106 



SOILS AND FERTILIZERS 



form tissue, it is removed from the juice and more nitrogen 

 is absorbed to reestablish equilibrium. 



The substances that are used by plants in large amounts 

 are absorbed in greater quantity than those that are not 

 required in making tissue, or in other ways removed from 

 solution in the plant juices. The unused substances that 

 remain in the plant juices prevent, by their presence, the 

 further absorption of those particular substances from the 

 soil water. It is important that substances like nitrogen, 

 phosphoric acid, potash and lime shall be present in abundant 

 quantities in the solution from which crops draw their 

 nourishment. 



126. How roots aid in solution of soil. — In addition to 

 their function in the absorption of plant nutrients, there 

 can be no doubt that roots aid in the solution of these nutri- 

 ents from the soil. One way is by the excretion of carbon 

 dioxide, which when dissolved in water is an excellent solvent 

 for such substances as lime, potash and even phosphoric 

 acid when present in certain forms. The following table 

 shows the percentage of carbon dioxide in air drawn from 

 the bottom of the large soil tanks that have previously been 

 mentioned. ' One of these tanks produced a crop of corn 

 during the summer when the analyses were made, the other 

 tank was kept bare of vegetation. 



Table 26. 



- Percentage of Carbon Dioxide in Air of Soil 

 Planted to Corn and of Bare Soil 



