VARIATIONS IN PASTURES 135 



does not undergo constantly the changes and fluctuations which 

 take place in arable soils. It would seem probable too that a 

 crop which is almost continually requiring food would render 

 impossible any considerable accumulation of readily available 

 plant food in the surface soil, such as takes place under certain 

 conditions in arable soils. In this connexion, it may be noted 

 that we are at present without precise knowledge as to the form 

 in which the pasture grasses take up their nitrogen. Recent 

 work on the assimilation of nitrogen by plants has shown that 

 perhaps many more types of compounds are available as 

 sources of nitrogen than was formerly supposed 1 but almost 

 all the experimental work has been carried out with cereals and 

 leguminous plants. There is definite evidence, however, that 

 ammonium salts, as well as nitrates, can serve directly as food, 

 at all events for some species of pasture grasses. In the case of 

 the grass plots at the Rothamsted Experimental Station which 

 receive heavy dressings of ammonium salts annually, it has 

 been found 2 that nitrification takes place only to a very slight 

 extent and is probably confined to the immediate neighbourhood 

 of the scattered particles of calcium carbonate present in the 

 soil, since the soil generally is acid. None the less, on these 

 plots a fairly heavy crop of coarse grass is grown, consisting 

 almost entirely of three species — Holcus lanatus, Alopecurus 

 pratensis and Arrenathemm avenaceum — forming tufts with bare 

 patches of peaty decayed vegetation here and there. 



The continual occupation of the land by a crop undoubtedly 

 has a most important influence on the physical condition of the 

 soil. The surface of grass land is disturbed only to a minimum 

 extent and consequently its physical condition and texture are 

 quite different from that of a well-tilled arable field on the same 

 soil. This has far-reaching effects. The undisturbed condition 

 of the surface and consequent slight aeration have a large share 

 in determining what will be the predominant types of bacteria ; 

 and one of the evident results of the defect is that those types 

 are favoured which cause the decay of organic matter to proceed 

 much less quickly than in well-aerated soils ; and there is 

 always a certain accumulation of humus. This is especially 

 seen in grass land on heavy clay soils and on soils deficient in 



1 See Hutchinson and Miller, Jour. Agric. Set. vol. iv. p. 282, for biblio- 

 graphy. 



2 Hall, Miller and Gimingham, Proc. Roy. Soc. B. 80, 1908. 



