320 NEW ZEALAND NATURE-STUDY BOOK 
The chemical composition of clay is not so simple as that 
of sand. Pure clay consists of silica and alumina, both 
composed of oxygen, with a certain proportion of water ; 
hence as a source of plant food it would be as useless as 
sand. But clay is rarely pure—a number of impurities 
being almost invariably present—these usually contain 
elements such as calcium, potassium, magnesium, and iron, 
which are so essential for the nutrition of plants. Clay is 
firm and tenacious, does not easily become heated, is able 
to retain moisture, and so is specially adapted for resisting 
a drought. 
A soil composed entirely of clay would be damp and 
cold, and would tend to become muddy, for the rain, as it 
fell on the clay, would be unable to drain away. As one of 
the constituents of soil, however, clay possesses many 
useful properties — thus it retains moisture, thereby 
preventing the soil from becoming dry ; it makes the soil 
firm and tenacious ; it absorbs and holds many of the useful 
products resulting from the decomposition of manures ; and 
it contains many substances useful as plant food. 
XVIIL—SAND AND CLAY. 
Useful lessons could be given under this heading either 
to senior or junior classes, (the class depending on the 
method of treatment)—the subject will furnish plenty of 
opportunity for establishing comparisons and contrasts. 
From what has already been said, there will be no difficulty 
in working out the following. 
Sand is composed of little grains, will not bind together 
even when wet, offers but little hold for the roots of plants, 
is exceedingly porous and hence will not hold moisture, is 
derived from the disintegration of rocks, and settles down 
quickly in water. 
