160 EXPEEIMENTS WITH PLANTS 



be used to wash down through the soil and out through the drains. 

 But, if only a little be used, it remains in the soil and finally evaporates 

 at the surface, thus adding its dissolved salts to the alkali already present 

 in the soil.^ In addition to the injurious effects mentioned above, the 

 alkali salts (especially the black alkali) destroy the tilth and keep the 

 soil in a permanently paddled condition; this is noticeable at the surface, 

 where a thick, hard crust forms on alkali lands. 



Each kind of crop needs a somewhat different food 

 from any other kind. A cereal crop takes from the 

 soil only one -half as much nitrogen and about one- 

 fourth as much potash as root crops. Clover will grow 

 where Wheat cannot and will leave the land fit for 

 Wheat again. This is partly on account of its deep 

 roots, which take food from a considerable depth and 

 raise it to the surface, partly on account of its power 

 of taking nitrogen from the air and partly because the 

 roots and stubble, etc., improve the tilth of the land. 



In practice, crops with similar needs are not raised 

 in succession on the same land: deep-rooted crops al- 

 ternate with shallow-rooted, etc. : white crops (cereals) 

 are usually alternated with green crops (Clover, etc.). 

 This alternation is known as the rotation of crops. 

 Find out as much as you can about the rotation of 

 crops in actual practice.^ 



From what has just been said, it is evident that 



1 See Hilgard ; "Origin, Value and Reclamation of Alkali Lands ; " Year- 

 Book of the Dept. of Agriculture, 1895. Also the following bulletins of the Cali- 

 fornia Experiment Station : Bulletin No. 133, R. Loughridge, "Tolerance of 

 Alkali by Various Cultures"; Bulletin 128, Hilgard, "Nature, Value and Utiliza- 

 tion of Alkali Lands." 



2 Rotation of crops also tends to destroy weeds, fungi, insect pests, etc. 



