14 SOIL CONDITIONS AND PLANT GROWTH 



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 placed in a 

 small vessel absorbed during the course of the month seven-eighths 

 of the air present. De Saussure, however, denied that plants assimi- 

 lated gaseous nitrogen. Boussingault's pot-experiments showed that 

 peas and clover could get nitrogen from the air while wheat could not 

 (4S) and his rotation experiments emphasised this distinction. He 

 himself did not make as much of this discovery as he might have done, 

 but Dumas (87) fully realised its importance. 



Liebig, as we have seen, maintained that ammonia, but not gaseous 

 nitrogen, was taken up by plants, a view confirmed 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 

 compound. The soil was burnt to remove all trace of nitrogen com- 

 pounds while the plants were kept throughout the experiment under 

 glass shades, but supplied with washed and purified air and with 

 pure water. In spite of the ample supply of mineral food the plants 

 languished and died : the conclusion seemed irresistible that plants 

 could not utilise gaseous nitrogen. For all non-leguminous crops this 

 conclusion agreed with the results of field trials. But there remained 

 the very troublesome fact that leguminous crops required no nitro- 

 genous manure and yet they contained large quantities of nitrogen, 

 and also enriched the soil considerably in this element. Where had 

 the nitrogen come from ? The amount of combined nitrogen brought 

 down by the rain was found to be far too small to account for the 

 result. For years experiments were carried on, but the problem re- 

 mained unsolved. Once again an investigation in agricultural chemistry 

 had been brought to a standstill for want of new methods of attack. 



The Beginnings of Soil Bacteriologfy. 



It had been a maxim with the older agricultural chemists that 

 " corruption is the mother of vegetation ". Liebig had taught that 

 nitrogenous organic matter decayed in the soil by a chemical pro- 

 cess " eremacausis " with formation of ammonia, the essential nitro- 



