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These yields were obtained in areas which had received no fertilizers 

 for several years. Yields as low as 5 bushels have been obtained from 

 plots where soil structure has been allowed to deteriorate. Fertilizer 

 application at the rate shown in Table III raised the yield on the cor- 

 responding plot in that year to only 1 1 bushels. The soils under different 

 rotations show very great differences in physical properties, especially 

 as regards drainage. The conditions of soil and crops shown in Figure 

 1 and the measured physical properties including porosity, aggregation, 

 and diffusion all correlate well with yields, indicating that the yields 

 are definitely limited by poor soil aeration. On the continuous corn plots 

 the corn is very poor and shows many deficiency symptoms for the 

 various nutrients even though these plots regularly receive 150 pounds 

 per acre of 0-20-20 and 300 pounds of 20-0-0. In this case physical 

 properties of a soil definitely limit the growth of the plants and the 

 over-all availability of the plant nutrients, and addition of fertilizers 

 does not raise the yield to a level where farming would be profitable. 

 These studies furnish a striking example of the inadequacy of a strictly 

 chemical approach in studies on crop production and emphasize the 

 close interrelation between all plant growth factors, physical as well as 

 chemical. 



To summarize, oxygen or proper soil aeration is definitely needed 

 for normal root growth, and for normal absorption of nutrients by roots. 

 Nutrients may be abundant and available as shown by chemical tests, 

 but largely unavailable to plants if the soil air supply is not adequate. 

 Soil structure is the governing property which not only controls air 

 supply but also affects the growth of roots as they penetrate the soil. 

 Large amounts of organic matter, added for the purpose of improving 

 soil structure, may cause temporarily harmful rather than beneficial ef- 

 fects through production of reducing conditions in the soil. Thus, soil 

 structure must be taken into account in interpreting the results of fer- 

 tilizer trials or applications, and at the same time it must be realized 

 that soils can be improved not alone through addition of fertilizers but 

 through improvement of their physical properties as well. 



