86 
FLORIDA STATE HORTICULTURAL SOCIETY. 
be repeated to the extent they have been, 
so that this source, when exhausted, will 
probably never be replaced. The phos- 
phatic deposits, as those through the 
southeastern portions of the United 
States are large and will give an abund¬ 
ant supply for a long time, but they are 
not inexhaustible. The source of supply 
from bones etc, which however, is insuf¬ 
ficient to supply the demand, can be count¬ 
ed upon as a steady source, and the same 
applies to slag, a by-product in iron smelt¬ 
ing. Viewed as a whole, there is noth¬ 
ing alarming in the present situation of 
the potash and phosphoric acid supply. 
In regard to nitrogen, the fact con¬ 
fronts us that we are continually using 
up this element of plant food without an 
adequate return to the soil. The constant 
cropping of the land, combined with our 
present system of sewerage disposal, 
which prevents the return to the soil of 
such a large and legitimate nitrogen sup¬ 
ply, the fact that so many crops are sold 
away from the land, are sufficient to in¬ 
dicate the loss, ■ without considering the 
destruction of nitrogen compounds by de¬ 
nitrifying bacteria, the leaching of the 
soil and other sources of loss. The 
world’s supply of the two richest sources 
of nitrogen, guano and saltpeter, is being 
rapidly exhausted. At the present rate 
of consumption of the latter, one billion 
tons a year, it will be exhausted, as claim¬ 
ed by some, in 30 years. New methods 
for the production of nitrogen compounds 
available for fertilizing purposes may be 
discovered, and some efforts have already 
been made in that direction with electrici¬ 
ty, but this does not promise sufficient as 
yet to count on it to any extent to relieve 
the situation. 
Nature has provided, to a certain ex¬ 
tent, for the return of nitrogen to the soil. 
Arising from the chemical changes going 
on, on the surface, there are always traces 
of ammonia compounds and compounds of 
nitrous and nitric acids in the air, which 
are washed down by rains. Electrical dis¬ 
charges in the air bring small amounts 
of the free nitrogen into combination so 
that it may be returned to the soil by the 
rain. In 1891 it was discovered that 
there were certain micro-organisms in the 
soil which could bring the free nitrogen 
of the air into combination without the 
aid of any other plant life. Experimen¬ 
tation on this line has been undertaken 
to make practical use of these organisms 
but so far without result. 
The fact that leguminous plants had a 
very beneficial effect on the soil is a very 
old observation and they have been used 
for this purpose without the knowledge 
of the reason for this. It was demon¬ 
strated in 1895 that there was actually 
an increase in nitrogen after such a crop. 
It was also noticed that these plants would 
grow in a soil that contained practically 
no nitrogen. Hellriegel, in 1886 an¬ 
nounced that leguminous plants derived 
their nitrogen from the atmosphere and 
in 1888 Wilfarth and Hellriegel demon¬ 
strated that the growth of such plants in 
a nitrogen-free soil, occured after the de¬ 
velopment of nodules on the roots. Pre¬ 
vious to this time, although these nodules 
had been noticed, they had been con¬ 
sidered as a diseased condition of the 
roots. When the true solution of 
the question began to appear, it was 
easily demonstrated that a leguminous 
plant, grown in a soil absolutely free 
from nitrogen, as in pure quartz sand that 
had been heated to redness, and watered 
with a solution devoid of nitrogen in any 
