oh. i SOIL 5 



layers of which are in an advanced state of decomposi- 

 tion, the soil in these forests shows little sign of these 

 organic remains. The cause of this is that, in evergreen 

 j forests, the leaf- fall is a much more gradual process than 

 I in forests the leaves of which all fall practically at the 

 same time ; that, owing to the relatively high tempera - 

 j ture, decomposition goes on much more quickly than 

 in colder latitudes ; and that, owing to torrential rains, 

 surface layers are much more liable to be washed away 

 than where the rainfall is gentle. 



According to their mineralogical origin, soils may 

 be broadly divided into clayey, limey, and sandy soils 

 respectively, but there are also numerous mixtures which 

 may modify their chief characteristics sufficiently to 

 earn for them separate nomenclatures. 



Clayey soils, which consist of at least 50 per 

 cent of clay, are rich in mineral constituents, and are 

 capable of absorbing a large quantity of water. This 

 faculty has, however, its drawbacks, for the absorption 

 of water to a great degree means the exclusion of air, 

 for it does not give up readily the water which it takes 

 up. The soil, therefore, becomes w T ater-logged, cold, and 

 badly aired, and when it does dry it splits and cracks, 

 and in so doing is apt to injure the roots and rootlets 

 of plants growing in it. When underlying a more 

 porous soil it first absorbs all the moisture which it 

 derives from it, but does not let any through. If the 

 water can then find an egress owing to the inclination 

 of the strata, a spring is formed at the junction of the 

 porous layer and the clay ; but if the strata are cup- 

 shaped the water, finding no egress, saturates the upper 

 porous layer and the soil becomes sw^ampy. The addition 

 of lime to a clayey soil renders the soil more permeable, 

 and in agriculture the liming of clayey soils is necessary 

 for the cultivation of certain crops. Humus will also 

 correct these physical defects to a certain degree and 

 make the growth of certain trees possible, which other- 

 wise would not be able to exist on heavy clay. 



In the Tropics, the soil known as " cotton soil," which 



