Y-axis: ℔ per acre
An equally difficult problem arises in connection with the length of time the process will continue. Decomposition of the nitrogen compounds never seems to be complete in the soil; it dribbles on interminably. In the year 1870 Lawes and Gilbert cut off a block of soil from its surroundings and undermined it so that the drainage water could be collected and analysed. The soil has been kept free from vegetation or addition of nitrogen compounds from that time till now; yet it has never failed to yield nitrates, and the annual yield falls off only very slowly ([Fig. 24]). This same peculiarity is seen in the yield of crops on unmanured land: it decreases, but very gradually; even after eighty years the process is far from complete, and there is no sign that it will ever come to an end.
TABLE XVII.—APPROXIMATE LOSS OF NITROGEN FROM CULTIVATED SOILS: BROADBALK WHEAT FIELD, ROTHAMSTED, FORTY-NINE YEARS (1865-1914.)
| Rich Soil: Plot 2. Lb. per Acre. | Poor Soil: Plot 3. Lb. per Acre. | |||
|---|---|---|---|---|
| Nitrogen in soil in 1865 | ·175 per cent. = 4340 | ·105 per cent. = 2720 | ||
| Nitrogen added in manure, rain (5 lb. per annum), and seed (2 lb. per annum) | 10,140 | 340 | ||
| Nitrogen expected in 1914 | 14,480 | 3060 | ||
| Nitrogen found in 1914 | ·259 per cent. = 5950 | ·095 per cent. = 2590 | ||
| Loss from soil | 8530 | 470 | ||
| Nitrogen accounted for in crops | 2500 | 750 | ||
| Balance, being dead loss | 6030 | -280 | [J] | |
| Annual dead loss | 123 | - 6 | [J] | |
| [J] Gains.Possibly the result of bacterial action. | ||||
A further remarkable fact connected with the decomposition of the nitrogen compounds is that it seems invariably to be accompanied by an evolution of gaseous nitrogen. Apparently there are two cases. Under anaerobic conditions many of the soil organisms have the power of obtaining their necessary oxygen from nitrates, thereby causing a change in the molecule which leads in some cases to liberation of gaseous nitrogen; but the same result seems to be attained in aerobic conditions, especially when carbon is being rapidly oxidised.
It is possible that the reaction is the same, and that in spite of the general aerobic conditions there is locally an anaerobic atmosphere. But it is also possible that some direct oxidation of protein or amino-acids may yield gaseous nitrogen. However it is brought about it affects a considerable proportion of the entire stock of nitrogen, and it becomes more serious as cultivation is intensified. Thus, on the Broadbalk plot receiving farmyard manure the loss is particularly heavy; on the unmanured plot it cannot be detected. The nitrogen balance-sheet is shown in [Table XVII.]
The oxidation of carbonaceous matter, however, is not invariably accompanied by a net loss of nitrogen; in other circumstances there is a net gain. In natural conditions there seems always to have been some leguminous vegetation growing; the gain may, therefore, be ascribed to the activity of the nodule organism. In pot experiments, however, it has been found possible, by adding sugar to the soil, to obtain gains of nitrogen where there is no leguminous vegetation, and this is attributed to the activity of Azotobacter.
The nitrogen cycle as observed in the soil is as follows:—
| Protein | |||
| By certain organisms and by growing plants | Ammonia | Mechanism uncertain | By Azotobacter, Clostridium, nodule organisms, etc. |
| Nitrite | |||
| Nitrate | Gaseous Nitrogen | ||
| By denitrifying organisms | |||