Nov. 1896.] BOTANICAL SOCIETY OF EDINBUKGH 9 



Seeing that the free nitrogen of the air was regarded by 

 the highest authorities to be unavailable for plant nutrition, 

 it became necessary to cast about and find what stores of 

 combined nitrogen were available in the world. 



I have already referred to the ammonia, which, as 

 carbonate of ammonia, is a constant though minute con- 

 stituent of the atmosphere. Its amount has been often 

 estimated, and the estimates show extraordinary variations, 

 from 1 part in twenty millions to as much as oh parts in 

 one million ; depending on the locality where the sample of 

 air was taken. Over the land it is more than over the sea; 

 and it is greatest near towns where coals are being burned, 

 and in places where organic matter is decaying. According 

 to Angus Smith the ammonia in its rain-water over England 

 is just about 1 part per million; in Scotland it is only half 

 as much. In towns in England it is 5 parts on an average; 

 in Scottish towns it is somewhat less, but in Glasgow it is 

 9 parts per million. It is very soluble in water, and is 

 washed out of the air by rain. After thunderstorms there 

 is also found nitric and nitrous acids, or their salts, in the 

 air, and these too are washed down by rain. Iiain-gauges 

 at various observatories in Europe, notably at Eothamsted 

 in England, have been in use for many years, to determine 

 not only the quantity but also the quality of the rain that 

 falls throughout the year ; and the total combined nitrogen 

 brought to the earth by them has been found to be some- 

 where between 4 and 10 lbs. per acre. That is a very 

 welcome addition to the nitrogenous food of the soil; but it 

 forms only about one-tenth of what is removed by a 

 moderate cereal crop, and is insufficient to recoup the soil 

 for the loss which it is constantly incurring by drainage. 

 Always there is nitrogen in some form of combination 

 flowing down the rivers to the sea, and the store of it on 

 the land is being diminished. It forms an important 

 manurial constituent for the nourishment of seaweeds, and 

 these again are the food of fishes, many of which are 

 brought back to the land to be consumed as food : but any 

 such restoration goes but a very little way in making good 

 the drain of nitrogen in some form of combination which 

 the land is constantly suffering. 



Still more serious is the loss of combined nitrogen from 



