SOIL REACTION AND MOISTURE 13 



As has been said, apart from abundance of chalk or 

 other carbonates, soils are naturally acid. It is clear that 

 even soils of an extremely acid character can support vege- 

 tation, though of a specialised character. Such soils are 

 agriculturally barren ; of the direct effect of a high hydrogen 

 ion concentration on the plant, however, we know very 

 little. It may raise the hydrogen ion concentration of the 

 root cells slightly (Truog, 1919), and this no doubt affects 

 metabolism and growth. It may also affect the entry of 

 salts and of water into the root. But the poverty of acid 

 soils is not due to the acidity alone. Such soils, if derived 

 from granite or schist rocks, are also very poor in nutrient 

 salts. Acid clay soils are physically very sticky and heavy. 

 The " liming " of a soil, so important in agricultural 

 practice, not only reduces hydrogen ion concentration, but 

 adds nutrient bases, alters the adsorptive properties, par- 

 ticularly of clay soils, and makes the soil lighter. Much 

 remains to be done before the actual effects of acidity 

 as such can be properly understood. 



State o£ Soil Moisture. — Moisture is retained in the soil 

 in a variety of ways. After rain more water is present than 

 the soil can hold ; the excess sinks under the action of gravity 

 and drains away, and this may be referred to as gravitational 

 water. There then remains a fraction held in the minute 

 crevices between the fine particles and as films round 

 them to which the term capillary moisture is applied. 

 Moisture is also retained absorbed by the soil colloids, and 

 as water of hydration in the silicates of the clay. This 

 fraction has been called the hygroscopic moisture^ being held 

 to be equivalent to the moisture taken up by a dry soil 

 from a saturated atmosphere. The hygroscopic moisture 

 is held by very high imbibition forces, amounting to as 

 much as 1000 atmospheres. The surface tension retain- 

 ing the capillary moisture is relatively small, equal to 

 2 or 3 atmospheres. The gravitational water is not held 

 at all. 



The determination of the proportions of these fractions 

 is a matter of difficulty, and recently Bouyoucos (1921) has 



