20 SOIL CONDITIONS AND PLANT GROWTH 



Crop per acre. 



Bushels. 

 Normal manure . .* , . "/.. -.,.. . . . . 43 



Manure without lime . ; . ... . . .41 



,, ,, potash . . . . . . .31 



,, phosphate . ... . . . 26^ 



,, nitrogen . 14 



Soil without manure ........ 12 



Other experiments of the same kind showed that nitrogen 

 was the dominant for all cereals and beetroot, potassium for 

 potatoes and vines, phosphates for the sugar cane. An excess 

 of the dominant constituent was always added to the crop 

 manure. The composition of the soil had to be taken into 

 account, but soil analysis was no good for the purpose. In- 

 stead he drew up a simple scheme of plot trials to enable 

 farmers to determine for themselves just what nutrient was 

 lacking in their soil. His method was thus essentially 

 empirical, but it still remains the best we have ; his view that 

 chemical manures are always better and cheaper than dung is, 

 however, too narrow and has not survived. 



The second controversy dealt with the source of nitrogen 

 in plants. Priestley had stated that a plant of Epilobium 

 hirsutum place'o r ln a small vessel absorbed during the course 

 of the month seven-eighths of the air present. De Saussure, 

 however, denied that plants assimilated gaseous nitrogen. 

 Boussingault's pot-experiments showed that peas and clover 

 could get nitrogen from the air while wheat could not (45), 

 and his rotation experiments emphasised this distinction. 

 He himself did not make as much of this discovery as he 

 might have done, but Dumas (88) fully realised its importance. 



Liebig, as we have seen, maintained that ammonia, but 

 not gaseous nitrogen, was taken up by plants, a view con- 

 firmed by Lawes, Gilbert, and Pugh (164) in the most rigid 

 demonstration that had yet been attempted. Plants of several 

 natural orders, including the leguminosae, were grown in 

 surroundings free from ammonia or any other nitrogen com- 

 pound. The soil was burnt to remove all trace of nitrogen 

 compounds while the plants were kept throughout the ex- 



