42 



ACTION OF ANIMAL AND VEGETABLE LIFE 



Africa, the veld in many districts is thickly studded with ant-heaps, 

 ranging from a foot to three or four feet in height, some nearly hemi- 

 spherical in shape, others columnar. The subterranean galleries of 

 the nest extend to a depth of six or eight feet and have a consider- 

 able lateral extension. When such land is ploughed and cultivated, 

 it is generally noted that the sites of the ant-heaps are decidedly more 

 fertile than the surrounding soil. 



Chemical examination shows that this is due to the material of an 

 ant-heap being decidedly richer in plant food than the surrounding 

 soil. For example, ant-heap material and the veld soil taken a few 

 feet away, collected by the writer near Christiana, in the Western 

 Transvaal, yielded the following figures on analysis : 



It is to be noted that the ant-heap material contains more than 

 four times as much nitrogen, nearly four times as much " available '' 

 potash and six times as much available phosphorus pentoxide as the 

 neighbouring soil. It is also richer in lime, magnesia and total 

 potash. 



Ant-heap material proves in practice to be very fertile, and when 

 mixed with a little sand, or sandy soil, so as to remedy its somewhat 

 too coherent nature, is admirable for seed beds. 



4. Vegetation. This acts in several ways : 



(i) Mechanically. The roots of a plant penetrate the rocks or soil, 

 rendering them porous, and so admitting air and water. Boots, indeed, 

 are capable of exerting considerable disruptive force, as is sometimes 

 strikingly shown by the dislocation of pavements, walls, or buildings 

 by the growth of trees. Plants also tend to prevent the access of sun- 

 light and air to the surface of rocks, and so favour their denudation by 

 moisture. 



(ii) Chemically, (a) During life, by the solvent action of the roots, 

 which secrete an acid liquor capable of dissolving many constituents 

 of rocks, (b) After death, by decaying and producing both acids of 



