Causes of Fertility or Barrenness of Soils. 
189 
Phosphoric acid is a most important ingredient of soils, and its 
application when deficient has been most successful. It is a 
compound of oxygen with the metal phosphorus ; it occurs 
united with various bases, as lime, magnesia, potash, soda, &c. ; 
such combinations being known as phosphates. Phosphate of 
lime, the most common form, causes the high manuring pro- 
perties of bones. In this state it is insoluble, but when the bones 
are digested in sulphuric acid, a chemical change takes place, 
and the resulting superphosphate, as it is called, contains a con- 
siderable quantity of soluble phosphate ; hence its action is more 
rapid than simple bones ; the latter are more lasting, and were 
formerly applied with the greatest advantage to worn out 
pastures. In this way the extensive dairy districts of Cheshire 
have been improved. For ages before, the milk (which contains 
a large percentage of bone eartli) and the young stock sold off, 
had abstracted all the phosphates from the soil, and the pastures 
were yearly becoming more exhausted. There have recently 
been discovered certain soils on the upper greensand, very rich 
in phosphates derived from the decomposition of coprolites or 
fossil manure, and their fertility is very remarkable. The bed 
from which they are derived is quarried, and forms a most 
valuable manure. 
Sulphuric acid is also found in the ashes of plants, and, united 
with various bases, exists in most soils. It has been found very 
beneficial Avhen imited with lime, as a top dressing for clover, 
especially on sandy soils. 
Magnesia is the oxide of magnesium, possessing basic proper- 
ties, and closely resembling lime in its nature. It occurs in most 
soils ; all limestone rocks contain more or less as carbonate ; sandy 
soils are most deficient, and its application to such in the form 
of sulphate (Epsom salts) has been found successful. The 
magnesian limestone consists of nearly equal parts of carbonate 
of magnesia and lime ; many fertile soils are found resting on it, 
which seems to refute the prevalent opinion of its poisonous 
tendency, when existing in large quantities. 
The alkalies. Potash and Soda, may be considered together, as 
they possess similar properties, and it is even believed are in 
certain cases capable of replacing each other in the cells of a 
plant. They are oxides of metallic bodies, potassium and sodium, 
and occur in the soil as salts, united with carbonic, silicic, sul- 
phuric, phosphoric, or hydrochloric acids. The percentage of 
each found in plants is very small, yet their jnesence in the soil 
is most important ; all plants contain them. The fertile nature 
of wood ashes is due to carbonate of potassa ; sea weeds owe 
their manuring effects principally to the alkalies ; clay soils are 
rich in alkalies, often occurring as silicates in an insoluble form, 
