90 . THE SOIL. 



Hydrated silicic acid loses its solubility in water by 

 simple drying, and it frequently happens that the 

 drainage of a marshy field will cause the siliceous plants 

 (reeds and horsetail) to disappear. The action exerted 

 upon the soil by hydrate of lime, or by lime slaked or 

 fallen to powder in the air, is twofold. On a soil rich 

 in humus constituents the lime combines, in the first 

 place, with the organic compounds present, which have 

 an acid reaction ; it neutralises the acid of the soil, 

 thereby causing the speedy disappearance of many 

 weeds, such as bog-moss (S-phagnum} and reed-grasses, 

 which flourish in a sour soil of this kind. Simple con- 

 tact with acids powerfully promotes the oxidation of 

 metals (copper, lead, iron), while contact with an alkali 

 prevents it (iron coated with a dilute solution of carbon- 

 ate of soda will not rust). Upon organic substances, 

 the action is the very reverse: acids prevent, and 

 alkalis promote, oxidation or decay. Excess of lime 

 causes the aforesaid destruction of the humose con- 

 stituents. 



In the same degree as the acid humus, by the action 

 of lime, disappears from the ground, the absorptive 

 power of the latter for hydrated silicic acid is increased ; 

 and the excess of this acid present loses its mobility in 

 the soil.* 



The action of lime, as we see, is so complex, that 

 from its favourable influence upon one field, it is 

 scarcely ever possible to form an opinion of its probable 

 action upon another field, the condition of which is 

 unknown. This is possible only when the causes of its 

 favourable action in the first case are clearly understood. 



"When lime has improved the condition of a field, 

 simply by neutralising the acid state of the soil, and 



* In an experiment made specially for the purpose, it was found that a 

 litre (about a quart) of forest soil, containing SO per cent, of humose con- 

 stituents, absorbed from a solution of silicate of potash only 15 milli- 

 grammes of silicic acid. But the same soil mixed with 10 per cent, of 

 washed chalk (carbonate of lime) absorbed 1140 milligrammes; and when 

 mixed with 10 per cent, of slaked lime instead of chalk, the absorptive 

 power was increased to such a degree, that a litre absorbed 3169 milli- 

 grammes of silicic acid. 



