26o CAUSES OF FERTILITY AND STERILITY [chap. 



Paring and Burning, 



When the poorer clay soils were first taken into 

 cultivation, a beginning was generally made by " paring " 

 the surface with the breast plough, and " burning " the 

 clods as soon as they were sufficiently dry. The clods 

 were made up into heaps a yard or so in diameter with 

 the brushings of the hedges and all the rough surface 

 vegetation, together with as much clay as was judged 

 prudent. Each heap was then allowed to burn slowly 

 and char the clay, without permitting the heat to rise 

 sufficiently to vitrify the clay or dissipate such valuable 

 material as the alkalis of the ash. The resulting ashes 

 effected a great improvement in the soil : the clay was 

 partially dehydrated, or at least coagulated, thus pro- 

 viding a certain amount of coarse material to ameliorate 

 the texture ; in the charred clay also, some of the potash 

 was rendered more available, while the plant residues 

 provided mineral salts and alkalis to promote nitrifica- 

 tion. The drawback to the process is the inevitable 

 loss of nitrogen to the soil; but any one who has 

 noticed how freely crops grow on the patches of 

 arable land where couch heaps have been burnt the 

 season before, will see that, for the time being, the 

 fertility of the soil is increased by the process. Other 

 advantages of burning lie in the destruction of weeds 

 and insect life of all kinds, and although it has been 

 almost wholly discontinued at the present day, the 

 older writers on agriculture are unanimous as to its 

 beneficial effects in bringing poor clay land into cultiva- 

 tion. Recent investigations also show that heating the 

 soil to low temperatures, such as that of boiling water, 

 bring about a great increase in its productivity, probably 

 owing to a rearrangement of the bacterial flora of the 

 soil, for complete sterilisation is only effected at higher 



