322 NEW ZEALAND NATURE-STUDY BOOK 
When carbonate of lime is present in quantity ranging 
from 5 to 20 per cent. the soil is called a marl; whena 
higher percentage is present, the name given isa calcareous 
soil. 
For nearly all purposes of cultivation a loamy soil is best 
—and one, moreovery which does not contain an excessive 
percentage of either sand or clay—in other words a medium 
loam. Hence the operations of the skilful cultivator are 
directed towards bringing the soil into this condition. 
CHEMICAL INGREDIENTS OF SOILs. 
We have already seen that soils contain silica, alumina, 
and lime. Other ingredients usually present are magnesia, 
oxide of iron, potash, soda, phosphoric acid, sulphuric acid, 
and chlorine. One of these, chlorine is a gas ; all of the 
others are oxides, that is, compounds of oxygen with some 
other element, though in the case of the acids, water also 
enters into the composition. 
Chlorine is an element, (gas). 
Magnesia is a compound of the metal magnesium, and 
oxygen. 
Oxide of iron is a compound of the metal iron, and 
oxygen. » 
Potash is a eeneund of the metal Borate and 
oxygen. 
Soda is a compound of the metal sodium, and oxygen. 
Sulphuric acid is a compound of the mineral sulphur, 
oxygen, and hydrogen. 
Phosphoric acid is a compound of phosphorus, oxygen, 
and hydrogen. 
With the exception of oxide of iron, none of these 
substances usually exist free in the soil—that is, soils do — 
not contain potash, soda, or lime as such, but hold them in 
