Essential ^Plant Food. 



Each of the three plant foods, nitrogen, phosphoric 

 acid and potash, are called essential ingredients in 

 fertilizers, as they are the elements first exhausted 

 from the soil by plants. There are eleven other ele- 

 ments just as essential to perfect plant growth as 

 these three, but the soil never becomes depleted of 

 them, and it is not necessary to supply them, except 

 in rare cases. Sometimes lime and iron are supplied 

 to the soil, though not regularly. Lime is used to 

 set free nitrogen, phosphoric acid and potash, when 

 they are known to be in the soil in insoluble condi- 

 tion and in large amounts. But, as lime adds no 

 necessary ingredient, its continued use alone will ex- 

 haust a soil. If a soil is known to lack iron, this may 

 be added to make green foliage and to deepen color 

 of oranges. 



If a soil becomes unproductive under good tillage 

 it is because one or more of the three essential plant 

 foods has become exhausted. Hence commercial fertil- 

 izers have come to be composed of various amounts 

 and forms of nitrogen, phosphoric acid and potash. 

 Commercial fertilizers are simply concentrated forms 

 of plant food. A good top soil contains every element 



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