THE MAKE-UP OF THE SOIL 13 



analysis about what crops will do well on this land. How- 

 ever, neither of these analyses is a perfect guide, and the only 

 way to find out which crops will do well and which kind of 

 plant food is lacking is to test the soil with different crops and 

 different fertilizers. 



Life in the Soil. — We are apt to think of the mineral 

 and vegetable matter in the soil as being dead substances. 

 However, there is a real life in the soil. There are many kinds 

 of plants too small to be seen with the naked eye living in the 

 dead organic matter and on the soil grains. These are called 

 soil bacteria, and many kinds are known. Each kind has a 

 work of its own to do. We do not know yet the real use of 

 a great many of these bacteria. We know that there are some 

 kinds whose business it is to make the vegetable matter decay 

 and put the substances of which it was composed in shape to 

 be used again for plant food. Other bacteria catch nitrogen 

 as it circulates through the soil as air and hold it for the use 

 of plants. Doubtless we have all seen the little knobs on the 

 roots of red clover, beans, or some other plant that belongs to 

 that group of plants which has blossoms like the garden bean 

 or pea. Such plants are called legumes. The knobs on the 

 roots are called nodules (See Fig. 20). In these nodules are 

 many bacteria which take nitrogen from the air and give it 

 to the plant, and also build it up into their own cells. (A 

 bacterium has but one cell.) While the bacteria furnish 

 nitrogen to the plant, the plant gives the bacteria such food 

 as starch and sugar to live on. So it is that the plants and 

 the bacteria are helpful to each other. When the clover or 

 other legume dies, there is left for the next crop a great deal 

 of nitrogen in the soil. As nitrogen is a costly plant food and 

 very important, the farmer is always anxious to have a good 



