120 PERMANENT AND TEMPOEAEY PASTUEES. 



the subsoil more dense, and the rain or water of irrigation pene- 

 trates with greater difficulty. During the droughts of summer 

 the moisture rises up less easily from the subsoil, and thus, from 

 physical causes, the production settles down to a normal level. 

 In time the chemical condition of the land also undergoes a 

 material change ; not only is the layer of soil which is occupied 

 by the roots rendered incapable of supplying a sufficiently large 

 amount of the elements necessary to the vegetation, but, owing 

 to the continued accumulation of vegetable debris, the layer of 

 soil in which the roots live at length becomes sour, even where 

 the earth may originally have been calcareous, and may still 

 be so in the underlying layers, so that the good plants tend to 

 disappear and give place to a vegetation which is characteristic 

 of sour land.' 



After giving the reasons and experiments which prove his 

 case, Monsieur Joulie adds : 'From all that has been stated we can 

 now draw the following practical and economical conclusions : — 



' 1st. That the cultivation of roots and cereals deprives the 

 soil of nitrogen, whilst that of grass and leguminous plants, 

 temporary or permanent, on the contrary, causes it to accumulate 

 in the soU. That nitrogen being the most expensive manure to 

 buy, it is not economical to devote part of the land permanently 

 to arable and part to grass, for while the one uses up the nitrogen, 

 the other accumulates it in excess. On the contrary, it is pre- 

 ferable to alternate on the same piece of land the cultivation of 

 roots and cereals with that of grass leys, so as in a measure to 

 repair by the second the loss of nitrogen which the first cause to 

 the soil. By this means cultivation can be kept up indefinitely 

 without purchased nitrogen, provided that the land be maintained 

 in a fit state of richness as regards the mineral elements which 

 are indispensable to healthy vegetation. 



' 2nd. The practical application of this principle is, that the 

 temporary occupation of the land by a grass ley for two or three 

 years, which takes its turn in the rotation of crops, should be 

 preferred. We thus secure the improvement of the soil obtain- 



