42 CHYMISTRY APPLIED TO AGRICULTURE. 



best soil may be stricken with barrenness if the salts be- 

 come too abundant in it. 



Thorough ploughing contributes largely to the fertility 

 of lands ; but in order that it may produce its best effects, 

 it is necessary to have regard to some circumstances 

 which are generally but too little attended to. 



Ploughing divides and softens the soil, mixes thorough- 

 ly its constituent principles, destroys weeds, and disposes 

 them to decay ; and frees the ground from those insects 

 which often abound in it. 



The ploughings should be more numerous, and con- 

 ducted with more care, upon a heavy soil, than upon 

 one which is light and porous. Clayey soils should be 

 ploughed only when dry ; when they have imbibed water 

 they form a soft paste, on which ploughing has no other 

 effect than to trace furrows in the mud. Sandy and cal- 

 careous lands may be ploughed at all times. Deep plough- 

 ings are very advantageous to lands which are of the same 

 nature to a considerable depth, since, in addition to the 

 good effects arising from the operation itself, those parts 

 of the soil which have become impregnated with the 

 manures, that the rains have carried down below the sur- 

 face, are thrown up to contribute to the nourishment of 

 vegetation. Deep ploughings are likewise useful in those 

 lands where the upper layer, being of too clayey and 

 compact a nature, rests upon a bed of sand or carbonate 

 of lime, which by this operation is brought to the surface 

 and mingled with the upper layer, thus rendering it more 

 fertile than it could be made by any other means. An 

 equally good result is obtained from deep ploughing in 

 the reverse case, that is, Vhen a soil, too sandy or calca- 

 reous, rests upon an argillaceous bed. 



But deep tillage does not belong to all soils, nor is it of 

 use under all circumstances. For instance, if a soil is 

 situated upon a vein of earth charged with black oxide of 

 iron, or upon a bed of marl, the mixture which would be 

 produced by deep tillage would reduce the land to almost 

 entire sterility for two or three years. I have myself ex- 

 perienced this result, and I speak from personal knowl- 

 edge. Near a forest of oaks upon one of my estates, the 

 land, which had been cultivated, was of an argillaceous 

 character for about six inches in depth ; under this lay 

 a bed of very dark brown earth, of five or six inches in 

 thickness, and composed of silex, clay, and oxide of iron. 



