HUMUS 



105 



we can take rain water, which we know has no plant food in 

 it, and by adding chemicals alone we can grow in this water 

 plants that will live for years. Small trees have been grown 

 in this way, but of course it is very little worth while, except 

 to show what can be done. We must grow our plants in the 

 earth, and humus is necessary for us. 



But when we take our crops from our gardens we are using 

 up the humus, and giving 

 nothing in return. If we 

 wish to keep the soil in 

 good condition, we must 

 therefore find some means 

 to restore humus to it. 

 The ordinary gardener 

 uses three methods. 



The first is to grow, in 

 each part of the garden 

 that is unoccupied by a 

 crop, plants that can be 

 dug into the soil, and 

 rotting there, make hu- 

 mus. This is called green 

 manuring. The best 

 plants to use for this pur- 

 pose are the clovers and 



their family, called the legumes. These plants give most 

 nitrogen to the soil, and in a very peculiar manner. Long 

 ago the Romans knew that legumes did good to the soil, 

 but they did not know why. Not many years ago some 

 one discovered that legumes have on their roots little 

 knots, or knobs. These were at first considered to be signs 

 of disease ; but they were presently discovered to be colonies 

 of the bacteria which best make soil nitrogen. They use 



FIG. 58. Nitrogen factories. Bacterial 

 colonies on roots of bean. 



