CONTRIBUTIONS FROM THE WISLEY LABORATORY. 185 
agree rather with those obtained on the heavy Rothamsted soil by 
Hall. This, however, certainly cannot be taken as definitely refuting 
Voelcker's conclusions, for his experiments extended over a number 
of years, and the author's so far cover only one season. Moreover, 
the winter and spring of 191 9-1 920 were abnormally wet, so that 
the conditions supposed by Voelcker to have acted against the full 
utilization of the extra nitrogen in the leguminous crop, i.e. an 
insufficient supply of moisture, or some allied effect, can hardly be 
supposed to have occurred in this experiment ; it may well be that 
the author's results will be reversed in a dry season. There can, 
however, be no doubt that under suitable moisture conditions the 
more nitrogenous legumes are usually better green manures for 
immediate effect than the less nitrogenous non-legumes, even on 
light soils, but until the Wisley trials have been repeated for several 
years on the same plots it must be left an open question whether 
the more lasting effects in building up the soil humus and improving 
the texture of the soil are similarly related. 
It must also be pointed out that Voelcker's experiments differed 
from the author's in another important respect. The green crops 
at Woburn were grown during the spring and summer, and turned 
under in early autumn, their effect being measured by the yield of 
winter wheat harvested in the succeeding year, whereas the author's 
green crops were autumn-sown, and were turned in before spring- 
sown roots. The methods of green manuring in the Woburn and 
Wisley experiments were thus so different as to make comparison of 
the results a matter of some difficulty. 
That the relative values of different green crops in the author's 
experiments were determined in no small degree by the amounts of 
nitrogen in them, is strongly indicated by the results shown in 
Tables X. and XL, where the amounts of nitrogen added to the 
soil by turning under above-ground portions of the different green 
crops are shown, based on analysis of samples of these crops, the 
yields of turnips being also again shown for comparison. 
Table X. 
Spring Section. 
Amounts of nitrogen added to the soil by turning under green crops. 
Nitrogen in Green 
Yield of Turnips 
Green Crop. 
Crops. 
(Roots). 
lb. per acre (mean). 
Tons per acre (mean) . 
Crimson Clover 
182 
io-5 
Vetch 
112 
9*7 
Red Clover . 
67 
9*3 
Rye 
73 
8-6 
Rape .... 
58 
6-4 
Turnips 
48 
5-7 
Mustard 
20 
5-9 
Black Medick 
11 
4'4 
VOL. XLVH. o 
