284 BULLETIN OF THE BUSSEY INSTITUTION. 
appreciable influence in helping the plants to obtain support from the 
soil-nitrogen. ‘This point, it will be observed, is a distinct and separate 
question to be examined by itself. It has nothing to do with the fact 
that the traces of ‘active nitrogen in rain-water are by themselves 
wholly inadequate for the growth of an ordinary crop. Nor indeed 
has it any immediate bearing upon the subject of the present paper. 
The experiments which have been recorded above show conclusively, 
on the one hand, that the soil-nitrogen is useful to plants under certain 
conditions, such as are found in nature; while, upon the other, they 
illustrate the fact, which Wolff had proved before, that soils devoid of 
vegetable mould, or of some other compound of nitrogen, will not sup- 
port crops that can be put in comparison with those that are readily 
and constantly obtained from soils that contain peat or loam, or some 
other source of nitrogen. 
The influence exerted by the impurities in rain-water upon the growth 
of plants in soils destitute of nitrogen is really so exceedingly small that 
it is hardly worthy of mention. I have satisfied myself by many trials 
that, regarded as a source of error, the influence of these impurities is 
so minute that it may safely be neglected excepting in the most refined 
experiments made for special purposes. There is an experiment of 
Hellriegel * which well illustrates this point. In an artificial soil that 
contained every thing, except nitrogen, necessary for the support of a 
maximum crop of barley, Hellriegel got a crop that weighed 0.184 
gramme, when the pot was watered with the purest distilled water ; 
while from a precisely similar pot, that was watered with rain-water, he 
got a crop that weighed 0.200 gramme. But froma third pot, similarly 
filled, and containing in addition an amount of nitrogen, in the form of 
a nitrate, equivalent to 84 lbs. of that element in 1,000,000 lbs. of soil, — 
he obtained a crop that weighed 17.776 grammes. With regard to 
this matter, it should be noticed that the sands employed in my experi- 
ments naturally required and received more water to keep them moist, 
when they were used by themselves, than when peat or loam had been 
mixed with them. 
T have not the least doubt but that, besides supplying nitrogen to plants, 
the humus of the soil may at times serve many other useful purposes, 
some of them dependent upon its chemical properties and others upon 
* Steeckhardt’s “ Chemische Ackersmann,” 1868, 14. 18. 
