40 RESULTS OF EXPERIMENTS AT ROTHAMSTED, 



Indeed, the greatest difficulty in the way of the supposition that 

 much nitrogen is available to plants by the nitrification of the nitrogen 

 of the sub-soil, is the want of sufficient aeration. Independently of the 

 greater or less porosity of the sub-soil itself, and of the channels 

 formed by worms, it is obvious that, wherever the roots go, water and 

 its contents can follow ; and that, with deep-rooted plants, and free 

 growth, there will be active movement of water, and there must be of 

 air also, in the lower layers of the soil. In experiments made in 1882, 

 there remained in the Melilotus soil, less water than in the Trifolium 

 repens soil where there had been less growth, in amount corresponding, 

 down to a depth of 54 inches, to a loss of 540 tons per acre, or nearly 

 5J inches of rain ; and, as already stated, in 1883, the Vicia saliva soils 

 showed, down to 108 inches, less water than the Trifolium repens soil, in 

 amount corresponding to between 600 and 700 tons per acre, or to 

 between 6 and 7 inches of rain. Obviously too, the still deeper 

 rooting, and still freer growing, Medicago saliva, would remove still 

 more water. 



Although much experiment, and much calculation, have been 

 devoted by several investigators to the estimation of the degree of 

 aeration of soils and sub-soils of different character, the data at 

 command do not justify any very definite conclusions on the subject. 

 The results seem to indicate a probable range of aeration from about 

 30 to over 50 per cent, of the volume of the soil. But these estimates 

 do not take into account the varying amounts of water in the soil or 

 sub-soil. In the case of the sub-soils above referred to, each layer of 

 9 inches in depth retained from about 2 to nearly 4 inches of water, 

 the amount varying very much, according to the nature of the sub-soil, 

 and especially according to the amount of growth, and the consequent 

 withdrawal of water from below, and its evaporation, chiefly through 

 the plant, but partly also from the surface soil. The amount must, 

 obviously, also vary very much according to the character of the 

 season. 



It may be stated that, supposing the sub-soil contained at one time, 

 air equal to one third of its volume, this would, not suffice for the 

 nitrification of as much nitrogen as was taken up each year, for several 

 years in succession, by the Merficaqo sativa ; or during the two years in 

 the case of the Red-clover on the Bean-exhausted land. But the 

 nitrogen is not taken up all at once, though much of it will be within 

 a few months of the year, during which period there would be the 

 most active withdrawal of water from below, and evolution by the 

 plant, and evaporation by the surface : soil. The replacement of this 

 sub-soil water by an equal volume of air would, however, still not 

 suffice. The question obviously arises how far, or how rapidly, the 

 used up oxygen will be replaced, and on this point there is very little 

 experimental evidence to aid us. 



Thus, then, the evidence is clear, that the nitrogen of raw clay 

 sub-soils, which constitutes an enormous store of already combined 

 nitrogen, is susceptible of nitrification, provided the organisms are 



