428 On the Continuous Groioth of Wheat on the 
crops — a yield which differs very little from the average of 
some of the great wheat-growing countries of the world : the 
yield of the United States, India, and China being, it is stated, 
from 12 to 13 bushels per acre. 
Now the Rothamsted soil certainly contains a very much less 
stock of fertility than the soils upon which Avheat is grown in 
other countries ; it is therefore impossible to attribute the com- 
paratively large yield in our experimental crops to any other 
cause than to the clean state of the land. The amount of food 
at the disposal of the plant is small, but it is not shared to 
any great extent with other plants. 
By way of illustration of what may occur where the land is 
not kept clean, we may mention that last autumn a portion of 
the crop at the top of our wheat field was left standing and 
allowed to shed its seeds, the soil not being disturbed in any 
way. About 14 bushels per acre fell upon the land, and up to 
a certain period the self-sown wheat was fairly plentiful. By this 
time, however, — less than one year — the weeds have almost 
destroyed the crop ; and if the seed is left to sow itself again, 
it is very probable that every plant will be driven out before 
next summer. 
The large produce of both wheat and barley upon the un- 
manured land in the Woburn experiments, also shows how much 
the crops grown upon the ordinary cultivated land of the country 
are reduced by weeds. It is true that weeds do not exhaust a 
soil, as, in their decay, the fertility which they have taken up 
becomes again available ; but they take up nitric acid, which, 
during their growth, reverts to the form of organic nitrogen. 
When this occurs in regard to the soil-nitrogen it is merely 
so much nitric acid employed in growing weeds instead of 
wheat ; but when such active nitrogen as salts of ammonia, or 
nitrate of soda has been applied to the land, the loss is much 
more serious. A high price is paid for these substances, in 
consequence of the nitrogen they contain being in a very active 
form, competent to produce crop growth ; whereas if weed 
growth takes place instead, the purchased nitrogen does not 
become available as food for the crop until the weeds decay. 
' Mineral Manures loithout Ammonia. 
In our previous paper on the Growth of Wheat we were only 
able to give the produce of this plot for 12 jears, as during the 
first 8 years of the experiment manures containing ammonia 
had been used in considerable quantities. We are now able to 
give the produce of this plot for 32 years, during which period 
a liberal supply of all the necessary mineral food of the wheat 
