28 PHYSICAL CONSTITUTION OF SOIL 



that which it exhibits when mixed with sand. The humus 

 of soils is made up of very various matters, consisting as it 

 does of the remains of decayed plant tissue, and of its 

 products in various stages of decomposition. Not only do 

 the coarser parts of the humus tend to lighten a clay with 

 which they are mixed, the colloid products already mentioned 

 act apparently in the same direction. Schloesing mixed 

 with clay 2, 4, and 6 per cent, of calcium hamate, and found 

 that with each increase in the proportion of humate the clay 

 became more pulverulent on drying. It is not difficult to 

 conceive possible explanations of this action of humus on 

 clay, but experimental proof is as yet wanting. 



Hydrated ferric oxide is another colloid substance which 

 in sandy soils undoubtedly plays the part of a cementing 

 agent. In rocks and soils of the Red Sandstone formations 

 its influence is plainly marked. Perhaps however the most 

 obvious example of the cementing action of ferric oxide is 

 afforded by the formation of iron-pan in moor soils. In moor 

 soils, and especially in those covered by heather, the iron 

 has been dissolved out of the surface soil by the action of 

 the humic acids, the sand at the surface being left remarkably 

 white. The iron passes in solution into the subsoil, where 

 it is reprecipitated, with the result that the sand at a certain 

 depth is cemented together, and an iron-stone produced l . 



1 The precipitation of the iron oxide may be brought about by contact with 

 calcium carbonate contained in the subsoil ; or, possibly, the whole action of 

 solution and precipitation is determined by alterations in the condition 

 of the soil. In autumn and early winter, with a soil saturated with water, 

 the iron may enter into solution as a ferrous salt, and be carried below ; 

 and in summer time, in a dry and aerated soil, it may be precipitated as 

 ferric oxide. The iron-pan would then be formed at the line at which 

 oxidation chiefly occurred. 



