52 



THE FARMERS HANDBOOK. 



NOTES ON THE WEATHERING OF SOILS. 



As a general rule, it is not easy to trace any close relationship between 

 the soil and the rock from which it has been derived. This is especially the 

 case when the soil has been exposed to atmospheric influences for a long 

 period, when vegetation has taken a good hold, and when the land has been 

 under cultivation. The changes brought about by all these processes ren- 

 dering the mineral matter in the original rock more soluble, and the further 

 action of plant-growth, rain, drainage, &c, remove the more soluble salts 

 from the soil to such an extent as to materially alter the original proportion 

 of the ingredients. 



The same thing happens when the soil is transported from its original- 

 position, either by rain or wind, or when the original soil is overlaid by 

 transported soil from other rock formation or by flood deposits. 



If the weathering or other agents of destruction have been sufficiently 

 active, and have acted for a sufficient length of time, the resulting soil from 

 a pure limestone rock becomes indistinguishable from that derived from 

 sandstone. 



Where, however, the rock has weathered in situ fairly rapidly, where 

 vegetation has uot taken hold and the rainfall is not excessive, it is possible 

 to trace the effects of weathering in the different layers of soil. . 



This has been shown by Dr. Jensen in the case of samples of granite soil 

 taken from a spot overlying the country rock, where the weathering process 

 could be traced through the different soil-layers right down to the unchanged 

 rock. 



The result is given in the accompanying table. 



Table showing alterations in composition at different depths of a granite 

 soil formed in situ (Bathurst Experiment Farm). 



This table is instructive as showing that the subsoil in granite areas is 

 invariably richer in mineral plant-food, especially potash, than the surface 

 soil. The phosphoric acid gets slightly and the nitrogen considerably less 

 at a depth, but the lime and potash are much higher. It follows from this 

 that when a granite surface soil gets depleted in potash a good supply can 

 be introduced by deeper ploughing. In sandstone country this is not true 

 to nearly the same extent — a fact which makes a poor granite soil an 

 infinitely better asset than a poor sandstone soil. 



