SOIL-STRUCTURE. 87 



finest particles, to render the texture of a clay soil more open." In general, 

 it increases the water-content of dry, bare areas and tends to decrease the 

 water-content of moist areas. The latter is chiefly the result of raising the 

 level, and is often comphcated by decreasing aeration and the possible produc- 

 tion of harmful substances through partial decomposition. The effect of 

 humus is most marked in the weathering of rock and in dry sand and gravel 

 areas, where the action is ciunulative throughout the whole course of develop- 

 ment. The increase in the number or size of the individuals in each successive 

 stage results in more material for humus production, and this increases the 

 water-content steadily from the initial to the climax stage. While the holard 

 increases, the echard also increases from less than 1 per cent in sand and gravel 

 to 12 to 15 per cent in loam, so that the chresard increases less rapidly than the 

 total water-content. The ultimate effect in each stage is to favor the invasion 

 of plants with greater water requirements, and hence with greater powers of 

 competition and duration. They readily become dominant and their prede- 

 cessors disappear or become subordinate. 



The penetration of roots tends to make hard soil looser in texture and to 

 increase the available water, while it decreases the permeability of sand and 

 raises the holard correspondingly. It is so intimately associated with humus 

 in its effects that it is difficult if not impossible to distinguish between them. 



(8) Reaction by compacting the sail. — ^This is an indirect effect due to the 

 reaction of the conmumity upon the water-content. It constitutes a reaction 

 of primary importance in the case of heath on sandy soils, and perhaps also 

 in the "hard" lands of the Great Plains. In heath-sand the final outcome is 

 the formation of a rock-Uke layer at a depth of 2.5 to 3 dm. This is the layer 

 known as "ortstein." There is still much doubt as to the process by which it 

 is formed, and it seems probable that it may arise in different ways. Graebner 

 (1909) assumed the usual formation of "ortstein" to be as follows: 



The humus substances characteristic of heath-sand remain in solution only 

 in pure or in acid water, but are precipitated in the presence of the soil salts. 

 They pass through the heath and sand ahnost imchanged, but are precipitated 

 where the sand lies in contact with a substratum richer in min eral salts. Here 

 is formed a brown layer which further accumulations of hmnus precipitates 

 convert into the true "orstein" which may reach a decimeter in thickness. 

 The primary effect of "ortstein" is mechanical in that it stops the downward 

 growth of roots completely. It seems to have an influence apart from this 

 also, inasmuch as roots grow poorly even when they pass through openings in 

 the layer. The horizontal growth of roots is also foimd where the layer is 

 not sufficiently compact to prevent penetration. This effect seems to be due 

 to poor aeration caused by a lack of oxygen. 



The effect of "ortstein" upon the course of succession is to handicap deep- 

 rooted plants, such as shrubs and trees, and to retard or prevent the appear- 

 ance of the final stages. Instead of producing or favoring the progression of 

 stages, as most reactions do, it limits development and tends to make the 

 heath the climax association. A somewhat similar result occurs in grassland 

 communities in arid or semiarid regions, where the penetration of water is 

 limited to the root layer (plates 19 b, 24 a). The soil beneath becomes 

 densely compacted into a layer known as "hardpan." As a result, deeper- 

 rooted species are eliminated and the area comes to be dominated by the 



