70 AGRICULTURE 



not to be looked for. But if the seed of 

 broom or lucern be sowed in the Weald it will 

 be found that the resulting plants at once 

 provide themselves with abundance of 

 nodules, and grow vigorously. One must 

 therefore conclude that leguminous plants 

 generally find in the soil bacteria which are 

 capable of entering into association with a 

 wide range of plants, and of immediately 

 forming colonies on species, and even genera, 

 that they can never previously have en- 

 countered. 



Beside the free nitrogen, there is a certain 

 amount of combined nitrogen present in the 

 atmosphere, the actual amount varying with 

 such a circumstance as proximity to a town, 

 where the atmosphere is more or less charged 

 with coal smoke and other impurities. Some 

 of this combined nitrogen (nitric acid) has 

 been formed in the atmosphere through the 

 agency of electrical discharges, while some 

 (dust, ammonia, etc.) has a terrestrial origin. 

 These substances are carried to the earth in 

 rain and snow and so get washed into the 

 soil, where their nitrogen becomes available 

 as the food of plants. Very careful estimates 

 at various places, notably Rothamsted, have 

 been made with regard to the amount of 

 combined nitrogen that is washed out of the 



