A STUDY OF THE AGRICULTURAL SOILS OF 

 CAPE COLONY. 



PART I. INTRODUCTORY. 



Van Helmont, three centuries ago, demonstrated, to his own satis- 

 faction, that water was the sole source of plant food. Digby, fifty yeara 

 later, ascribed the nutrition of plants to a mysterious principle in the air, 

 and refused to consider water as anything more than a mere vehicle for 

 the conveyance of this principle. Yet another half-century, and both 

 these theories were rejected by Tull, who put forth the view that the soil 

 particles constituted all the nutriment that the plant needed. He held, 

 however, that, in order to* be available as nutriment, these particles had 

 to be extremely minute. Scarce a century has passed since Thaer promul- 

 gated the doctrine that the source of plant food was nothing other than 

 humus, a doctrine that commenced to be undermined by Saussure while 

 still in its infancy, and was completely pulverised by Liebig when pro- 

 pounding his mineral theory half a century later. According to Liebig 

 not the organic, but the inorganic, or mineral constituents of the soil served 

 to build up the vegetable structure. 



Fifty years have passed since the day of the great German chemist', 

 and at present the tendency is to lay emphasis on no single one of the 

 various views alluded to above, but to regard them all as partially true. 

 The worth of the mineral constituents of plant food is almost universally 

 accepted ; but not to the entire rejection of organic matter as a valuable 

 agent in determining a soil's fertility. The mechanical subdivision of the 

 soil by sifting, sedimentation, or elutriation, is the modern counterpart 

 of the views put forward by Tull. In the fixation of atmospheric nitro- 

 gen by bacteria we may trace a reflection of Digby 's theory; and the re- 

 cent investigations of Whitney and others in the United States of America 

 have led them to conclusions which, in an embryonic state, lay hidden 

 within the water theory of Van Helmont. 



It is, nevertheless, erroneous to affirm however much it may seem 

 to be the case that modern investigators have reverted to the opinions 

 held by their predecessors two or three centuries earlier. It is not only in 

 connection with agriculture that chemical science may appear to have 

 moved along a circular pathway, and yet the striking theories which have 

 resulted from, the investigations into the nature of radio-activity, for in- 

 tance, do not by any means imply that the scientists of our day have re- 

 treated to the alchemists' notions respecting the philosopher's stone and 

 the transmutation of metals. We may be trending back to the same ver- 



