THE FOOD OF PLANTS 



29 



Bean Plants and Nitrogen. The plants of the bean fam- 

 ily have small knotty and wartlike growths on their roots. 

 These growths are called tubercles, and are caused by 

 very small germs, or bacteria. The tubercles are the 

 homes of bacteria. Through the activity of these bac- 

 teria, nitrogen is taken from the air and stored in the 

 tubercles. This is called "fixing" the nitrogen. When 

 the plants die, a part of the nitrogen 

 fixed in their tubercles remains in the 

 ground and enriches it. If the living 

 tops of these plants are plowed under, 

 the soil is still more enriched in nitro- 

 gen. Plants used in this way are 

 called green manures. 



Experimenting with Fertilizers. As 

 has already been stated, we cannot tell 

 positively from the appearance of a 

 plant what fertilizer it most wants ; 

 but we can learn by an easy and cheap FlG 25 

 experiment. Suppose that we have a 

 field of maize which is well cultivated and watered, but 

 still produces small crops. Some fertilizer is almost cer- 

 tainly wanted; but we might lose considerable money 

 by treating the whole field with a fertilizer not especially 

 needed. 



So we mark off plots two meters square ; and on one 

 plot put one kilo of manure; on another, two kilos; on 

 another, one half kilo of ashes; and on another, one kilo. 

 Guano, fish, or fish-refuse, and bean or peanut tops can 



Tubercles on the 

 roots of a bean plant 



