The Legumes
Categories:
ORGANIC MATTER
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 consti
uents 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 be added to the soil's supply for future crops. These
plants, known as legumes, embrace the clovers, alfalfa, the vetches,
peas, beans, and many others of less value. They provide not only the
organic matter so much needed by all thin soils, but at the same time
they are the means of adding to the soil large amounts of the one
element of plant-food that is most costly, most unstable, and most
deficient in poor soils. Their ability to secure nitrogen for their own
growth in poor land also is a prime consideration in their selection
for soil improvement, assuring a supply of organic matter where
otherwise partial failure would occur.