TRANSACTIONS OF SUB-SECTION K. 711 
reached an available form. As it may also be accepted that the plant itself, 
apart from bacterial action, neither converts any of the combined nitrogen 
it obtains into gas nor brings into combination any of the free nitrogen of 
the air, there is neither gain nor loss of soil nitrogen when the growth of 
the plant is returned to the soil. 
(2) Various bacteria are capable of bringing atmospheric nitrogen into 
combination and so increasing the stock of soil nitrogen. They may either 
live in symbiosis with higher plants (Pseudomonas) or exist free in the soil 
(Azotobacter, Clostridium). 
(3) Another group of bacteria, in the process of breaking down organic 
matter, liberate the nitrogen in the free state and so reduce the stock of soil 
nitrogen. 
(4) Natural drainage waters contain nitrates which have been derived 
from the soil nitrogen by bacterial oxidation. 
(5) The rain annually contributes a certain amount of combined 
nitrogen to the soil, The amount is greater in the proximity of towns ; the 
average amount at Rothamsted is 3°84 lbs. per acre per annum, and other 
results would show that this is a very representative figure for pure country 
air. 
The interplay of these factors gives rise in practice to the following types 
of cases :— 
A. The nitrogen content of land under arable cultivation declines when 
the produce is entirely removed and no organic nitrogen is added as manure. 
The unmanured plot on the wheat-field at Rothamsted shows the following 
results :— 
Broadbath, Plot 3. Nitrogen, lbs. per acre. 
| In Soil, | Im Soil, | Lossin | Added by | Removed by | Unaccounted | 
| 1865 | 1893 | 28 Years Rain and Seed! Crop | for 
| 2,722 lbs. | 2,437 lbs. | 285 Ibs. 167 lbs. _ 428 lbs. | — 24 lbs. | 
Thus the crop almost exactly accounts for the decline in nitrogen in the 
soil + that which has been brought down by the rain. Since there are also 
losses due to drainage and to the nitrogen contained in the weeds removed 
from the plot, there must be some recuperative actions (No. 2) at work to 
maintain the balance. These, however, are not large. Taking similar 
figures for plot 7, which receives 86 lbs. of nitrogen annually as ammonium 
salts ;— 
Broadbatk, Plot 7. Nitrogen, lbs. per aere. 
| InSoil, | In Soil, | Loss in | Added in | Added by | Removed Icilasucen ted 
1865 1893 ain and 
28 Years Manure | abet | in Crop | for 
3,034 Ibs. 2,971Ibs./ 63 Ibs. | 2,408 Ibs. 167 Ibs, | 1,212 Ibs. —1, 426 Ibs 
In this case the recuperative agencies are not sufficient to balance the 
losses by drainage water, weeds, &c. 
B. When land rich in organic compounds of nitrogen is subjected to 
arable cultivation, the destructive agencies (No. 3) become very active and 
the land loses nitrogen rapidly. This may be illustrated by the results 
obtained from plot 2 on the same field, which receives 14 tons of farmyard 
manure, containing approximately 200 lbs. of nitrogen, every year. 
