34 



HOW TO KEEP SOIL FERTILE 



not vary these amounts much, nor can she use one ma- 

 terial instead of another. When the carpenters that are 

 building a house run out of nails, their work must stop 

 until more nails are provided; or, if they try to make 



their nails hold out by 

 using less than the 

 usual number, the 

 house will not be strong 

 and may be blown 

 down by the first hard 

 wind. Just so, if one 

 of the soil materials 

 that go to make up a 

 plant runs short, the 

 plant must stop grow- 

 ing, or the growth 

 made will be unhealthy, 

 and the plant will fail 

 from disease. 



Soil Water Holds 

 Plant Food Figure 

 17 shows some wheat 

 plants growing in wa- 

 ter in two jars. The 

 right-hand jar contains 

 rain water, and the left- 

 hand jar contains well 

 water, that is, water that has come out of the soil. 

 Rain water has almost nothing dissolved in it, because, 

 when water evaporates into the air, the dissolved ma- 

 terials it contained are left behind. In this experiment, 



FIG. 17. Plants growing in water. 



