FERTILIZERS 1 7 



ESSENTIAL PLANT FOOD. 



23. Each of the three plant foods, nitrogen, phos- 

 phoric acid and potash, are called essential ingredi- 

 ents in fertilizers, as they are the elements first 

 exhausted from the soil by plants. There are eleven 

 other elements 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 is sup- 

 plied to the soil, though not regularly, to set free 

 nitrogen, phosphoric acid and potash, when they 

 are known to be in the soil in insoluble condition 

 and large amounts. But, as lime adds no necessary 

 ingredient, its continued use alone will exhaust a 

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

 added to make green foliage and to deepen the color 

 of oranges. 



If a soil becomes unproductive under conditions of 

 good tillage and water, it is usually because one or 

 more of the essential plant foods has become ex- 

 hausted. Hence commercial fertilizers have come to 

 be composed of various amounts and forms of nitro- 

 gen, phosphoric acid and potash. Commercial fer- 

 tilizers are simply concentrated forms of plant food. 

 A good top soil contains every element essential to 

 plant growth and is a fertilizer, but it is not suffi- 

 ciently concentrated to pay for handling and trans- 

 portation. 



24. Each of the three plant foods, nitrogen, phos- 

 phoric acid and potash, have their respective market 

 values for each 1 per cent., or unit, of 20 pounds to 

 the ton. If a ton of fertilizer contains 3 per cent, of 

 an element that means 60 pounds. The purchaser 



