the soil for the roots of plants that are to follow.
Thus this group of plants, known as legumes, have been used for
generations as a method of increasing the nitrogen content of soils.
Nitrogen, incidentally, is the most costly element to buy in commercial
fertilizers. The soil-improving benefits of legumes may be secured by
growing them either for harvest as a source of animal food or for plowing
under as a means of utilizing them entirely for the development of soil
fertility.

In reading of the studies of soil fertility that were made by George
Washington at Mount Vernon, we learn of the improvement that he made in
the relatively poor soils of that area by growing plants of the legume
family. The actual reason why such improvement was brought about was not
known in Washington's time, but the results were apparent. Today, the
value of legumes as soil builders is well recognized and we understand
much more definitely than Washington did the reasons for their being so
helpful in increasing crop production.

Many soil areas do not contain the particular type of bacteria necessary
to the fixation of atmospheric nitrogen by legumes. This is frequently the
cause of failure in growing alfalfa, soybeans, cowpeas and less well known
members of the legume family. Each legume has its own type of
nodule-forming bacteria. In order to assure the presence of the proper
bacterial family, means often must be employed to add them to the soil
where the specific crop is to be grown. This may be accomplished by adding
soil from an area where the legume does well to the new area, or the seed
may be inoculated with commercial cultures before seeding. Either method
is effective. If soil is used it should be drilled in or spread on a
cloudy day to prevent the destructive action of the sun's rays on the
exposed minute forms of plant life we call bacteria.

If it is not known that the legume to be planted has been grown
successfully in a given field within the previous several years, the
precaution of adding the proper bacteria should be taken. In some
sections, such legumes as red, alsike, crimson and white clovers have been
grown for many years and the bacteria for these plants are well
distributed. There, inoculation is not necessary for these crops, but it
probably should be practiced if other legumes such as alfalfa, cowpeas or
soybeans are to be grown on land for the first time.

_The Value of Humus._--In addition to the chemical elem