REACTIONS OX LAND 89 



by plants, probably does not occur in nature. A temporary decrease 

 may follow a season of luxuriant growth, such as is noted above after 

 a burn or in consequence of exceptionally favorable climatic condi- 

 tions. Even in the case of crops, the amount absorbed each year may 

 be a very small part of the total present, so that cultivation for long 

 periods may produce no appreciable deficiency (Hall, 1905). 



The formation of heath sand probably furnishes an example of 

 reduction in nutrient content as a consequence of the formation of 

 acids by humus. These render the mineral nutrients soluble, and 

 they are then removed by the percolating water, beginning at the top. 

 A somewhat similar process takes place in sandy soils in regions of 

 high rainfall, the leaching action of the rain apparently being pro- 

 moted by the acids derived from partial decomposition. 



Reaction upon Air Content. The amount of air in the soil is known 

 to bear an inverse relation to the water content, decreasing as the 

 latter rises, and the reverse. It increases markedly as a result of all 

 animal activities that disturb and loosen the soil, and particularly so 

 by reason of the fact that these are so often accompanied by tunnel- 

 ing. The loosening effect of plant roots and rhizomes also promotes 

 the entrance of air in some measure. 



The chief response of plants and of soil animals is to modify the 

 composition of soil air. This involves the absorption of oxygen and 

 the release of carbon dioxide. Since the density of the medium pre- 

 vents ready exchange with the atmosphere above the soil, the air 

 content is regularly lower in oxygen and higher in carbon dioxide 

 than ordinary air. In soils neither wet nor packed, the reduction of 

 oxygen as a consequence of the respiration of roots is rarely serious, 

 though this is not at all true of waterlogged soils. On the other hand, 

 carbon dioxide diffuses less readily through the pores of the soil and 

 may accumulate to a harmful extent in the deeper layers of many 

 compact soils, though the greatest quantity is to be found in w^et soils. 



Reaction by increasing the amount of soil oxygen is a property of a 

 number of blue-green and yellow-green algae that grow in the upper 

 layer of moist or wet ground, or even on the latter, but the effect is 

 probably too limited to influence any but the minute organisms of the 

 soil. 



Reaction in Terms of Acids and Toxins. A voluminous literature 

 has grown up around the moot questions of bog xerophytes, toxic 

 exudates, and soil toxins, though the questions themselves have by 

 this time been answered mostly in the negative. It has been shown 

 that the effect of bog water can be largely explained on the basis of 

 deficient aeration, and that the evidence for the secretion of toxic 



