TEMPORARY PASTURES 121 



for their growth. So they grow vigorously. But little by 

 little the soil becomes more compact, the subsoil more dense, 

 and the rain or water of irrigation penetrates 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 accumu- 

 late in the soil. 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 preferable 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, pro- 

 vided that the land be maintained in a fit state of richness as 

 regards the mineral elements which are indispensable to healthy 

 vegetation. 



