REACTIONS ON LAND 87 



Soil Water, Solutes, and Gases 



These three sets of factors are most intimately associated in the 

 soil, and the modification of one leads to the change of others. This 

 is truest of water content, since the amount of this determines in large 

 measure the concentration of the soil solution, as well as the pore space 

 available for air or other gases. Since water is the chief factor in 

 plant as well as community response, it is more or less affected by 

 nearly all reactions. In addition, the increase or decrease of the total 

 water or holard may be a direct outcome of the activity of plants 

 themselves, and this effect may operate upon the available water or 

 chresard, as well as upon the total amount present. 



Reaction by Increasing the Water Content. No flowering plants 

 are known that increase water content as a direct reaction, though 

 they may bring this about by the formation of dew and the condensa- 

 tion of fog. The sole plants to exert a direct effect are a few mosses, 

 notably Sphagnum, and perhaps such algae as the ground forms of 

 Nostoc. The power of the peat mosses to absorb and retain relatively 

 enormous amounts of rain and dew is unique. Because of this prop- 

 erty, Sphagnum is able to waterlog or flood small areas, with profound 

 effects upon the course of succession and the accumulation of organic 

 material. 



All reactions that enhance absorption or hamper percolation or 

 evaporation increase water content indirectly. Root systems are every- 

 where of paramount importance in promoting penetration of water into 

 soils, this reaction being most significant in finer or "harder" types. 

 The fibrous roots of grasses are the most effective in augmenting 

 water content, but the deeper-seated ones of perennial forbs and woody 

 plants operate especially in connection with percolation. Humus is 

 likewise a major agent in fostering absorption, and its action is prac- 

 tically universal; even in the initial stages of the xerosere, its small 

 but cumulative effect is decisive. 



INIuch more obvious, but greatly restricted in extent, are the conse- 

 quences arising from the disturbance of the soil by rodents in particu- 

 lar. Not only do the mounds of loose dirt affect the amount of water 

 that enters and leaves the soil, but the openings and tunnels exert a 

 further action, especially on slopes. It seems to be a common assump- 

 tion that the reactions of burrowing rodents increase erosion by sup- 

 plying loose material and in particular by the wearing of tunnels into 

 gullies. However, constant search for evidence of the latter effect has 

 disclosed but a few doubtful instances, while on the other hand there 



