THE ABSORPTION OF MINERAL ELEMENTS 



79 



the roots. It increases the solubility of many substances 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. Roots of plants absorb more 

 cations than anions, which causes an increase in the acidity of a 

 medium. Prianishnikov has shown that the cereals can feed upon 

 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 such phosphates. An important factor in dis- 



Fig. 32. — Marble plate showing traces left by roots (after Nalhansohn). 



solving mineral substances in the soil is the activity of bacteria. 

 Many of these soil organisms, for example, bacteria causing the 

 disintegration of cellulose, among other products excrete different ■ 

 acids, chiefly butyric and lactic. Hence the solution of rock phos- 

 phate proceeds more easily and rapidly in humus soils rich in bac- 

 teria than in poor soils. In order to separate the activity of roots 

 from those of bacteria, experiments must be carried out in sterile 

 cultures. Unfortunately, there is not at present a technique 

 which would allow the sterilization of soil without fundamentally 

 changing all its properties; that is why we have not been able to 

 apply sterilization methods in soil-culture experiments. 



Finally, the concentration of hydrogen ions in the soil affects 

 the availability of the insoluble substances. The experiments of 

 Prianishnikov have shown that cereals can feed better upon rock 

 phosphate in acid soils than in the slightly alkaline black soils. 

 As the amount of calcium bicarbonate is one of the most important 

 factors regulating the reactions in the soil, its content in the soil 



