h iron, which
is deposited as a yellowish or red mineral on objects over which it
flows. Ponds fed by these springs accumulate deposits of the mineral in
the muddy bottoms. Some of the most valuable deposits of iron ore have
accumulated in bogs fed by iron-impregnated spring water. In a similar
way lime deposits called marl or chalk are made.

THE SOIL IN FIELDS AND GARDENS

City and country teachers are expected to teach classes about the
formation and cultivation of soil. It is surprising how much of the
needed materials can be brought in by the children, even in the cities.
The beginning is a flowering plant growing in a pot. A window box is a
small garden. A garden plot is a miniature farm.

_Materials to collect for study indoors._ A few pieces of different
kinds of rock: Granite, sandstone, slate; gravelly fragments of each,
and finer sand. Pebbles from brooks and seashore. Samples of clays of
different colors, and sands. Samples of sandy and clay soils, black pond
muck, peat and coal. Rock fossils. A box of moist earth with earthworms
in it. _Keep it moist._ A piece of sod, and a red clover plant with the
soil clinging to its roots.

_What is soil?_ It is the surface layer of the earth's crust, sometimes
too shallow on the rocks to plough, sometimes much deeper. Under deep
soil lies the "subsoil," usually hard and rarely ploughed.

_What is soil made of?_ Ground rock materials and decayed remains of
animal and plant life. By slow decay the soil becomes rich food for the
growing of new plants. Wild land grows up to weeds and finally to
forests. The soil in fields and gardens is cultivated to make it
fertile. Plants take fertility from the soil. To maintain the same
richness, plant food must be put back into the soil. This is done by
deep tillage, and by mixing in with the soil manures, green crops, like
clover, and commercial fertilizers.

_Plants must be made comfortable, and must be fed._ Few plants are
comfortable in sand. It gets hot, it lets water through, and it shifts
in wind and is a poor anchor for roots. Clay is so stiff that water
cannot easily permeate it; roots have the same trouble to penetrate it
and get at the food it is rich in. Air cannot get in.

Sand mixed with clay makes a mellow soil, which lets water and air pass
freely through. The roots are more comfortable, and the tiny root hairs
can reach the particles of both kinds of mineral food. But the needful
third element is decay