WEIGHT OF SOIL PER ACRE 47 



soil by the treading of horses and men in the furrow, and the 

 pressure of the sole of the plough. The formation of this 

 consolidated layer is clearly aided by the accumulation of 

 fine clay immediately beneath the ploughed soil. Such an 

 impervious layer is of course most injurious to the growth 

 of crops, and if formed requires to be broken up by a subsoil 

 plough following the ordinary plough in the same furrow. 



The difference in the weight of the surface soil and subsoil 

 is most marked in the case of the old pasture land at Rotham- 

 sted ; here the surface nine inches is only three-quarters the 

 weight of the second nine inches. This is an excellent illustra- 

 tion of the lightening of a soil laid down to grass brought 

 about chiefly by the accumulation of vegetable residues in 

 the surface soil. 



In cases in which the sampling of the subsoil was carried 

 out at Rothamsted to a greater depth than 36 inches, it 

 appeared that below this depth there was little further in- 

 crease in the consolidation and weight of the soil. 



The sandy soil at Woburn is seen to be, at every depth, 

 of greater weight than the heavy loam at Rothamsted, thus 

 supplying a further illustration of the comparative lightness 

 of clay soils. 



The apparent specific gravity, and the weight per cubic 

 foot of the dry soils at the surface, and at the depth of 27 

 to 36 inches were as follows: 



Apparent Weight 



Specific Gravity. per Cubic Foot. 



Ib. 



Rothamsted Old Pasture, first 9 inches ... 1-144 ... 71-3 



fourth ... 1-642 ... 102-3 



Rothamsted Arable Land, first 9 inches ... 1-434 ... 89-4 



fourth ... 1-627 ... 101-4 



Woburn Arable Land, first 9 inches ... 1-550 ... 96-6 



fourth 1-715 106-9 



