80 Elementary Principles of Agriculture 



rine, calcium, and magnesium. The substances named 

 occur in nearly all soils in quantities sufficient to supply 

 the plants abundantly. Other substances, as potassium, 

 phosphorus and nitrogen, are more important, and must 

 be supplied when necessary. (See table of fertilizing 

 substances in feed-stuffs in Appendix.) 



112. The Form in Which Plants Take Up Their Mineral 

 Food. These "elements" occur in the soil as compounds 

 with other substances. The soil is composed mostly of 



Fig. 44. Pot cultures of alfalfa, showing effect of adding different fertilizers. 

 D9, nothing; D10, nitrogen; Dll, potassium, and D12, phosphorus. Pho- 

 tograph from Oklahoma Agricultural and Mechanical College. 



insoluble compounds, which the plants cannot use. The 

 particles are very slowly changed into soluble compounds, 

 and in this form are absorbed by the plants. The amount 

 or per cent of soluble matter in the soil water at any 

 one time is exceedingly small, as shown by the analysis 

 of natural waters. In fact, if the amount should exceed 

 ten parts in a thousand the effect would be unfavorable 

 on the growth of the plant. The total amount of, say, 

 potash in the soil may be several per cent of the total 

 soil weight, yet the amount in solution at any time may 

 rarely exceed fifty parts per million of water. It is well 



