centuries. We also occasionally hear of worn out soils, but it would 

 be more correct to say badly managed or spoilt soils, because there 

 is no evidence that the productivity of a soil ever declines under 

 suitable treatment. 



In tracing changes in the fertility of soil, we may content our- 

 selves with following up the changes in the amount of nitrogen 

 present, because though phosphoric acid, potash, and lime are im- 

 portant factors in plant nutrition, these elements are not susceptible 

 to the gains and losses from external operations like cultivation, 

 by which the stock of nitrogen is so greatly affected. 



There are various processes at work which will diminish or add 

 to the stock of nitrogen in the soil, and these may be summarised 

 as follows: 



1) The growth of plants simply removes some of the nitrogen 

 that has reached an available form, and if the crop is taken off at 

 harvest, there is so much direct loss to the soil. As it may also 

 be accepted that the plant itself, apart from bacterial action, neither 

 converts any of the combined nitrogen it obtains into gas, nor 

 brings into combination any of the free nitrogen of the air, there 

 is neither gain nor loss of soil nitrogen when the growth of the 

 plant is returned to the soil. 



2) Various bacteria are capable of bringing atmospheric nitrogen 

 into combination, and so increasing the stock of soil nitrogen. 

 They may either live in symbiosis with higher plants (Pseudomonas), 

 or exist free in the soil (Azotobacter, Clostndium}. 



3) Another group of bacteria in the process of breaking down 

 organic matter liberate the nitrogen in the free state, and so reduce 

 the stock of soil nitrogen. 



4) Natural drainage waters contain nitrates which have been 

 derived from the soil nitrogen by bacterial oxidation. 



5) The rain annually contributes a certain amount of combined 

 nitrogen to the soil. The amount is greater in the proximity of 

 towns; the average amount at Rothamsted is 3.84 Ib. per acre per 

 annum, and other results would show that this is a very repre- 

 sentative figure for ordinary country air. 



In practice most of these factors giving rise to gain or loss are 

 at work together ; which of them will predominate will depend upon 

 the style of farming and cultivation the land receives. Some of 

 the Rothamsted plots with their long-recorded history afford an 

 opportunity of estimating the interplay of the various factors. 



A. The simplest case to take is that of land under arable cul- 

 tivation when nothing is restored to the soil. The unmanured plot 



