KEMOVAL OF NUTRIENTS FROM THE SOIL 301 



concentration of potassium is offered in single solution, this 

 nutrient is more or less neglected by the seedlings. This rela- 

 tionship, by which the calcium ions make the potassium physi- 

 ologically available, is spoken of by True as synergism x and 

 probably has a great deal to do with the penetration of nu- 

 trient ions into the plant. It is no doubt of considerable im- 

 portance in acid soils where the active calcium is low. 



159. The absorptive capacity of different crops. — 

 Cereals have the power of utilizing the potassium and phos- 

 phorus of the soil to a considerable degree, but they generally 

 require fertilization with nitrogen salts. Most of the cereals, 

 such as wheat, rye, oats, and barley, take up the principal 

 part of their nitrogen early in the season, before the nitrifica- 

 tion processes are sufficiently operative to furnish a large 

 supply of nitrogen ; hence nitrogen is the fertilizer constituent 

 that usually gives good results, and should be added in a 

 soluble form. Wheat, in particular, needs a large amount of 

 available nitrogen early in its spring growth. Since it is a 

 " delicate feeder," it does best after a cultivated crop or a 

 fallow, by which the nitrogen has been converted into a soluble 

 form. Oats can make better use of the soil nutrients and do 

 not require so much manuring. Maize is a very "coarse 

 feeder/' and, while it removes a large quantity of plant nu- 

 trients from the soil, it does not require that this shall be 

 added in a soluble form. Farm and other slowly acting ma- 

 nures may well be applied for the maize crop. The long 

 growing period required by maize gives it opportunity to 

 utilize the nitrogen as it becomes available during the summer, 

 when ammonification and nitrification are active. Phosphorus 

 is the substance usually most needed by maize. 



Grasses, when in meadow or in pasture, are greatly bene- 

 fited by manures. They are less vigorous ' * feeders ' ' than the 

 cereals, have shorter roots, and, when allowed to grow for 

 more than one year, the lack of aeration in the soil causes the 



1 The term is used here in the sense of cooperation. 



