32 ELEMENTS OF FARM PRACTICE 



for many centuries. So it is necessary to learn only about 

 the ones that are most likely to be required. 



Names. — The four elements that are most likely to 

 become limited in the soil are nitrogen, phosphorus, potas- 

 sium and calcium. The names of these elements are not 

 hard to pronounce or learn. Every boy and girl should 

 know these names, what part each element plays in the 

 growth of plants, and a practical means of maintaining 



Figure 13. — A crop ol alfalfa, one of the best farm tertilizers. 



a supply of them in the soil. Boys and girls who have 

 studied physiology and have learned how to pronounce 

 and to know the meaning of such words as occipital, parietal, 

 cerebellum, etc., or who have learned to pronounce and 

 know the meaning of such words as subtrahend and min- 

 uend in arithmetic will not have difficulty in learning to use 

 the words, nitrogen, phosphorus, potassium and calcium. 



Nitrogen is needed by all plants. Nearly 80% of the 

 air is nitrogen, and this is the chief source of nitrogen for 

 the soil. The problem of the farmer is to get the nitrogen 

 out of the air and into the soil. It is present in the soil 



