THE NATURE OF PLANT NUTRIENTS 



15 



sea-fowls. Besides nitrogen, it generally also contains considera- 

 ble quantities of both phosphorus and potassium. The recently- 

 developed industries fixing nitrogen from the air have supplied 

 products which largely replace the natural substances. These 

 may be obtained as substances containing ammonia, cyanamid, 

 nitrate, and urea. Certain plants, as the legumes, may draw 

 upon the store of nitrogen in the air through the agency of bac- 



Indispensable in appre- 

 ciable amounts 



PRINCIPAL SOURCE 



From the carbon dioxide of 

 the atmosphere. 



From oxj'gen and carbon di- 

 oxide of the atmosphere 

 and from water. 



From Mater. 



From nitrate or ammonia 

 in soil. Can be obtained 

 from gaseous nitrogen by 

 leguminous plants. 



From phosphates in soil. 



From salts in the soil. 



From salts in the soil. 



From salts in the soil. 



From ferrous or ferric salts 

 in the soil. 



From sulfates in the soil. 



• From salts in the soil. 



Indispensable in small 

 amounts, exerting 

 stimulating effects or 

 correcting soil condi- 

 tions 



Fig. 6. — Sources of plant nutrients. 



terial action. These plants are used quite generally as cover 

 crops which, when plowed into the soil, greatly increase its nitrogen 

 content. The return of barnyard manures to soils not only adds 

 nitrogen but also both phosphorus and potassium. 



Phosphorus is most frequently introduced into the soil as the 

 relatively insoluble tri-calcium phosphate, as the soluble super- 

 phosphate, or as basic slag, which is a by-product of the steel 



