114 



PLANT PHYSIOLOGY 



plants grown in this. The cultures should be kept so that the roots 

 are in the dark, and it has been found best to keep the solution 

 slightly acid. The seed contains much stored food and as soon 

 as the roots have reached a sufficient length the cotyledons should 

 be removed ; otherwise there will be no visible effect of a deficient 

 element for a week or two. The concentration of the solution 

 must not be too strong or the roots will become plasmolyzed 

 (Chap. XIX), and for the reasons discussed in the following 

 chapter the salts should be in a fixed ratio which varies with the 

 plant and the stage of its development. Various solutions for gen- 

 eral purpose work have been prepared and found suitable for 

 laboratory work. Among these are: 



Shive and Tottingham have worked with a three-salt solution 

 which has proved very satisfactory. That of Shive (1915) con- 

 sists of monopotassium phosphate, calcium nitrate, and magne- 

 sium sulphate with a trace of iron phosphate. It will be ob- 

 served that these solutions contain no boron or manganese. These 

 elements are needed in such small quantities that salts which 

 have not been purified by very special processes generally contain 

 enough of boron and manganese to prevent any of their deficiency 

 symptoms from appearing. 



Crop Rotation.— Since different plants take from the soil dif- 

 ferent materials, it would seem inadvisable to continue using 

 the same soil for the same crop for several consecutive years. It 

 is a common agricultural practice to grow plants in a consecutive 

 series or to follow what is called a system of "crop rotation." If 

 the failure of a crop is due to the withdrawal of certain essential 

 elements, fertilizers should be able to make the rotation unnec- 

 essary. While the addition of fertilizers does enable the farmer 

 to disregard rotation to a certain extent, the time ultimately 

 comes when the crop deteriorates. 



This has led to a second theory of crop rotation, which has been 

 developed by the U. S. Bureau of Soils. This is known as the toxin 



