14 SOILS AND MANUEES 



and some kinds of bacteria, which are normally present 

 in the soil, possess the power of fixing the free nitrogen. 

 The compounds so formed undergo various changes and 

 ultimately become available to the higher plants. Certain 

 other kinds of bacteria, which are also commonly present, 

 are able to enter into a symbiotic association with plants 

 belonging to the order leguminosse, the pea and bean 

 family, and in that condition produce compounds of 

 nitrogen which nourish the plant. This, of course, is a 

 striking exception to the general rule that plants do not 

 utilise the free nitrogen of the air. It is of great import- 

 ance, and will be more fully considered later on. The 

 facts have been ascertained by numerous careful experi- 

 ments, and confirmed by observations on a large practical 

 scale. At Bothamsted, it was found that nitrogen accumu- 

 lated in pasture soils at the rate of about 50 Ibs. per acre 

 annually. In arable soils, under a four-course rotation, 

 on an average of forty years, the quantity of nitrogen 

 annually removed in the crops exceeded that supplied in 

 the manures by some 32 Ibs. per acre. The nitrogenous 

 compounds in the organic matter thus accumulated in 

 the soil (gradually undergo decomposition, the nitrogen 

 is converted into nitric acid and so becomes available for 

 the growth of plants. 



The air, therefore, is the ultimate source of all the 

 nitrogen, though plants cannot as a rule avail themselves 

 of it directly. 



Water Vapour. The proportion of water vapour in the 

 air depends upon the temperature and pressure at the time 

 and place, and is therefore extremely variable. It is 

 increased by evaporation from the soil and free water sur- 

 faces. It is diminished by condensation, whereby dew and 

 clouds are formed. Clouds do not consist of water vapour, 

 properly so-called, but of fine, suspended particles of liquid 

 water. It is necessary, therefore, when dealing with the 



