266 BULLETIN OF THE BUSSEY INSTITUTION. 
their food, and are made moderately comfortable in respect to their 
physical surroundings. ‘The experiments were made in earthen 
flower-pots. Five kernels of dwarf maize (pop-corn) were planted in 
each pot; and each of the trials was made in duplicate. 
It is apparent that the experiments of Wolff really afforded a capital 
illustration of the importance of the soil-nitrogen for the support of 
plants. But, as has been said already, they were made to elucidate 
another matter ; and, at the time when they were made, it was commonly 
believed that most soils contained considerable amounts of ammonium 
compounds available for the growth of plants. So long as that belief 
lasted, there was no special motive to consider the results from the present 
point of view; and Wolff’s experiments consequently stood for no more 
than one item of proof, among many others, that plants can perfectly well 
make use of artificial supplies of ammonia. But so soon as it had 
been shown by the researches of Boussingault,* and of Knop and 
Wolf,+ that there is usually no more than an insignificant trace of am- 
monia in the soil, the experiments of Wolff acquired a new meaning, 
for they point just as clearly to the presence in the soil of a supply of 
nitrogen useful for plants, as they do to the possibility of growing 
plants in the ashes of soil to which nitrates and ammonium salts have 
been added. 
The results both of Wolff and myself agree perfectly with those 
obtained by Hellriegel } and by Lawes, Gilbert, and Pugh,§ who, for 
the sake of controlling certain experiments made by them for other 
* “ Agronomie,” 3-220. 
+ “ Landwirthschaftliche Versuchs-Stationen,” 1861, 3. pp. 109, 207. In five 
different samples of soils, Knop and Wolf (p. 209) found no more than from 
0.0001 to 0.00087 gramme of ammonia for every 100 grammes of the soil, 
regarded as absolutely dry. 
In numerous samples of rain-water (p. 120) they found from 0.00003 to 
0.0003 gramme ammonia to 100 grammes of the liquid; or, in other words, 
from 8 ten-millionths to 3 millionths. In the locality where the experiments 
were made, Meeckern, near Leipzig, there was usually no more than from one to 
two millionths of ammonia in the rain-water, though occasionally the propor- 
tion rose to three-millionths. 
In dew and hail they found two-millionths of ammonia, and in snow from one 
to three-millionths. Well-water usually contains no ammonia, and the waters 
of rivers and ponds contain rather less than rain-water (p. 127). 
t Hoffmann’s “ Jahresbericht,” 1861, 4. 112. 
§ “Philosophical Transactions of the Royal Society of London,” 1861, 151. 
pp. 484, 535, 557, 574, and figure. 
