iv] WHAT THE NEW CROPS DID 57 



a great deal of food, but this did not necessitate a 

 great net loss to the soil. For the carbon, hydrogen 

 and oxygen (the elements most largely retained in 

 the body tissues) came from the inexhaustible supplies 

 in the atmosphere, while of the other constituents, 

 retained to a less extent and largely passed off into 

 the manure, the nitrogen was mainly drawn from the 

 atmosphere through the agency of the clover, and 

 only the phosphorus, potassium, calcium, etc., came 

 exclusively from the soil. This manure when put 

 on to the land actually enriched it in nitrogenous 

 organic matter, and went far to replace the mineral 

 substances withdrawn by the previous year's crops. 

 So far as nitrogen was concerned, the system was 

 permanent: crop production went on at a higher 

 level than was ever before possible, and this new 

 level was determined in principle by the amount of 

 nitrogen fixed by the quadrennial clover crop, and in 

 practice by the amount that was returned to the soil 

 in the manure. 



But there still remained the loss of phosphorus, 

 which was intensified as the cities grew and imported 

 more and more dairy produce, meat, bread, etc., from 

 the country. At the end of the eighteenth century, in 

 spite of all the improvements, an ordinary yield of 

 wheat was probably only about 23 bushels, little 

 more than good farmers had been accustomed to 

 get for 250 years past. The improvements had been 



