HOW TO KEEP SOIL FERTILE 35 



the water in both jars was boiled before being 1 put in. 

 The rain water was boiled to drive off the gases it had 

 taken from the air, and the well water was boiled to evap- 

 orate a part of the water and thus to make stronger the 

 solution of mineral matters it contained. From the 

 picture, we see that the plants in the jar containing 

 the well water are growing faster than those in the 

 other jar. This is because the well water contains 

 in solution the materials that the wheat plant needs for 

 food. The rain water, on the other hand, contains al- 

 most no food material. The only reason the plants can 

 grow in the rain water at all is that the seed contains a 

 small amount of food. When this food is used up, the 

 plants will soon starve. 



Make-up of Rich Soil. Chemists have found that 

 certain quantities of about a dozen different materials 

 are used by plants for food. The one that is used in 

 largest proportion is carbon, and, as we learned in Lesson 

 4, this comes from the air. All the others come from the 

 soil. But of all the substances that come from the soil, 

 only three often run short; most soils contain a large 

 surplus of all the others. The three that often fail are 

 called nitrogen, phosphoric acid, and potash. These are 

 the only substances, then, that the farmer or gardener 

 needs to put on his land often to keep it " rich," that 

 is, to give it all it needs to produce good crops ; and, since 

 of these nitrogen is most often lacking, it must be sup- 

 plied most generously to the soil. 



Manure When plants of any kind grow on the land, 

 they take out of the soil what they need for food. If we 

 put these plants back on the land and let them decay, 



