The Nitrogen Compounds of the Fundamental Rocks

Extract

It has been shown that many rocks, when samples are taken from such a depth as precludes all possibility of weathering, contain nitrogen in amounts comparable to those present in soil, especially when the strata consist of indurated clays like the Lower Lias, Oxford, Kimmeridge or London Clay. Since the rocks also contain carbon the nitrogen is without doubt of organic origin; the carbon and nitrogen compounds represent the humus present in the clay when it was deposited, and are in fact the mineralised remains of that organic matter. When soil is produced by the weathering of such rocks these carbon and nitrogen compounds will remain wholly or in part in the soil and may there account for a considerable proportion of the total nitrogen they contain. It thus becomes of some importance to ascertain if this nitrogen is ever likely to become available for the plant by the normal processes of bacterial oxidation, or whether it has passed into such a state of combination as to be susceptible of no further change under such conditions as prevail in the soil. If the compounds are too mineralised or bituminised to be attacked by bacteria it would explain the fact that much of the nitrogen present in soils seems to remain permanently beyond the reach of plants. For example the soil of Broadbalk wheat field at Rothamsted, which had been cropped continuously with wheat for 50 years without the addition of any manure, still showed in 1893 very nearly 0·1 per cent. of nitrogen, equivalent to about 2500 lbs. per acre in the top 9 inches of soil; yet the average wheat crop on this plot only contains about 17 lbs. of nitrogen, of which 5 lbs. is supplied annually by the rain, without taking into account any further additions by the action of Azotobacter and kindred organisms.