Atmospheric Nutrition of Plants. 
537 
examined soil that had been well penetrated by air, without finding 
large quantities of nitrates present. 
So much with regard to the indirect influence of the atmosphere in 
the nutrition of plants. 
The indirect influence of the air on mineral matters, especially in 
clay-soils, is also most important. Clay-soils frequently contain an 
abundant store of mineral food, which is as essential as atmospheric 
food to the growth of plants ; but in many cases it is so locked up 
that it is of little or no avail to the growing plants, and it is only 
when the air thoroughly penetrates the soil that it becomes available. 
It is by the introduction of atmospheric oxygen and carbonic acid 
that many mineral compounds entering into the double silicates of 
alumina arc gradually decomposed, and the alkalies, lime, potash, 
soda, &o., arc rendered assimilable by plants. In consequence of this 
remarkable action, the surface of the soil becomes more porous and 
powdery ; and a proper aeration reduces a stiff clay-soil into fine 
earth, at the same time producing the food so much wanted for the 
growth of the plant, and that condition which enables the plant to 
avail itself more abundantly of atmospheric food ; for the more 
porous the soil is, the more readily it absorbs ammonia and carbonic 
acid from the atmosphere. The atmosphere supplies an abundant 
store of food in a direct way to all plants ; it also supplies nutriment 
in au indirect way by operating upon the organic matters of the soil, 
producing carbonic acid and nitrates. It operates upon the mineral 
constituents of the soil, rendering them soluble, and therefore avail- 
able as food for plants ; and lastly, while effecting all these beneficial 
changes in the soil itself, it materially improves its mechanical con- 
dition, whereby it acquires the power of availing itself more perfectly 
of the food existiug in such great abundance in the atmosphere. 
Discussion. 
Mr. Frere : You tell us that rain furnishes 22 lbs. of ammonia and 
nitric acid to the acre. You have also spoken of the nitrogen in the 
roots of an acre of clover being equivalent to 8 cwt. of guano ; for 
the purpose of connecting the two statements, will you also mention 
the number of pounds of nitrogen in 1 cwt. of guano ? 
Dr. Voelcker : Good Peruvian guano contains from 14 to 15 lbs. 
of nitrogen to the cwt. I mentioned only one example of the quantity 
of nitrogen in roots and other vegetable remains that are left in the 
soil ; but it of course varies. I am at present engaged in an inquiry 
into the growth of clover, and I shall follow it up by endeavouring to 
ascertain how far it is useful as a preparation for the wheat crop. 
Mr. Frere : I gather from these premises that a dressing of 4 cwts. 
of guano would produce about 60 lbs. of nitrogen in different forms, 
or three times as much as the rainfall will produce. 
Dr. Voelcker : Exactly so. 
Mr. Holland, M.P., in allusion to the large amount of nitrogen in 
the roots of clover, said that in growing and managing the clover crop, 
it was desirable to pay as much attention as possible to increasing the 
