RESPECTIVE VALUES OF ORGANIC AND INORGANIC MANURES. 221 



action in the soil of the two classes of manures from three points 

 of view, viz. the mechanical, chemical, and biological. 



i. Mechanical. — Let us first examine their effect on the mechan- 

 ical or physical condition of the soil. Now all organic substances 

 contain humus : that is, they leave behind, after they have decayed 

 in the soil, that valuable residue that we have learnt to associate 

 with the use of farmyard manure or leaf-mould &c. ; and this humus, 

 which more than any other ingredient is always tending to oxidize 

 and diminish in the soil, has a very marked effect on its working. 



Humus opens a clay soil by loosely binding together the finer 

 particles to which its plastic nature is due. These particles are so 

 fine as to assume when extracted from the soil an almost gelatinous 

 nature, and when they are spread out, or " deflocculated " through- 

 out the soil they make the whole into a sticky unworkable mass, 

 giving the clay what is known as its colloid property. It is this 

 property that enables the potter so to work his clay that he can 

 mould it at will, and upon which the brick-maker relies when he 

 puddles his brick-earth before moulding his bricks. 



All practical men are painfully aware of this characteristic of 

 a heavy soil when they try to work it after rain before it is suffi- 

 ciently dry, and their aim in cultivation is to counteract it as far 

 as possible and so to manage their heavy soils that they may crumble 

 down into a fine and workable condition. 



Humus greatly assists in this endeavour by collecting together 

 or "flocculating" these fine particles; perhaps the simplest in- 

 stance of this action of humus is that afforded when a piece of old 

 grass land naturally rich in humus is ploughed up, especially if the 

 soil be heavy. After the winter it will crumble readily, so as to 

 harrow down to a mellow seed-bed while on a neighbouring piece of 

 the same soil that has been arable for some time a number of large 

 unyielding clods will probably be noticed. 



Again, on a light soil, humus, by acting in a contrary direction, 

 has an equally beneficial effect ; in this case it loosely binds together 

 the coarser particles, imparting to the soil more of a spongy or 

 retentive nature. 



This mechanical effect of humus leads to the following advantages 

 in soils well supplied with this important ingredient : — 



The land retains much heat otherwise lost, and, being warmer, 

 the crops consequently ripen earlier. 



The land dries more quickly, yet retains its moisture better in 

 drought. 



The superfluous moisture from storms and heavy showers is held 

 as by a sponge and passes downwards gradually, thus all wash is 

 avoided ; the water-retaining power of a soil well supplied with 

 humus is 20 per cent, greater than that possessed by a mineralized 

 soil. 



In dry weather the land cools sooner and more dew is precipi- 

 tated. The air passing over humus-fed soil is cooler and moister 



VOL. XLI. o 



