4 SOILS: PROPSRTIMS AND MANAGEMENT 



Elements obtained from Elements coming directly from^ 



air or water the soil itself 



Carbon Nitrogen Magnesium 



Ox\^gen Phosphorus Iron 



Hydrogen Potassium Sulfur 



Nitrogen Calcium 



Carbon is obtained very largely by the plant directly 

 from the air as carbon dioxide (CO2) ; while oxygen 

 comes directly from the atmosphere or from water, which 

 is also the source of at least a part of the hydrogen utilized 

 in vegetative groviiih. The other elements, except in 

 the case of leguminous crops, are taken wholly from the 

 soil solution itself. 



Wliile all these elements found in the soil must be 

 available in order that plants may grow normally, only 

 a very few ever become limiting factors. The three 

 elements most likely to be lacking in a soil from a food 

 standpoint are nitrogen, phosphorus, and potassium. 

 They may be designated as the primary elements for 

 plant growth. The other elements are usually present 

 in amounts many times greater than will ever be needed 

 by crops. Calcium, while necessary in large quantities 

 in a soil, is largely an amendment, and very seldom may 

 limit plant growth because of being in too minute quantity 

 to supply the food needs of a crop. The liming of a soil 

 is for other purposes than the supplying of calcium for 

 plant nutrition. Sulfur is supposed, in certain soils, 

 to limit plant growth because of its insujfficiency, but 

 ordinarily it is never found in a minimum quantity. 



Nitrogen exists in the soil largely as a portion of the 



1 Sodium, silicon, and aluminium are found in plants, but are 

 not essential to proper growth. 



