140 MINERAL SALTS ABSORPTION IN PLANTS 



E. Absorption of Ions from Soil 

 1. Anions 



As far as is known, anions are absorbed by plants almost 

 entirely from the soil solution. A reservoir of certain anions is 

 present as insoluble substances but this is not true of nitrate. During 

 the growing season, soils normally show a decline in the amount 

 of soluble nitrogen compounds present, and this is followed by a 

 slow replacement, through nitrogen fixation and the breakdown of 

 organic substances. The periodic growing of legumes, which are 

 symbiotically associated with nitrogen-fixing bacteria, followed by 

 "digging-in" of the crop ("green manuring") is a well-known 

 method of increasing the available nitrogen in soils. Application of 

 nitrogen-containing fertilizers is essential for the maintenance of 

 high crop yields on arable land under modern agricultural practice. 



Phosphorus exists in soils mainly as insoluble phosphates from 

 which it can be released to some extent by other anions. Dean and 

 Rubins (1947) found that the effectiveness with which various 

 anions cause release of phosphate was in the order: hydroxyl> 

 citrate > fluoride > tartarate > arsenate > acetate. Soil micro- 

 organisms assist in rendering phosphate available for plants by the 

 organic acids they excrete, and also by causing enzymatic degrad- 

 ation of organic phosphorus-containing substances such as nucleic 

 acids, phosphatides and phytin. 



The concentration of phosphate in soil solutions is so low that 

 doubts have been expressed whether plants can absorb it quickly 

 enough from this source alone to sustain normal growth. Tidmore 

 (1930) showed, however, by experiments involving flowing culture 

 solutions, that even rapidly growing plants can absorb adequate 

 amounts of phosphate from media containing as little as 0-5 parts 

 per million of soluble phosphorus. Phosphorus uptake by Zea 

 mays was increased when the concentration of the solution was 

 raised to 1 part per million, but growth was not improved. If 

 phosphate is absorbed solely from the soil solution, the solution 

 must be replenished frequently during growth of a crop. Stout and 

 Overstreet (1950) calculated from experiments with potted plants 

 that the soil solution might need to be completely recharged with 

 phosphate 10 times/day during the growing season to support the 

 growth of some crops. 



A large part of the soluble phosphates added to soils as fertilizers 



