II] 



THE SOIL AND PLANT FOOD 



25 



balanced by losses. Thus limits are finally reached, 

 and no soil becomes absolutely destitute of nitrogen 

 or very rich in it; few, if any, of our British soils 

 (leaving out sand dunes and peat bogs, which are not 

 true soils) contain less than 0'05 per cent or more 

 than TO per cent of nitrogen. 



We may summarise the changes of the nitrogen 

 compounds of the soil in diagram form thus: 



Complex Nitrogen compounds 



Gaseous 

 Nitrogen 



The conversion of the complex nitrogen compounds 

 into nitrates is the process whereby plant food is 

 made, and the fixation of gaseous nitrogen is the 

 means whereby the stock of nitrogen compounds is 

 maintained. Both these are obviously indispensable 

 to plant growth and to the fertility of the soil. The 

 other change, the evolution of gaseous nitrogen from 

 complex nitrogenous compounds, appears on our 

 present knowledge to be sheer waste and to serve 



