42 SCIENCE AND SOIL 



10 pounds of phosphorus. The phosphorus required by animals 

 is, as a rule, supplied in the plants that serve as animal foods. 



The percentage of phosphorus in the earth's crust is small when 

 compared with the requirements of plants, especially when we also 

 consider that the phosphorus accumulates in the more concen- 

 trated and more salable products, as in the seed or grain, and also 

 in the flesh, bone, and milk, of animals. 



Phosphorus is usually taken up by plants in the form of phos- 

 phates, but within the plant it enters into organic combination 

 as shown above. 



The six elements thus far discussed in some detail carbon, 

 oxygen, hydrogen, nitrogen, phosphorus, and sulfur are all non- 

 metallic. Three of them oxygen, hydrogen, and nitrogen are 

 gases in the free state, and the other three carbon, phosphorus, 

 and sulfur are nonmetallic solids. Four other elements are also 

 absolutely essential to the growth of all agricultural plants. 



Potassium and magnesium. These are metallic elements which 

 have very important functions in plant growth and which are re- 

 quired in considerable amounts. Both are stored in the seed in 

 relative abundance, and are found in the ash of grains in the form 

 of phosphates, although still larger amounts of potassium are 

 stored in the coarser parts of plants (as in straw, cornstalks, etc.). 



It is not known that potassium and magnesium are essential 

 constituents of protoplasm, but, like nitrogen and phosphorus, 

 they are found in largest proportions in the embryo tissues. It is 

 suggested that one of their essential functions may be as carriers of 

 nitrogen and phosphorus in the form of definite salts (as nitrates 

 and phosphates) capable of reaction with certain products result- 

 ing from the fixation of carbon, oxygen, and hydrogen. Certainly 

 it is not sufficient that phosphorus, for example, shall merely be 

 carried in the form of some soluble phosphate into the laboratory 

 (the leaf) of the plant, but the compound must be such that the 

 metallic base will release the phosphorus at the proper time, in 

 order that it may enter the organic combination and thus become 

 a part of the living organism. 



It is known that organic acids are developed by the plant with 

 which potassium and other bases carried into the plant in the form 

 of nitrates, phosphates, etc., may unite, and do unite, at some time 



