Chapter Twenty-Two 

 FERTILIZERS 



Plants absorb some of each of the elements found in the 

 soil solution, without regard to their need; in fact, poisonous 

 ones may be absorbed in quantities sufficient to kill the 

 plant. This subject will be discussed first from the point of 

 view of soil culture and later in the more specialized water 

 culture. Boron (an essential element) has been absorbed in 

 amounts injurious to the plant. For example, at one time it 

 became a serious problem when a small amount as an im- 

 purity in one of the fertilizer constituents caused tomato 

 flowers to drop, with a considerable loss to the growers. 

 Selenium (an unessential element available in certain soils) 

 may be absorbed not only in large enough amounts to be 

 toxic to some plants, but also, which is more important, in 

 quantities which are not toxic to the plants but are sufficient 

 to cause serious injury to animals feeding on selenium-con- 

 taminated plants. 



Of the more than forty elements found by analyzing plants 

 only fourteen are known to be essential for their growth. 

 Some of these are needed in quantities of less than one-tenth 

 of an ounce in a ton of dry matter, yet without this small 

 amount the plants can not grow and produce seeds. 



The fourteen elements may be divided into two groups. 

 Three of the elements, carbon, hydrogen, and oxygen make 

 up most of the plant (more than 98 per cent in corn), and 

 they are obtained from the air and water, as described in 

 Chapter 12. The other eleven elements, nitrogen, phos- 

 phorus, potassium, calcium, magnesium, iron, sulphur, man- 

 ganese, boron, zinc, and copper are obtained from the supply 

 in the soil. Except for nitrogen these eleven elements may 

 be recovered from the plant ash. 



164 



