EDAPHIC OR SOIL FACTORS: PHYSICAL 211 



incUnata initial stages, with a rich interminghng of transported alpine 

 plants and of xerophytes from dry, neighboring slopes such as Oxytropis 

 pilosa and Carex nitida, are driven out in the Swiss Rhine valley, in one 

 or two decades, by impenetrable thickets of Hippophae, which in turn 

 give way to the P. silvestris forest. 



Soil Crumbing. — The habitability of a soil depends not only upon 

 its dispersion but quite as much upon the relation of the particles to 

 each other. The finer the grain of a soil the greater the importance of 

 these conditions. In this respect two conditions are recognized: 

 single-grain structure and aggregate or crumb structure (Fig. 110). 



After prolonged, heavy rainfall finely granular soils appear doughy 

 or sticky: they are "muddy. " Rain water with small mineral content 

 passes between the soil particles and 

 decreases their cohesive force. They shift 

 their position under the effect of the beating 

 raindrops and fill up the vacant spaces. The 

 soil changes to single-grain structure. The 

 converse effect is produced by heat and 

 especially by frost. 



The sudden expansion of the soil, caused 

 by freezing of the soil water, cracks up the 

 densely packed soil particles. This expan- 

 sion extends into the minutest chinks. The 

 smallest soil particles collect in groups; they 

 form aggregates or "crumbs" (Fig. 1106). Fig. ho. — Single grain 

 Crumbing of the soil is brought about also *rb":,^:'toe'(?fc:i'Sir' 

 by the action of soil organisms, by tillage 



of the soil, and especially by the influences of various soluble 

 salts (salts of carbonic acid, sulphuric acid, chlorides, etc.). On the 

 other hand, neutral carbonates and soluble salts of silicic acid tend 

 toward deflocculation. For this reason soils deposited in pure water 

 (glacial Dryas clay) are very densely packed. 



The farmer and the forester combat the single-grain structure of 

 their soils and endeavor to maintain a crumb structure which is 

 essential to good tilth. For the plant sociologist, the relation of root 

 layering to structure of the soil offers a fruitful field for research. In 

 such work, water content, water conduction, and aeration of the soil 

 should be included as controlling factors. 



2. SOIL WATER 



According to the manner in which it occurs in the soil, the soil 

 scientist distinguishes between attached water, i.e., water fixed in the 



