36 PHYSICAL CONSTITUTION OF SOIL 



the soil 1 ; that a jelly-like substance should greatly shrink 

 on drying is of course what we should naturally expect. 



The facts just mentioned are well known in agriculture. 

 The shrinking and cracking exhibited by clay soils in dry 

 weather, and by moor lands rich in humus, are familiar 

 occurrences. The result of this shrinking of the soil is at 

 first harmful, for the roots of plants are torn, and the 

 cracks in the soil allow of a speedier drying of the subsoil. 

 The after results are, in the case of a clay soil, decidedly 

 beneficial. The fissures established in a time of drought 

 afford in future easy lines of drainage. The texture of the 

 clay is also improved to a considerable depth, the drying and 

 remoistening being favourable to the formation of compound 

 particles. Air is also admitted to the subsoil in exceptional 

 quantity, and oxidation in the subsoil is consequently promoted. 



The swelling of dry clay, or of peat, under the influence 

 of rain is also familiar to us. The overflow of peat bogs in an 

 exceptional rainfall has often led to serious results. Hilgard 

 mentions a 'dry bog' soil which increased 30 per cent, in 

 volume when saturated with water. He also mentions that 

 the alkali soils of America, containing sodium carbonate, 

 contract strongly when the powdered soil is wetted. This 

 interesting fact still requires explanation. 



Nature and Origin of Tilth. We are now sufficiently 

 acquainted with the constitution and properties of soil to 

 consider the important question of tilth. By tilth we doubtless 

 primarily understand the pulverulent condition of the soil 

 which results from successful tillage. We shall here, however, 



1 Schiibler found that precipitated magnesium carbonate lost 15.4 per cent, 

 of its volume on drying. This substance is not usually reckoned as a colloid ; 

 it is however exceptionally light and voluminous. 



