TEMPORARY PASTURES 111 



elements necessaiy for theii- 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 accumu- 

 lation 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 

 veeetation which is characteristic of sour land.' 



After giving the reasons and experiments which prove his 

 case, Monsieur JouUe 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 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 measiu-e 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 obtainable 

 from the cultivation of Leguminosas (clover, lucerne, vetches, &c.). 

 But as this class of plant will not succeed on every soil, temporary 

 "leys" with gramineous (grass) herbage ought to give, where 



