CHAPTER XII 



RELATION OF THE PLANT TO THE CHEMICAL 

 COMPOSITION OF THE SOIL 



"The soil is not only a sponge, from which the plant 

 may obtain water, but it is also a storehouse of plant 

 food and a laboratory in which the plant food is pre- 

 pared and dissolved for the plant." Osterhout, Ex- 

 periments with Plants. 



108. In the preceding chapter, the relation of the 

 plant to the water contained in the soil, and the means 

 by which the water supply may be increased, have been 

 discussed. These tillage operations not only cause the 

 water to be retained for the use of the plants, but dis- 

 solve the mineral food elements in the soil. While the 

 amount, the kind, and the condition of these soil foods 

 affect very greatly the fertility or agricultural value of 

 a soil, we should remember that, without resort to 

 means for improving the mechanical condition, many 

 soils, naturally rich in plant food, would yield poor 

 crops. We should study closely the relation of the 

 chemical composition of the soil to the fruitfulness of 

 the crops. 



109. The Essential Elements. By growing plants with 

 their roots in a medium of known composition, plant 

 physiologists have determined which elements of the 

 soil are really necessary for the healthy, normal growth 

 of the plant. By the same means they have been able 

 to determine the effect of other substances. For these 

 tests, the plants are usually grown in vessels thoroughly 



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