AVAILABLE NITROGEN OF THE SOIL. 283 
ing of their indirect nutritive influence upon vegetation. 
By these chemical transformations the organic nitrogen 
may pass into the two compounds which, in the present 
‘state of knowledge, we must regard as practically the ex- 
clusive feeders of the plant with nitrogen. The rapidity 
and completeness of the transformation depend upon 
circumstances or conditions which we understand but im- 
perfectly, and which are extremely important subjects for 
further investigation. 
Difficulty of estimating the Available Nitrogen of any 
Soil.—The value of a soil as to its power of supplying 
plants with nitrogen isa problem by no means easy to 
solve. The calculations that have just been made from 
the analytical data of Boussingault regarding the soil of 
his garden are necessarily based on the assumption that 
no alteration in the condition of the nitrogen could take 
place during the period of growth. In reality, however, 
there ig no constancy either in the absolute quantity of 
nitrogen in the soil or in its state of availability. Por- 
tions of nitrogen, both from the air and from fertilizers, 
may continually enter the soil and assume temporarily the 
form of insoluble and inert organic combinations. Other 
portions, again, at the same time and as continually, may 
escape from this condition and be washed out or gathered 
by vegetation in the form of soluble nitrates, as has al- 
ready been set forth. It is then manifestly impossible to 
learn more from analysis, than how much nitrogen is avail- 
able to vegetation at the moment the sample is examined. 
To estimate with accuracy what is assimilable during the 
whole season of growth is simply out of the question. 
The nearest approach that can be made to this result is to 
ascertain how much a crop can gather from a limited vol- 
ume of the soil. 
Bretschneider’s Experiments.—W e may introduce here 
a notice of some recent researches made by Bretschneider 
in Silesia, a brief account of which has appeared since the 
