46 



by diffusion only, can th© nearly coraplate removal of 

 nitrates b© accounted for. Usually the phosphoric acid 

 doee not fall relatively so low in soil as nitrates. Thl8 

 may be <^ue to the slower diffusion, or to the continuous 

 solubility either in th© field or in the laboratory, or to 

 all combined. 



^ile the* movement of soil nutrients for any consider- 

 able diBtance in a reaeonabi© length of time is due to the 

 mechanic al transfer by the v.ater of capillarity or pei*®©- 

 lation, the transfer to the rootlets after t#i© root zone is 

 reached is probably chiefly due to diffusion- Capillary 

 water passing to th© surface may hp, this r.eans be largely 

 or nearly wholly divested of the salts which the i>iants 

 require. Under such cireuia stances the analysis of the 

 surface soil for soluble plant food would giv^ little 

 indication of the nutrients used by the plants 



Ta^ amount of plant food nutrients in '<=^ soil sample 

 aay not even approximately represent th© amount used by 

 the plant, FTowaver, th© amount of excess would probably 

 in Eost cases be some indication. V^ner© the amount was 

 abundant, the plant would be well supplied, but the 

 amou&t might run down to a certs^in fairly or equally low 

 limit, and the plant be nearly or equally as well supplied. 



