32 Agricultural Chemistry. 



may be used directly by the plant or ultimately converted to 

 nitrates. Ammonia from this source contributes but a small 

 part of the nitrogen required by crops. 



The average amount of nitrogen brought to the soil per acre 

 yearly by rain at the Rothamsted Experiment Station, over a 

 period of 1 8 years was as follows : 



Nitrogen as nitrates and nitrites 1 . 1 Ibs . 



Nitrogen as Ammonia 2 . 6 Ibs . 



Nitrogen in organic forms 1 . Ib . 



Total nitrogen 4 . 7 Ibs. 



Obviously this supply of nitrogen falls far short of the 50 to 

 100 pounds of nitrogen per acre required by different crops. 



Solids. The solids usually present in common air are sub- 

 stances which have been taken up by the wind and remain sus- 

 pended in finely divided condition. They include bacteria, yeast 

 spores and other microscopic forms of plant life. These furnish 

 the nitrogen already referred to as brought to the soil in ' * organic 

 forms." The air contains dust particles from finely divided soil 

 and this constituent is prominent in dry regions. Spray from 

 bodies of salt water, when taken into the air by wind, evaporates 

 and leaves small quantities of salts suspended. These consist 

 principally of sulphates and chlorides of sodium, potassium, cal- 

 cium and magnesium. Salts may be returned to the soil by rain 

 in considerable amounts near the sea coast. Common salt is 

 brought to the soil in this manner at the rate of 186 pounds per 

 acre yearly at Georgetown, British Guiana ; at Rothamsted, Eng- 

 land, which is farther inland, the amount is about 24 pounds per 

 acre. Sulphur is an important element in the growth of plants, 

 and is brought to the soil by rain in the form of sulphates taken 

 up from the sea. These supplies of plant food may become im- 

 portant factors in the growth of crops. It has been estimated 

 that the chlorine in rain water at Rothamsted is sufficient for 

 crops, with the possible exception of mangels, and that the sul- 

 phur supplied in this way meets the demands of most cultivated 

 crops. This high supply of sulphur may, however, be derived 



