PHYSICAL PROPERTIES OF SOIL. 29 



(3.) From inundations, whether natural or artiliciaL 

 Water derived from the atmosphere acts most favourably', 

 provided the supply is suitably distributed over the different 

 seasons of the year, and the soil is capable of retaining mois- 

 ture sufficiently long during dry weather. Where these con- 

 ditions are wanting, ground water is likely to act more favour- 

 ably, because it produces a more even degree of moisture in 

 the upper layers of the soil. Natural inundation water is, in 

 many cases, objectionable, because it renders the soil too wet 

 at one time, and too dry at others. Artificial inundation, or 

 irrigation, produces very favourable results, but it is generally 

 expensive. 



D. Gases. 



The gases, such as air, carbon dioxide and ammonia, have 

 been dealt with in section I. of this chapter. It is only neces- 

 sary to add, that the amount of air in the soil varies within 

 wide limits, and that the amount of carbon dioxide depends on 

 the quantity of organic matter in the soil, and the rate at 

 which it is decomposed. 



4. PJiysical Properties of Soil. 



The principal physical properties of importance in sylvi- 

 culture are the following : — 



a. C'o)isisfefic//, 



or binding power, the cohesion between the different 

 particles of the soil. It depends on the chemical composition 

 of the different parts, the degree of division, and the quantity 

 of moisture in the soil. Generally, it is greatest in clay and 

 smallest in sand. An admixture of humus moderates both 

 extremes. 



b. Sliriiikinij, 



or the reduction of the volume of the soil imder the process 

 of drying. It causes cracks in the soil, followed by the 

 exposure of the roots. Heavy soils crack more than light 

 soils. 



