VII.] PHYSICAL EFFECTS OF MANURE 217 



is always tending to oxidise and diminish, and the 

 humus, considered merely from the physical side, 

 contributes largely to the fertility of the soil. In the 

 first place, it improves the texture of all soils ; to sands 

 it gives cohesion and water-retaining power, while by 

 loosely binding together the finest particles of clay soils 

 it renders them more porous and friable. When a piece 

 of old grass land, even on the stiffest of soils, has been 

 ploughed up it is easy to see the beneficial effect of 

 the humus that has been accumulated ; after the winter 

 the plough slice will crumble naturally so as to harrow 

 down at once to a mellow seed-bed, whereas a neigh- 

 bouring piece of the same soil that has long been under 

 arable cultivation will only show a number of harsh 

 intractable clods. The importance of a good seed-bed 

 to the future well-being and ultimate yield of the crop 

 can hardly be exaggerated ; it is the basis of all good 

 farming ; so that even when the fertilising properties of 

 farmyard manure have been replaced by artificial 

 manures, some other means, such as the ploughing-in 

 of green crops, must be resorted to in order to maintain 

 the stock of humus. Of course, the value of humus — 

 and in this respect of farmyard manure — will vary on 

 different soils and with different crops ; cereals, for 

 example, are comparatively unaffected by its absence, 

 as may be seen by the manner in which Mr Prout 

 grows cereals almost continuously on his strong soil 

 with artificial fertilisers only, but root crops are very 

 dependent on a mellow seed-bed. This may be seen on 

 the Rothamsted plots ; the wheat which has now been 

 grown on the same land for sixty-five years, comes as 

 well and yields as big crops on the plots receiving only 

 artificial manures as it does on the plot receiving dung, 

 but on the mangold field the result is different. Where 

 artificial manures containing no organic matter have 



