CONTRIBUTIONS FROM THE WISLEY LABORATORY. 187 
Table XII. 
Nitric Nitrogen in Soil ; Sampled on May 27, 1920. 
Autumn 
Section. 
Spring Section. 
Green Crop. 
Nitric 
N. ppm. 
Turnip 
yield tons 
per acre. 
Green Crop. 
Nitric 
N. ppm. 
Turnip 
yield tons 
per acre. 
Rye . 
Oats . 
Mustard 
Vetches 
Turnip 
Crimson Clover . 
Rape 
Control 
4-0 
4-1 
5'i 
4*3 
(7'4) 
3- 4 
4- 9 
3*5 
ii-4 
II «o 
10 -4 
10-3 
(7-5) 
7-0 
6-4 
6-6 
Crimson Clover . 
Vetches 
Red Clover 
Rye 
Oats . 
Rape 
Turnip 
Control . > 
22 -6 
26-1 
IO -O 
13-6 
7-i 
9'5 
6-9 
20-4 
18-7 
13-4 
n-4 
io-6 
9-9 
7-i 
In every case, the soil from these plots had been found on April 
9, 1920 (after heavy rain) to contain less than 0-5 parts per million 
of nitric nitrogen, so that the above amounts of nitrates were produced 
during the seven weeks after that date. 
There is thus a good general parallelism between the nitrate 
content of the soil and the yield of turnips obtained. The figures 
for the plots green-manured with rape and with turnip are abnormal, 
and may indicate some special effect when the green manure crop 
is the same as, or closely allied to, the main crop, but careful investi- 
gation would be needed to establish this point. A similar, but less 
complete, parallelism was obtained between bacterial numbers and 
turnip yield, especially in the spring section, but the method of 
bacterial counts available at the time was unsatisfactory, and, more- 
over, many of the results had to be subsequently discarded owing 
to the daily fluctuations in bacterial numbers which are liable to 
occur in soils.* These fluctuations are such as to render non- 
comparable the results of bacterial counts made on samples not 
taken on the same day. 
Advantage was also taken of the few dry spells during the growth 
and decomposition of the green crops in the soil, to test the soils for 
any difference in moisture content. It was unfortunately not possible 
to take samples below the top 9 inches, so that the results are of 
somewhat limited value, but as far as they go there was no indication 
of any extra drying effect of vetches or the other leguminous crops. 
Indeed, if anything, the wow-leguminous crops seemed to leave the 
top soil drier than the others. 
Unfortunately this aspect of the subject — the physical effect of 
green manuring on soil texture and moisture content — was largely 
excluded from consideration in the author's trials, owing to the 
exceptionally wet season, which presented no opportunities for 
studying the behaviour of the plots during drought, and it has been 
* Cutler, Crump, and Sandon, Phil. Trans. Roy. Soc. in press, July 1922. 
