IX.] 



CAUSES OF STERILITY 



241 



Outside the ring 

 On the ring 

 Inside the ring . 



Carbon per 

 cent. 



3-30 

 2.99 

 2.78 



Nitrogen per 

 cent. 



0-28l 

 0266 

 O.247 



Nitrates per 

 million. 



2.44 



11*46 



1-03 



It will be seen that the unchanged soil outside con- 

 tains the most carbon and nitrogen; the ring itself 

 contains an intermediate amount, and the least is 

 contained within the ring after the luxuriant vegeta- 

 tion has passed away. The soil on the ring is in high 

 condition, because the organic residues it contains are 

 recently formed and will change rapidly; after they 

 have been cropped out, the land is less able to support 

 a crop, even though there is still much plant food left 

 in the soil. The last column in the table (the analysis 

 of a single example only) shows the difference in avail- 

 able nitrogen ; and though in a pasture there are never 

 many nitrates to be detected, so rapidly are they seized 

 upon by the crop, still the organic nitrogen compounds 

 in the soil must be in a more nitrifiable condition on 

 the ring to yield the results there shown. Doubtless an 

 investigation of the nature and distribution of the 

 bacteria and micro-fungi in and about a fairy ring would 

 throw further light on the varying fertility of such closely 

 neighbouring areas of soil, but no data are at present 

 available. 



Sterility of Soils, 



Few soils occurring in this country can be described 

 as absolutely barren, yet from time to time land is met 

 with which yields such poor crops that it may fairly be 

 designated as sterile. The causes of sterility are 

 various; amongst them may be enumerated both the 



Q 



