ing it the
porous condition so essential to productiveness. Improved physical
condition is likewise given to a sandy soil, the humus binding the
particles together.

2. To make the soil retentive of moisture. Yields of crops are
limited more by lack of a constant and adequate supply of moisture
throughout the growing season than by any other one factor. Decayed
organic matter has great capacity for holding moisture, and in some
measure should supply the water needed during periods of light
rainfall.

3. To serve, directly and indirectly, as a solvent of the inert
plant-food in the soil that is known as the "natural strength" of
the land. Its acids do this work directly, and by its presence it
makes possible the work of the friendly bacteria that are man's
chief allies in maintaining soil fertility.

4. To furnish plant-food directly to growing plants. Even when it
has been produced from the soil supplies alone, there is great gain
because the growing crop must have immediately available supplies.
Many of the plants used in providing humus for the soil are better
foragers for fertility than other plants that follow, sending their
roots deeper into the subsoil or using more inert forms of
fertility.

The Legumes.--Any plant that grows and rots in the soil adds to the
productive power of the land if lime is present, but plants differ in
value as makers of humus. There are only ten essential constituents of
plant-food, and the soil contains only four that concern us because the
others are always present in abundance. If lime has been applied to
give to the soil a condition friendly to plant life, we are concerned
with three constituents only, viz. nitrogen, phosphoric acid, and
potash. The last two are minerals and cannot come from the air. They
must be drawn from original stores in the soil or be obtained from
outside sources in the form of fertilizers. The nitrogen is in the air
in abundance, but plants cannot draw directly from this store in any
appreciable amount. The soil supply is usually light because nitrogen
is unstable in character and has escaped from all agricultural land in
vast amounts during past ages.

Profitable farming is based upon the great fact that we have one class
of plants which can use bacteria to work over the nitrogen of the air
into a form available for their use, and the store of nitrogen thus
gained can