74 
The Food of our Agricultural Crops. 
established the fact that, under these conditions, no increase of 
nitrogen in the plants takes place. 
In Germany, in 1883, Hellriegel commenced some experi- 
ments with various plants in pots : he used washed sand with 
mineral manures, adding to some, but not to all, combined 
nitrogen, generally as a nitrate, and found that, with most of the 
plants, the increase of growth was in proportion to the amount of 
nitric acid supplied, but that the leguminous plants did not 
derive the same benefit from the nitrogenous manures as did the 
others. He then applied to his plants an extract from a fertile 
soil, and found that some of the leguminous plants, but not all 
of them, grew luxuriantly under this treatment : lupins, for 
instance, would not grow with the soil-extract. He then took 
an extract from a soil where they were growing luxuriantly, and 
obtained good growth. Again, if he sterilised the soil-extract 
so as to destroy the organisms in it, it ceased to be beneficial. 
Hellriegel found that the plants contained much larger quanti- 
ties of nitrogen than there were in the soil-extract, while the 
amount in the plants was much too great to be attributed to 
errors of analysis. These experiments were so interesting and 
important that we considered it desirable to carry out some 
of a similar character, and this we have been doing for the last 
two years, confirming in every respect the results obtained by 
Hellriegel. 
It appears probable that although the leguminous plants 
do not directly utilise the free nitrogen of the atmosphere, they 
do in some way obtain nitrogen from it through the medium of 
lower organisms, the development of which is, somehow, con- 
nected with the growth of the leguminous plants. The question 
which I now propose to consider is the bearing which these ex- 
periments have upon practical agriculture. Are we to suppose 
that the large quantity of nitrogen which these plants are known 
to contain has its origin in the atmosphere, and not in the soil ? 
This is a very important question, as the opinion has already 
been expressed that we need not trouble ourselves about the 
source of the nitrogen in our leguminous crops, as it all comes 
from the atmosphere. Granting that the experiments recorded 
are perfectly trustworthy and correct — that, in the absence of 
nitrogen in the sand in which the seed is sown, and with a very 
minute amount of nitrogen supplied in the extract from a 
fertile soil, a gain of nitrogen in the plant takes place, we have 
still to inquire whether Ihe same process goes on in an ordinary 
agricultural soil which contains nitrogen in more or less abun- 
dance. 
In 1860, we published in this Journal some experiments 
