156 Mineral Nutrition of Plants 



tion on nitrification and the growth of sugar beets. They found that 

 in every case compaction decreased beet yields, and artificial aeration 

 tended to overcome the harmful effects of compaction, indicating that 

 low aeration was limiting yields. They also found that compacted soil, 

 in which large amounts of nitrogenous material had been incorporated, 

 had been prevented from rendering these nitrogenous substances avail- 

 able as nitrates, at least to a level comparable with that which might 

 normally be expected and which was demonstrated on the same soil 

 that had not been compacted. 



Trogdon (68) found in a study of effectiveness of various nitrogen 

 fertilizers that maximum utilization of added fertilizer elements was 

 not obtained if air space porosity was low. Wherever soil porosities 

 were low, tests showed the presence of reducing conditions in the soil 

 throughout much of the two rather wet seasons and definite phos- 

 phorus and nitrate deficiency symptoms were observed even where 

 700 pounds of ammonium sulfate and 800 pounds of 0-10-10 fertilizer 

 had been applied. 



It is, of course, well known that oxygen is required for the produc- 

 tion of nitrates, for the fixation of nitrogen, and, in fact, for the con- 

 tinued activity of most of the important types of microorganisms in 

 the soil. A fact which is not so widely considered, however, is that the 

 requirement for oxygen by microorganisms which are actively decom- 

 posing organic matter in the soil is comparatively high. It appears very 

 likely, in fact, that the commonly observed depression in plant growth 

 immediately after adding large amounts of green manure or other 

 organic matter to a soil may be due more to competition between the 

 higher plants and the microorganisms for the limited supply of oxygen 

 than to any other single factor. The full significance of this point re- 

 mains to be worked out, but it is worthy of further study. Trogdon 

 (68) found that, where large quantities of fresh organic matter were 

 added to soil in contact with fertilizer banded below the surface, there 

 was a reduction in yield as compared to no organic matter. This was 

 apparently associated with deficient oxygen supplies since definite nit- 

 rogen, phosphorus, and potassium deficiency symptoms were observed 

 in the plants and reducing conditions could be demonstrated in the soil 

 near the organic matter. It is likely that some of the peculiar results 



