522 SOILS. 



only a purely physical action. That potash, phosphoric acid 

 and nitrogen, while most essential as plant-foods, exert other- 

 wise little if any effect on general plant-distribution. He al- 

 ludes similarly to magnesia; and his final conclusion is that 

 " chemical are in general more potent than physical influences," 

 and that the most widely active influences are carbonate of 

 lime and chlorid of sodium. He does not, of course, deny the 

 potent influence of moisture upon plant distribution. 



Since these publications were made, many observers have 

 investigated the subject, and the broad distinction between 

 lime-loving or calciphile and lime-repelled or calcifuge plants 

 has been very generally recognized and discussed : but the 

 cause of this discrimination by plants is still more or less the 

 subject of controversy. Some still claim that the calcifuge 

 plants (such as the chestnut, the huckleberries and whortle- 

 berries, the heather and many other Ericaceae, most sedges, 

 etc.) are repelled by calcareous lands because they need a large 

 supply of silica, which they suppose cannot well be assimilated 

 in presence of much lime; hence they also designate the calci- 

 fuge plants as " silicophile " ; while others attribute the prefer- 

 ence of calciphile plants to the physical effects produced upon 

 the soil by lime, as outlined above ( chapter 20, page 3/9). 



The contention that the presence of much lime in soils ren- 

 ders silica insoluble and hence unassimilable by plants, is at 

 once negatived by the fact that waters exceptionally rich in 

 silica, partly simply dissolved by carbonic acid, partly in the 

 form of water-soluble alkali-silicates, are very abundantly 

 found in the arid region. This is especially the case in Cali- 

 fornia, where moreover a number of species of very rough- 

 surfaced horsetail rushes and grasses prove the ready absorp- 

 tion of silica when wanted, even in strongly calcareous soils. 

 P.ut the question is whether the supposed class of silicophile 

 plants is a reality or merely a theoretical fiction, based upon 

 the habit of speaking of " siliceous " soils as a class apart from 

 other and especially heavier or clay soils. As a matter of fact, 

 the siliceous soils usually so called are simply those poor in clay 

 and lime in other words, " light " lands, the outcome of the 

 weathering of quartzose rocks into sandy soils, which in the 

 humid region are always poor in lime because thoroughly 

 leached. In the arid region, on the contrary, sandy lands are 



