THE WORK OF ROOTS 145 



such as lava and basalt, though of course much more 

 slowly than soft rocks, such as limestone, etc. 



Clay (which consists of alumina combined with 

 silica and water) is of great service to the plant in 

 "fixing" plant -foods. We may illustrate this by filling 

 a lamp-chimney with clay or good garden soil (so 

 firmly packed that the liquid takes an hour or so to 

 run through) then pour in ammonia water at the top; 

 the water will be deprived of the ammonia in passing 

 through the soil and will come out at the bottom with- 

 out any odor of ammonia. The ammonia has been 

 "fixed" (principally by the compounds of alumina) in 

 the soil so that it cannot be readily washed out by 

 rain, etc. The same thing happens with dung liquor, 

 phosphoric acid (fixed by lime and magnesia and, to a 

 slight extent, by iron in the soil) or potash (for the 

 last two, test with litmus paper) . 



The carbonic acid of the soil and the alumina 

 and silica of the clay act, not as foods of the plant, but 

 as servants which store food; the clay (and likewise 

 humus) "fixes" soluble food which would otherwise be 

 washed out of the soil by the rain, the acids in the 

 soil (carbonic acid, humus acids, etc.) render this 

 "fixed" food soluble and available to the plant. The 

 air which circulates in the soil is of immense import- 

 ance in promoting chemical processes which prepare and 

 set free plant-food of all kinds and especially nitrogen 

 compounds (see pages 148 and 383) . Where the air sup- 



