24 THE CHEMISTHY OF THE FARM 



continued use of farmyard manure, or by the plough- 

 ing in of green crops, is a fact familiar to the farmer. 

 While humus increases the coherence of sand, it has a 

 contrary effect on clay, and the ploughing in of long 

 dung is one of the most effectual means of lightening 

 a heavy soil. 



In some sandy soils hydrated ferric oxide acts as a ce- 

 menting material. Calcium carbonate will also tend to 

 increase the coherence of sand. Well-cultivated mixed 

 soils (loams) consist Jargely of compound particles, made 

 up of various constituents held together by cementing 

 materials ; these coarse porous particles are highly 

 favourable to a good physical condition of the soil. 



3. Belations to Water. — In a natural soil consisting 

 of solid particles of fairly uniform size, the interspaces 

 are about 40 per cent, of the volume, whether the par- 

 ticles are large or small ; but if the particles are a mix- 

 ture of large and small (as gravel and sand) the volume 

 of the interspaces is much reduced. On the other 

 hand, if the particles are themselves porous, as in the 

 case of chalk, loam, and especially of humus, the 

 volume of the interspaces is much increased. It is 

 this volume of the interspaces which determines the 

 amount of water which a soil will contain when per- 

 fectly saturated, or the amount of air which it will 

 contain when dry. 



The influence of humus on the capacity of a soil for 

 water is remarkable. The surface soil of the wheat- 

 field at Rothamsted was sampled in January, 18G9, 

 when saturated with water : the unmanured land con- 

 tained in the first six inches 29 9 of water per 100 of 

 dry soil ; the land manured with farmyard manure for 



