278 CAUSES OF FERTILITY AND STERILITY [chap. 



be as far as possible a tenant's asset, to be bought by 

 him on entry and valued to him on leaving. The 

 difficulty which even an experienced man finds in 

 putting a value on so intangible an item makes it 

 almost impossible to assess the condition of a farm, 

 but it is desirable in every way that the outgoing 

 tenant should be encouraged to maintain the condition 

 of his farm by giving him due compensation for the 

 unexhausted value both of manures and foods used 

 in the latter years of his tenancy. 



" Fairy Rings" 



The significance of " condition " and its dependence 

 upon a supply of recently decayed organic matter is 

 well seen in the development of " fairy rings " in 

 pastures. " Fairy rings " are circles of dark - green 

 grass, common enough in poor pastures, which are 

 found to extend their size every year, leaving the grass 

 within the ring of a lighter colour and of generally 

 poorer aspect than that outside. On examining the 

 soil immediately outside a ring, it is found to be full of 

 the mycelium of one or two common species of fungi, 

 but the mycelium rarely occurs in the soil beneath the 

 ring itself, and never in that within the ring. The 

 ring appears to be dependent on the growth of 

 the fungus, which starts at one point and draws 

 upon the humus reserves contained in the soil. 

 Having consumed whatever humus is available, the 

 mycelium must proceed into the annular area of soil 

 immediately round the first patch, thus from year to 

 year it spreads outward. After the death of the 

 fungus, there is left behind in the soil it has just 

 occupied a quantity of organic matter, which readily 

 decays and becomes available for plant nutrition ; 

 thus a ring of luxuriant vegetation immediately 



