Page and Bodman 157 



sometimes observed in glass-house vegetable production can be ex- 

 plained as due to temporary partial reducing conditions brought about 

 by turning under large amounts of organic matter where temperature 

 and moisture conditions favor rapid decomposition. 



Where reducing conditions are developed in the soil either through 

 additions of large amounts of organic matter, through waterlogging, or 

 compaction, or a combination of these factors, it is, of course, well 

 known that important chemical changes are produced in the system. 

 Under reducing conditions, carbon dioxide is usually reduced to me- 

 thane; nitrates to nitrites, ammonia, or free nitrogen; sulfates to hydro- 

 gen sulfide; ferric to ferrous iron; and trivalent to divalent manganese. 

 There is considerable evidence that most of the reduction is brought 

 about by microorganisms and that the presence of readily decomposable 

 organic matter greatly favors the development of reducing conditions 

 wherever oxygen supply might be low. In most soils, however, every 

 effort is, or should be, made to prevent the occurrence of extreme re- 

 ducing conditions, so that too great emphasis on the nature of the 

 chemical status of the reduced soil is probably not warranted for this 

 discussion. 



Of more significance is the effect of even a partial reduction in the 

 supply of oxygen available to plant roots, caused either through utiliza- 

 tion of available oxygen by microorganisms, or through reduction in 

 the rate of supply through the soil pores. Since oxygen is necessary for 

 normal root functioning and absorption of water and nutrients, any 

 process which significantly reduces the supply will have an adverse 

 effect on root growth and availability of nutrient elements. 



MECHANICS OF SOIL AERATION 



The work reported in the preceding sections indicates that nutrient 

 uptake and availability, particularly of potassium, is affected by soil 

 aeration. As was mentioned earlier, the detailed mechanics of soil aera- 

 tion are not well understood, but workers in the field are quite well 

 agreed that practically all aeration occurring in the soil results from gase- 

 ous interchange by diffusion. Oxygen diffuses into the soil and carbon 

 dioxide diffuses out, simultaneously. Factors such as wind, barometric 

 pressure changes, or flushing by light rains or irrigations, apparently 



