Magnesium, a silver white solid.

Iron, a silver gray solid.

Of these elements the nitrogen, sulphur, phosphorus, potassium,
calcium, magnesium, and iron must not only exist in the soil but must
also be there in such form that the plant can use them. The plant does
not use them in their simple elementary form but in various compounds.
These compounds must be soluble in water or in weak acids.

Of these seven elements of plant food the nitrogen, phosphorus, and
potassium and calcium are of particular importance to the farmer,
because they do not always exist in the soil in sufficient available
quantities to produce profitable crops. Professor Roberts, of Cornell
University, tells us that an average acre of soil eight inches deep
contains three thousand pounds of nitrogen. The nitrogen exists
largely in the humus of the soil and it is only as the humus decays
that the nitrogen is made available. Here is another reason for
keeping the soil well supplied with organic matter. The decay of this
organic matter is hastened by working the soil; therefore good tillage
helps to supply the plant with nitrogen.

If the nitrogen becomes available when there is no crop on the soil it
will be washed out by rains and so lost. Therefore the soil,
especially if it is sandy, should be covered with a crop the year
through. Many lands lose large amounts of plant food by being left
bare through the fall and winter, especially in those parts of the
country where the land does not freeze. The phosphorus, potassium and
calcium also exist in most soils in considerable quantities, but often
are not available; thorough tillage and the addition of organic matter