Needs of Soil Ventilation. 205 



and decaying organic matter in the soil into nitric acid, 

 large amounts of oxygen are needed, for each of the three 

 known forms of microscopic life which do this work are 

 unable to live in its absence. 



239. A Water-logged Soil. One of the chief reasons for 

 the unproductiveness of a water-logged soil is the deficiency 

 of free atmospheric oxygen in it. When the soil pores are 

 filled with water and this water is stationary, that is, not 

 changing, the free oxygen which it may contain in the air 

 dissolved in it is soon used up and then the rate at which 

 oxygen from the air above the soil is able to make its way 

 downward through the soil-water and around and between 

 the soil grains is much too slow to meet the ordinary needs 

 of the roots of any crop. !N"ot only this, but, as pointed 

 out in (103) ? even the microscopic organisms in the soil 

 find so scanty a supply that they are obliged to decompose 

 the nitric acid for the oxygen it contains in order to supply 

 their needs. The chief need of draining wet lands, then, 

 is to secure to the soil a more rapid change of air. 



240. Floating Gardens. The instances where the Chinese 

 and Mexicans grow crops upon floating rafts of logs an- 

 chored in a stream or lake and thinly covered with soil 

 may seem to contradict the statements in the last paragraph 

 regarding a water-logged soil because, in these cases, the 

 soil is very wet in its lower portion and the roots of the 

 plants are continually immersed in a saturated soil or in 

 the water itself beneath. A little reflection, however, will 

 make it clear that the two cases are very different. Both 

 in the lake and in the running stream the water is chang- 

 ing continually so that a new supply, charged with fresh 

 oxygen, is being continually brought to the roots or very 

 near them. 



It is the abundance of oxygen which rain water and 

 that used for irrigation contains which prevents it from 

 killing crops when the water entering the soil is excessive. 

 As long as the water is moving through the soil, and a 



