108 The Nitrogen Problem, in Crop Production. 
tares, lupines, &c. — stand out in sharp contrast with all others. 
They do not depend for their growth on a supply of ready-made 
nitrogen compounds, but can obtain all the nitrogen they want 
from the air. In fact, they actually enrich the soil, because the 
roots and stubble, when decayed, produce valuable nitrogenous 
plant food. The following Rothamsted experiment shows that 
a clover crop will both yield more nitrogen to be sold off or 
fed, and also leave more quickly available nitrogen behind it, 
than a corn crop : — 
Wheat Glover 
Lb. of nitrogen in crop removed .... 12 27 
Lb. of quickly available nitrogen ( i.e in the form 
of nitrates) in the top 9 inches of soil remaining 
after the crop is removed ..... 52'4 145 - 7 
But the most convincing evidence is furnished by the 
Agdell field experiment. Here the rotation clover, wheat, 
roots, barley, is tried against fallow, wheat, roots, barley, 
and the beneficial effect of clover on the succeeding crops is 
well seen in Table II. 
Table II . — Effects of Clover on subsequent Crops in the 
Rotation , average 1884-1907 ( omitting 1898-1901, when 
Beans replaced Clover in the Rotation). 
Potash and Superphosphate Supplied, but no Nitrogenous Manures. 
1st crop, wheat 
2nd crop, 
swedes 
3rd crop, barley 
Grain 
Straw 
Grain 
Straw 
Bushels 
Cwt. 
Owt. 
Bushels 
Cwt. 
Yield after clover 
36-6 
339 
189 
19'8 
128 
Yield after fallow 
28 1 
263 
151 
141 
9 3 
Increase due to clover 
8-5 
7-6 
38 
5'7 
35 
Percentage increase 
30 
29 
25 
40-4 
37-5 
In this experiment the swedes are all drawn off ; had they 
been fed on the land, the barley would have shown a still 
further increase. 
No nitrogenous manure is supplied to this plot, but both 
potash salts, and phosphates are given. Another experiment 
shows that the beneficial results of clover depend on an 
adequate supply of these substances ; reference to Table III. 
shows that, on the unmanured plot, clover scarcely increases 
the yield of wheat and depresses the yield of barley and 
swedes, particularly the latter. 
