104 PLANT STRUCTURES 
ents is required. Should sand be in excess, the soil is 
impoverished, since silica contains no nutriment, and 
it is rendered too dry, as on account of the relatively 
small surface of the sand grains in proportion to 
their mass it retains but little water. Should there 
be too little sand, percolation of air and water is 
hampered; the soil tends to become water-logged 
and badly aerated, and turns sour. Should humus 
be absent, the nitrogen-producing bacteria cease their 
activities and the soil is sterile, as may be tested by 
digging up some subsoil, or soil from the deeper levels 
to which roots or other organic matter have never 
penetrated. An excess of humus, on the other hand, 
results in the accumulation of acid products inimical 
to bacterial growth: in consequence decay is arrested, 
and a mass of plant débris forms, highly charged (for 
humus is very spongy) with acid water and badly 
aerated, which is unsuitable for vegetable growth: we 
may study an extreme case of such conditions in our 
peat bogs. Should water be in excess in soils, air is 
forced out in proportion, and the roots cannot breathe. 
Too much air means a corresponding diminution of 
water, and the plants suffer from drought. 
“The soil is not merely a reservoir for the mineral 
nutrients of plants, but is the seat of complex physical, 
chemical, and biological actions which directly and 
indirectly influence soil fertility. These actions are 
intimately associated with the organic matter of the 
soil and its bacterial inhabitants. Mineralogy and 
inorganic chemistry, though helpful, are no longer 
capable of solving soil problems. Biochemistry and 
bacteriology, with their modern conceptions of 
colloids, absorption phenomena, enzymes, oxidizing, 
