apter that the plant foods, nitrogen,
phosphoric acid, potash and lime, are apt to be found wanting in
sufficient available quantities to supply the needs of profitable
crops. We learned also that lime is useful in improving the texture of
the soil and in making other plant foods available. Now the commercial
fertilizers are used to supply the soil with these four substances and
they may be classified according to the substance furnished as
follows:

Sources of nitrogen,
" " phosphoric acid,
" " potash,
" " lime.

SOURCES OF NITROGEN

Nitrogen is the most expensive of plant foods to buy, therefore
special attention should be given to producing it on the farm by means
of barn manures and legumes plowed under.

The principal commercial sources of nitrogen are: Nitrate of soda,
sulphate of ammonia, dried blood, tankage, dry ground fish,
cotton-seed meal.

_Nitrate of Soda_ or Chile saltpetre containing 15.5 per cent. of
nitrogen, is found in large deposits in the rainless regions of
western South America. In the crude state as it comes from the mine it
contains common salt and earthy matter as impurities. To remove these
impurities the crude nitrate is put into tanks of warm water. The
nitrate dissolves and the salt and earthy matter settle to the bottom
of the tank. The water with the nitrate in solution is then drawn off
into other tanks from which the water is evaporated, leaving the
nitrate, a coarse, dirty looking salt which is packed in
three-hundred-pound bags and shipped.

Plants that take their nitrogen from the soil take it in the form of
nitrate. Hence nitrate of soda, which is very soluble in water, is
immediately available to plants and is one of the most directly
useful nitrogen fertilizers. It is used for quick results and should
be applied only to land that has a crop or is to be immediately
planted, otherwise it is liable to be lost by leaching.

_Sulphate of Ammonia_ contains 20 per cent. of nitrogen. It is a white
salt, finer and cleaner looking than the nitrate. It is a by-product
of the gas works and coke ovens. The nitrogen in it is quite readily
available.

_Dried Blood_ contains 8 to 12 per cent. of nitrogen. This is blood
collected in slaughter-houses and dried by steam or hot air. It decays
rapidly in the soil and is a quick acting nitrogen fertilizer.

_Tankage_ contains 4 to 8 per cent. of nitrogen and 7 to 20 per cent.
of phosphoric acid. Slaughter-