288 HOW CROPS FEED. 
most care, unavoidably remain in a soil from which grow- 
ing vegetation is removed. The regular and great increase 
of total nitrogen in the vetch plot was certainly due in 
part to the abundance of leaves that fell from the 
plants, and covered the surface of the soil. But this ni- 
trogen, as well as that of the standing crops, must have 
come from the atmosphere, since the soil exhibited no 
diminution in its content of this element. 
We have here confirmation of the view that ammonia, 
as naturally supplied, is of very trifling importance to 
vegetation, and that, consequently, nitrates are the chief 
natural means of providing nitrogen for crops. The fact 
that atmospheric nitrogen becomes a part of the soil and 
enters speedily into organic and inert combinations, also 
appears to be sustained by these researches. 
Quantity of Nitrogen needful for Maximum Grain 
Crops.—Hellriegel has made experiments on the effects 
of various quantities of nitrogen (in the form of nitrates) 
on the yield of cereals. The plants grew in an artificial 
soil consisting of pure quartz sand, with an admixture of 
ash-ingredients in such proportions as trial had demon- 
strated to be appropriate. All the conditions of the ex- 
periments were made as nearly alike as possible, except as 
regards the amount of nitrogen, which, in a series of eight 
trials, ranged from nothing to 84 parts per 1,000,000 of soil. 
The subjoined Table contains his results. 
EFFECTS OF VARIOUS PROPORTIONS OF ASSIMILABLE NITROGEN 
IN THE SOIL. 
Nitrogen in| Yield of Grain, in lbs. 
1,000,000 
lbs. of soil. Wheat. Rye. | Oats. 
Found | Calculated | Found | Calculated | Found | Calculated 
0 0.002 —— 0.218 — 0.380 — 
Increase Increase Increase 
q 0.553 0.926 0.832 0.966 0.929 1.168 
14 1.708 1.851 1.944 1.9838 2.605 2.336 
21 2.767 MAT Grieg 2.669 2.899 8.845 8.503 
28 3.763 3.708 4.172 3.866 6.211 4.671 
42 6.065 5.554 5.162 , 5.798 7.039 7.007 
56 7.198 7.406 7.163 %.732 9.052 9.342 
84 9.257 9.257 8.698 8.698 9.342 « 9.342 
