THE WORLD’S CEREAL AND FOOD-CROPS 41 
Fertiliser. Weight. Fertiliser. Weight. 
Natural phosphates 6-912 Norwegian nitrate 
Superphosphate of of lime ... . 0312 
lime ‘ Calcium cyanamide 0-542 
Basic slag . ‘ Sulphate of ammonia 1-630 
Natural guano Sulphur . 1-609 
Salts of potash . Sulphate of copper 0-166 
Chilean nitrate of 
soda . 7-432 Total of above . 33-688 
This total of a little more than 33 million ordinary 
tons, probably somewhat short of the true amount, 
will undoubtedly be greatly increased as population 
grows, and the increase—s$0 long as it can be maintained 
~_will aid agriculture correspondingly. The elements 
most needed are nitrogen, potassium and phosphorus. 
Calcium is of course abundant. 
In regard to the first, nitrogen, it may be noted that 
the total nitrogen content of the earth’s atmosphere 
is about 3920 million million tons, and inasmuch as 
there is some return to the atmosphere through the 
agency of anaerobic bacteria, it may easily be drawn 
upon to the extent per capita necessary for nitrogen 
supplies for human needs. Owing to lightning, etc., 
rain brings down—directly and indirectly—in the form 
of ammonia, nitric nitrogen and organic nitrogen, the 
amount of no less than about 300 million tons annually. 
The amounts are about as follows:—Nitrogen in the 
form of ammonia, say about 2-8 lbs., in the form 
of nitrates and nitrites about 1-2 lbs., and in the 
form of organic nitrogen about 1-3 lbs., all per 
acre per annum. [his means a little over 1% tons 
per square mile. 
1 A general survey of “Recent Progress in Soil Biology and Bio- 
Chemistry” shows that much has still to be learnt regarding the 
fixation of nitrogen in the soil, and that much existing literature on the 
subject is invalid.—See S. A. Waksman, Soi/ Science, 25, 29; 1928.