REACTIONS ON LAND 79 



also. All plants contribute to the humus in some measure by the death 

 of the entire organism, annually or from time to time, by the annual 

 falling of leaves and the shoots of perennial herbs, and by the exfolia- 

 tion and decay of roots and underground stems. The amount pro- 

 duced depends upon the density and size of the population and upon 

 the rate and completeness of decomposition. It is small in the pioneer 

 stages of a sere, especially in xeric situations, and increases steadily 

 to reach a maximum in or before the climax. AVhatever the contribu- 

 tion made by animals, this is probably greatest in the subclimax or a 

 late serai stage. 



The admixture of organic matter not only permits the renewed 

 utilization of the nutrients absorbed by previous generations, but it 

 also produces highly important physical effects, especially upon the 

 water content or holard. At first thought it appears a contradiction 

 that humus should have opposite effects upon sand and clay and yet 

 improve the water relations of both. This is explained by its cement- 

 ing action, as a consequence of which it makes one more retentive 

 of water and the other more porous. In general, this tends to increase 

 the water content of dry areas and to decrease that of moist soils, 

 though the decrease is to be ascribed in large measure to raising the 

 level. On the other hand, the non-available water or echard of sand 

 is diminished relatively, while that of loam and clay is augmented. 

 Furthermore, penetration by roots, especially fibrous ones, and the 

 activity of burrowing animals, loosen hard soils and increase absorp- 

 tion, and conversely tend to compact sand and raise the water content 

 correspondingly (cf. Romell, 1921-1935). 



Reaction by Disturbing the Soil. Practically all the reactions of 

 this group are caused by the activities of animals, negligible excep- 

 tions being furnished chiefly by the growth of underground plant parts. 

 The number of genera and species concerned is very large, represent- 

 ing every major terrestrial group. Man naturally stands preeminent 

 in the variety and magnitude of his effects, mammals generally being 

 much more important than all other groups combined, with the pos- 

 sible exception of ants and earthworms. As to the activities or proc- 

 esses involved, digging of all sorts, with the attendant transfer of 

 material, is paramount, trampling, pawing, wallowing, etc., being of 

 quite secondary importance. In a large number of cases, the reaction 

 itself is not a direct object of the activity, but an outcome of a pur- 

 poseful coaction, as in the rooting of swine. IMoreover, the reaction 

 may be superficial or deep seated, temporary, recurrent, or perma- 

 nent, and in its effect significant or immaterial. As in the case of plants 

 these consequences depend largely upon the degree of aggregation, a 



