the underlying
rock and is usually distinguished from the upper portion by its lighter
colour, poorer texture, and smaller supply of available plant food. The
difference is due largely to the absence of humus. The character of the
subsoil has an important bearing on the condition of the upper soil. A
layer of sand or gravel a few feet below the surface provides natural
drainage, but if it be too deep, it may allow the water to run away
rapidly, carrying the plant food down below the roots of the plants. A
hard clay subsoil will render the top too wet in rainy weather and too
dry in droughts, because of the small amount of water absorbed. Such a
soil is benefited by under-draining. A deep and absorptive subsoil
returns water to the surface, by capillary action, as it is needed. The
subsoil finally contains a large amount of plant food, which becomes
gradually changed into a form in which plants can make use of it. Pupils
should find out the character of the subsoil in their various fields at
home and its effect on the fertility of the field.

FERTILIZERS

Along with water, the roots take up from the soil various substances
that are essential to their healthy growth. Potash, phosphoric acid,
nitrogen, calcium, sulphur, magnesium, and iron are needed by plants,
but the first three are particularly important. If land is to yield good
crops year after year, it must be fertilized, that is, there must be
added chemicals containing the above-mentioned plant foods. Land becomes
poor from two causes: the plant food in the soil becomes exhausted, and
poisonous excretions from the roots of one year's crops act injuriously
on those of the next season. Rotating crops will improve both conditions
for a while, but eventually the soil will require treatment.

Humus contains plant food and is also an excellent absorbent of the
poisonous excretions. It is added as barn-yard manure, leaves, or as a
green crop ploughed in.

The chemicals commonly used comprise nitrate of soda, bone meal,
sulphate of potash, chloride of potash, lime, ashes, cotton-seed meal,
dried blood, super-phosphate, rock phosphate, and basic clay.

EXPERIMENTS:

1. Sow wheat on the same plot year after year and note the result when
no fertilizer is used. Sow wheat on another plot, but use good manure.

2. Try the various commercial fertilizers on the school plots, leaving
some without treatment.

3. Examine the roots of clover, peas, or beans, and look for nodu