CULTIVATION OF WASTE LAND 381 



and a working proportion of carbonate of lime. Now, as we know, all 

 virgin soils are not rich, and only in a few parts of the world are to be 

 found those wonderful black soils that are often several feet in depth and 

 contain 10 to 20 per cent, of organic matter and 3 to 5 parts per thou- 

 sand of nitrogen. These soils are all calcareous, they occur in regions of 

 a moderate rainfall inducing grass-steppe or bush conditions, and the 

 annual fall of vegetation provides the organic matter which the Azoto- 

 bacter requires as a source of energy in order to fix nitrogen. Non- 

 calcareous soils under similar climatic conditions do not accumulate 

 nitrogen and become rich; in the absence of carbonate of lime the 

 nitrogen-fixing organisms are not active, and the soil only receives from 

 the annual fall of vegetation the nitrogen that was originally taken 

 from it. There is but a cyclic movement of nitrogen from the soil to 

 the plant and back again, whereas in the calcareous soils there is also 

 continuous addition of fresh nitrogen derived from the atmosphere, in 

 which process the carbonaceous part of the annual crop supplies the 

 motive power. 



The other leading case to be found at Rothamsted is that of certain 

 grassplots which have artificially been brought into an acid condition by 

 the continued application of sulphate of ammonia. In these soils nitri- 

 fication is suspended, the nitrification organisms have even disappeared, 

 though the herbage still obtains nitrogen because most plants are able 

 to utilize ammoniacal nitrogen as well as nitrates. The interesting 

 feature, however, is that the decaying grass on these acid soils passes into 

 the form of peat, a layer of which is forming upon the surface of the 

 soil, though nothing of the kind is found on adjacent plots where the 

 use of lime or of alkaline manures has prevented the development of 

 acidity. From this we may learn that the development of a surface 

 layer of peat, independent of waterlogging (when another kind of peat 

 forms even under alkaline conditions), is determined by the acidity of 

 the soil, when certain of the bacterial processes of decay are replaced 

 by changes due by micro-fungi which do not carry the breaking-down 

 of organic matter to the destructive stage. This affords us a clue to 

 the origin of many areas of upland peat in the British Isles, where the 

 remains of ancient forest roots and stumps of trees are found on the 

 true soil surface below the layer of peat, but where there is no water- 

 logging to bring about the death of the trees and the formation of peat. 

 We may suppose that when the land-surface became fit for vegetation 

 at the close of the glacial epoch it covered itself with a normal vege- 

 tation, chiefly dwarf forest, because of the rainfall and temperature. 

 The soil, however, being without carbonate of lime, would in time 

 become acid with the products of decay of the vegetable matter falling 

 to the ground, and as soon as this acid condition was set up peat would 

 begin to form from the grassy surface vegetation. The process would 



