430 
On the Continuous Growth of Wheal on the 
very liberal supply of minerals has only been competent to 
increase the yield by 1^ bushels per acre per annum ! 
The average of the total produce — straw and corn — of the 
mixed minerals' plot for the 32 years, has amounted to 2421 lbs., 
and of the unmanured plot to 2090 lbs., a difference of 331 lbs. in 
favour of the minerals. The amount of nitrogen in this 331 lbs. 
would be not more than 3 lbs. ; this represents the whole of the 
nitrogen which the wheat upon an acre of land — though fur- 
nished with an abundance of minerals — has been able to obtain 
from the soil and atmosphere in excess of that obtained by the 
wheat grown without manure ! 
In order to explain the causes which have produced these 
crops, it will be necessary to show what has taken place in the 
soil ; but before doing so, it may be as well to give a slight 
review of the crops themselves. 
We find the two crops running a parallel course, showing 
great differences in their yield as the seasons are favourable or 
unfavourable, but rarely differing from each other more than 
from 3 to 4 bushels per acre. The yield in both is slowly de- 
clining, as we find that during the first 8 years the unmanured 
produce gave, in two separate years, a crop of 20 bushels per 
acre ; and the mineral-manured plot on three occasions yielded 
a crop exceeding 20 bushels per acre. For the last 23 years, 
neither plot has given a produce of 20 bushels, and it is hardly 
possible that, without some change in the manures applied, a 
crop of this size can ever be grown again. 
The Rothamsted soil — like a great many cultivated soils — 
contains a large amount of the mineral food of plants ; it also 
contains organic nitrogen, that is to say, nitrogen in combina- 
tion with carbon, the residue of previous vegetation. This 
organic nitrogen does not appear to be available as food for the 
wheat plant, but every year a certain amount of it is converted 
into nitric acid, which combines with the lime in the soil. In 
this state it is very soluble in water, is readily washed out of the 
soil by heavy rain, and, further, is a most important and essential 
food of the wheat plant. 
The amount of nitric acid formed each year will vary, the 
formation being most rapid in the hottest weather, provided the 
soil is sufficiently moist. The amount of nitric acid which the 
wheat crop can take up will also vary, and in a cold and wet 
winter much will be washed beyond the reach of the roots of the 
plant. 
These facts, which are of universal application, enable us to 
explain some of the causes which tend to the production of 
good or bad crops of wheat. Analyses of the soil of these two 
