70 Physics of the Soil 



Furthermore, from these combinations, under favorable 

 conditions, plants are able to supply their needs. 



80. Functions of the Essential Plant Foods. From the 

 standpoint of plant physiology it is again unfortunate flint 

 little has yet been positively demonstrated regarding the 

 part played by each of the essential elements of plant food 

 taken through the soil and soil moisture. It is known that 

 nitrogen is an essential constituent of the protein com- 

 pounds of living tissues, and that to most of the cultivated 

 crops it becomes available in the form of nitric ac[d or of 

 a nitrate of lime, magnesia, potash or gome other base. Po- 

 tassium does not appear as an essential ingredient of plant 

 tissues or of its storage products like starch or gluten, but 

 Nobbe, Schroeder and Erdmann have shown that when 

 Japanese buckwheat, placed in nutritive solutions en- 

 tirely free from potash salts, after a few weeks' growth 

 came to a standstill and that all organs of the plant came 

 to be nearly or quite free from starch ; but when a potas- 

 sium salt was added to the solution starch began to develop 

 and growth became normal. 



In regard to phosphorus the clearest indications go to 

 suggest that it is usually taken into the plant in the form 

 of phosphates and, because its compounds are often asso- 

 ciated with the soluble albuminoids, that it assists in some 

 way in the transfer of these from place to place in the plant. 



Some compound of iron must exist in soil solutions and 

 must enter the plant before the normal development of the 

 green coloring matter, chlorophyll, can take place; so ex- 

 tremely small quantities, however, are needed that no soil 

 is ever lacking in sufficient available forms. 



Sulphur is apparently largely if not wholly taken into 

 the plant in the form of sulphates, and these are thought to 

 be decomposed by the oxalic acid, setting the sulphuric acid 

 free, which is then broken down and the sulphur appro- 

 priated to enter as an essential constituent of the albumin- 

 oid compounds. 



But little is known of the part played in plant life by 



