20 THE CHEMISTRY OP THE FARM. 



soil, rich in humus, is sure to be also rich in nitrogen ; 

 a soil destitute of humus will contain scarcely any nitrogen. 

 The fertility of virgin soils is largely due to the nitro- 

 genous humus which they contain. Humus appears to 

 possess in part an amide nature ; it yields ammonia and 

 soluble nitrogenous bodies when acted on by acids or 

 alkalis, and probably, to a much less extent, under the 

 action of water. 



2. Relation to Water. The power of absorbing 

 water from damp air known as hygroscopicity, is 

 scarcely at all possessed by sand, but to a greater degree 

 by clay and humus. Schlossing found that while dry 

 siliceous sand absorbed nothing, a strong clay absorbed 

 3'5 per cent., and a garden soil 5*2 per cent. Water thus 

 absorbed is insensible, and can have little direct influence 

 in plant nutrition. All soils condense water from the 

 atmosphere when their temperature is below the dew 

 point. 



Sand has the least, and humus the greatest capacity for 

 retaining water. Schloesing found that fine sand, saturated 

 with water, and thoroughly drained, retained 7 per cent.; a 

 clay soil 35 per cent. ; and a forest soil 42 per cent. Light 

 sandy soils thus suffer most from drought, while applica- 

 tions of farmyard manure, or the ploughing in of green 

 crops, increase the water-holding power of a soil by in- 

 creasing the proportion of humus. 



Capillary attraction, by which water is raised from 

 the subsoil to the surface in dry weather, depends on the 

 distance between the particles of the soil ; it is least in 

 open sandy soils composed of coarse particles, and 

 grea.test in the case of loam or clay. 



