or even 

 to increase it. 



Any method by which the fertility of the soil can be 

 increased may be included under the general term 



