150 Mineral Nutrition of Plants 



in general agreement with those obtained by Metzger and Hide (50) 

 and by Weldon and Hide (74) who found, respectively, that organic 

 carbon, and carbon and nitrogen content, were distinctly higher in 

 well-aggregated than in poorly aggregated fractions separated from 

 some Kansan soils. The percentages of carbon and nitrogen present, 

 moreover, were found to decrease with a decrease in the size of aggre- 

 gates except for the colloidal fraction, which was high in both elements. 

 Applications of manure to alfalfa grown on one of the soils caused in- 

 creases in the carbon content which were greater in the well-aggregated 

 fraction. 



These results differ from those of Clarke (31) who examined the 

 aggregation and total nitrogen content of certain South Australian soils 

 and concluded that nitrogen content is not significantly related to and 

 has no significant effect on water-stable aggregation. 



Breazeale and McGeorge (ij) in one of a series of investigations on 

 soil structure made by the soils group in Arizona, examined the effect 

 of puddling upon microbial nitrogen transformations in some alkaline- 

 calcareous soils. Puddling was brought about by soil manipulation, by 

 means of an electrical vibrator, at the moisture equivalent. Nitrification 

 was found to be completely stopped by such treatment during subse- 

 quent incubation, and denitrification became active. When nitrogen 

 was originally added either as sodium nitrate or ammonium sulfate, 

 a greater loss of nitrogen was observed from the puddled than from 

 the unpuddled soil and the loss was entirely as gaseous nitrogen. Brea- 

 zeale and McGeorge state that dry fallowing fully restores the nitri- 

 fying power of a puddled soil. Some benefit is obtained by the addition 

 of a dust mulch to the surface of puddled soils. This is explained by up- 

 ward movement, when the soil is moist, of dissolved ammonia into the 

 layer of loose soil, and also by the somewhat more friable condition 

 developed by the slower drying of the puddled soil itself. Both effects 

 produce a condition more favorable to microbial nitrification. 



In experiments upon the decomposition of organic matter in puddled 

 and unpuddled Gila clay loam, McGeorge and Breazeale (5/) found 

 that addition of dry and rotted alfalfa to unpuddled soils resulted in 

 excellent growth of barley seedlings. No nitrogen deficiency was ap- 

 parent. Seedlings on the puddled soil made poor growth, were yellow, 



