SAND AND CLAY 320 
combination with other substances. The chemical in- 
gredients in fact exist in the form of salts—a salt being a 
substance formed by the union of an acid with a base. 
For instance the base magnesia unites with the acid 
carbonic acid to form carbonate of magnesia, and the 
base lime unites with the acid phosphoric acid to 
form phosphate of lime. The resulting compounds 
carbonate of magnesia and phosphate of lime are known as 
salts. 
Of the various substances required for the nutrition 
of plants, there are four of which the available supply is 
likely to run short, so that the cultivator has to be 
continually making good the deficiency. These are 
phosphoric acid, potash, lime, and nitrogen. The 
three former are mineral ingredients, for they occur 
naturally'in the soil. Nitrogen is derived from the decay 
of organic matter in the soil, though small quantities in 
the form of ammonia—(a compound of nitrogen and 
hydrogen) and nitric acid (a compound of nitrogen, oxygen, 
and hydrogen) are brought down dissolved in rain. 
LOCAL AND TRANSPORTED SOIL. 
When a soil is derived from the decomposition or 
weathering of the underlying rock it is known as a local 
soil; when, however, it bears no relationship to the rock, 
lying beneath, but has been brought to its present position 
from elsewhere it is called a transported soil. 
An examination of the soil of a district, especially in 
cuttings, river channels, land-slips ete., where the exposures 
are fresh, will in most cases reveal its origin. The alluvial 
flats forming such extensive tracts in many of our river 
valleys furnish good examples of transported soil. In 
nearly all cases the agent concerned in the removal and 
deposition of soils is water. 
