204 THE FARMER OF TO-MORROW 



plant (the so-called carbohydrates), and all 

 are constantly being interchanged between the 

 plant and air, or water, in a never-ending cycle 

 of economy. 



Iron is a fourth element, "but the amount 

 required by plants is so small, and the amount 

 contained in the soil is so large that soils have 

 never been known to become deficient in 

 iron." Sulphur is used sparingly by plants, 

 frequently merely tolerated. In addition to 

 the supply in the soil, there are constantly be- 

 ing added, by rains and absorption, the sul- 

 phurous gases which pass into the air on com- 

 bustion or decay of organic materials, such as 

 coal, wood, grass, leaves, etc. 



To quote from Doctor Hopkins : 



"But there are five other essential elements 

 of plant food and these require especial con- 

 sideration in connection with permanent soil 

 fertility. They are potassium, magnesium, 

 calcium, phosphorus and nitrogen. There are 

 also five important points to be kept in mind 

 in relation to each of these elements : the soil's 

 supply, the crop requirements, the loss by 

 leaching, the methods of liberation, and the 

 means of renewal. 



"The neglect of one or more of these impor- 

 tant points in relation to one or more of the 



