514 Mr. A. D. Hall [May 24, 



WEEKLY EVENING MEETING, 



Friday, May 24, 1912. 



Sir William Crookes, O.M. LL.D. D.Sc. F.E.S., Honorary 

 Secretary and Vice-President,, in the Chair. 



A. D. Hall, Esq., M.A. F.R.S. 



Recent Advances in Agricultural Science — The Fertility of 

 the Soil 



From an ordinary coramonsense point of view the fertility of the 

 soil is best defined as that property for which a man pays rent— 

 the property which causes some land to let for £2 or £3 an acre, 

 whereas the adjoining land may be dear enough :it 10s. With 

 the causes of this fertility I do' not propose to deal at any great 

 length this evening more than to indicate that it is the outcome of a 

 very complex series of factors, nmong wiiich we can enumerate the 

 actual supply of plant food in the soil,its mechanical texture as condition- 

 ing the movements of water, and the particular micro-fauna and flora 

 inhabiting the soil, for upon these lower organisms depend the facility 

 with which the material contained in the soil will become available 

 for the nutrition of the plant. For the purpose of the present argu- 

 ment it will be sufficient to fix our attention upon the amount of 

 nitrogen in the soil as the main factor determining fertility, because, 

 in the first place, nitrogen is one of the necessary and most expensive 

 elements in the nutrition of the plant, and, secondly, because its 

 amount in the soil is subject to both gains and losses from causes 

 which are more or less under the control of the farmer. The other 

 essential elements which the plant has to draw from the soil — for 

 example, phosphoric acid and potash— are only subject to slight losses 

 by solution in the drainage water, and cannot be added to except 

 deliberately by the action of the farmer ; but in the case of nitrogen 

 we have in addition to the small stock of combined nitrogen in the 

 soil the vast store of free gaseous nitrogen with which both soil and 

 plant are in contact. We may take it as settled nowadays that t!ie 

 plant itself can make no use of nitrogen gas, but must draw coml)ined 

 nitrogen in one of its simpler forms, such as nitrates or ammonia, 

 from the soil. Among the bacteria of the soil, however, there are 

 two great groups, one of which is capable of breaking up compounds 

 of nitrogen and setting free the element as gas, whereas the other can 

 take free gaseous nitrogen from the atmosphere and bring it into a 

 combined form. Which of these two groups will be more active 

 depends upon the conditions prevailing in the soil, and goes far to 

 determine both its current fertility and the length of time during 

 which it will be capable of bearing crops. 



