158 Mineral Nutrition of Plants 



contribute little to the movement of soil gases. In the past, attempts 

 at studying soil aeration have been directed mostly toward studying 

 partial pressures of oxygen in the soil air, by measuring porosity 

 (usually at a standard moisture content) or by studying permeability 

 of soil to gases under a pressure differential. Each of these methods has 

 distinct shortcomings. Difficulties in measuring oxygen have already 

 been mentioned, but such work should be continued making use of 

 the newly-available oxygen analyzers. Determination of porosity gives 

 at best an indirect measure of aeration of a soil, particularly if all 

 measurements are made at some standard moisture condition, since 

 this exact moisture condition may only occur in the field rarely and, 

 hence, cannot indicate the aeration conditions under which the plants 

 are living. An improvement in this measurement is made possible by 

 measuring porosity at field moisture with the pressure pycnometer (52). 

 With this instrument it is possible to measure the pore space actually 

 filled with air at the moisture condition prevailing in the soil at the 

 time of sampling. Another shortcoming of porosity measurements is 

 that the effectiveness of air pore space in the soil is limited by the de- 

 gree to which there are open channels or continuous pores to and 

 through the surface so that free diffusion can occur. It is easy to see 

 how a soil with apparently very favorable porosity in the root zone 

 could be quite unsatisfactory for root growth if the surface were sealed 

 through formation of a crust. Possibly the most important benefit of a 

 mulch comes through its action in maintaining porosity unblocked 

 or sealed through to the surface, so that free diffusion into the soil can 

 occur. The chief drawback to characterizing aeration in terms of air 

 permeability is that gas flowing under a pressure differential may be 

 subject to such factors as turbulent flow, friction, and others which are 

 not of significance in free diffusion. 



In recent work, Blake (9, to) has studied the diffusion process. He 

 found, as have previous workers, that there is a linear relationship be- 

 tween diffusion and porosity, but, unlike most work reported in the 

 literature, he did not find a constant factor to express the relationship 

 between porosity and diffusion. The coefficient was different for soils 

 of different properties. Thus, two soils might have identical porosities 

 when expressed on a volume basis, but, if they differ in the character of 



