CONCENTRATION AND REACTION 9 



same order as that given by the extraction methods, which of 

 course deal with soils in a moist or wet condition. 

 Bouyoucos found great differences in different soils, as 

 might be expected. The freezing-point method is the most 

 expeditious yet invented, and the best, in that it permits of 

 the examination of the soil solution in situ and under different 

 conditions of moisture. 



In normal moist soils, then, the solution is certainly very 

 dilute. The solutes are constantly removed and con- 

 stantly renewed, though not necessarily at equal rates. 

 The removal takes place partly by the plant, and partly by 

 washing down of the solution to the subsoil in drainage 

 after rain. This action is complex, for certain constituents 

 — notably potassium and the phosphates — tend to be 

 retained in the soil by the adsorptive action of the colloids, 

 and by interaction with calcium compounds. This fact is 

 illustrated by the relatively small amount of potassium and 

 phosphorus found in the drainage water. The high figures 

 for calcium and nitrates show that these are easily washed 

 out (cp. Table II). 



Under natural conditions the material removed by the 

 plant is ultimately returned to the soil in plant remains or 

 in animal droppings. Furthermore, the material washed 

 into the subsoil tends to be brought up again by the action 

 of deeply penetrating roots. The soil minerals are subject 

 to the slow solvent action of the soil moisture. The most 

 serious wastage occurs in the loss of nitrates, and the balance 

 here is largely restored by bacterial action. 



Reaction oJ Soil Solution. — The effect of soil reaction 

 has had much attention drawn to it in recent years, conse- 

 quent on the recognition of the profound influence exerted 

 on many physiological processes by the reaction of the 

 medium. This influence depends on the strength of the 

 acid or base present. It is well known that although 

 equimolecular quantities of hydrochloric and of acetic 

 acids neutralise the same quantity of a base, the former is a 

 ^/ro«^ acid, while the latter is zoeak. The difference lies in 

 the greater degree of dissociation of the former, which leads 



