Studies in Forestry [CHAP. v. 



after inundations is often of this character. Sometimes, as in 

 the case of marshes, moors, and peat bogs, the accumulations 

 of humose soil is so great that the roots of trees are unable to 

 reach the mineral soil below it ; such formations usually take 

 place on sandy soil resting upon clay. 



Ferruginous soils are coloured brownish-red, and contain at 

 least 10 to 15% of ferric oxide or hydrated ferric oxide. But 

 even by 2 to 5% of ferric oxide the general character of sandy 

 soils may become affected, although considerably larger quan- 

 tities may fail to effect any noticeable alterations in clayey or 

 loamy soils. Thus moorpan, sometimes containing only about 

 2% of ferric oxide, is just as impenetrable to the roots of trees 

 as iron-band or limonite with its 60 to 70%. In both cases tree 

 growth is hindered until the impenetrable layer has been 

 broken through, so as to allow of the roots reaching the sub- 

 soil, and of the moisture circulating normally. 



III. The Physical Properties of Soils. 



To these, in a far greater degree than to the mineral com- 

 position of any soil, is due its capacity to sustain timber crops, 

 and to yield to them in due proportion the various forms 

 of nutriment requisite for their growth, development, and 

 reproduction. All the physical properties act and react on 

 each other in determining the quality of any given land. 



Cohesiveness or Tenacity is the resistance offered by a soil 

 to instruments used in separating its particles, or to force 

 employed in disintegrating them. This property is not alone 

 of importance in relation to air, moisture, and warmth, but 

 also indicates the resistance to be overcome by the root- 

 systems in penetrating into, and ramifying throughout, the soil. 



Clay soils have the greatest cohesiveness, sand the least ; 

 lime approaches more to the clay, while loam resembles rather 

 the sand. An admixture of humus tends to level all classes, 

 making clays and limes less tenacious, and loams and sands 



