ABSORPTION OF MINERAL ELEMENTS BY PLANTS 257 



It is possible to observe in the same way the dissolving effect 

 of the roots upon natural rock phosphates. Sometimes, in water- 

 culture solutions, nutrient elements are introduced in the form 

 of solid substances, which are gradually used by the plants during 

 their grow^th, as in Crone's solution, where phosphorus is supplied 

 in the form of water-insoluble Ca3(P04)2 and FeP04. 



The mechanism of the dissolving action of roots is not under- 

 stood as yet. An important function is performed by CO 2 

 excreted by the roots. It increases the solubility of many sub- 

 stances in water, especially the carbonates. Some investigators 

 (Maze, Schulov) have found that roots excrete malic acid as 

 well as sugar and other organic substances. In the dissolving 

 of insoluble substances, an important part is played by the 

 physiologically acid salts, for instance, ammonium sulphate 

 (see Art. 50). Roots of plants absorb more of their cations than 

 anions, which causes an increase in the acidity of the medium. 

 Prianishnikov has shown that the roots of cereals can dissolve 

 solid rock phosphate only in the presence of nitrogen in the form 

 of ammonium sulphate. If fertilized with nitrates, the roots 

 are not able to dissolve insoluble phosphates. 



An important factor in dissolving mineral substances in the 

 soil is the activity of bacteria. Many of these soil organisms, for 

 example, the bacteria, that cause the disintegration of cellulose 

 among other products, excrete acids, chiefly butyric and lactic. 

 Hence the solution of rock phosphate proceeds more easily and 

 rapidly in humus soils rich in bacteria than in poor soils. In 

 order to separate the activity of roots from those of bacteria, 

 experiments must be carried out in sterile cultures. Unfor- 

 tunately, there is not at present a technique that would allow 

 the steriUzation of soil without fundamentally changing all its 

 properties ; that is why it has not been possible to apply steriliza- 

 tion methods in soil-culture experiments. 



The concentration of hydrogen ions in the soil hkcwise affects 

 the availability of the insoluble substances. According to 

 experiments first carried out by Engelgard, cereals absorb rock 

 phosphate better from acid podsol soils than from slightly alka- 

 line black soils, as the amount of calcium bicarbonate is one of 

 the most important factors regulating the availability of the 

 insoluble substances. It has been observed that an excess of 

 calcium salts, which produces an alkaline reaction of the soil. 



