18 per cent of phosphoric acid. The availability of the
plant-food in this fertilizer has been the subject of much discussion.
The chemist's test which is fair for acid phosphate is admittedly not
fair when used for basic slag. Field tests, at experiment stations and
on farms, are our best sources of knowledge. When the soil is slightly
acid, each 1 per cent of phosphoric acid in the slag appears to be
about as valuable as each 1 per cent of the available phosphoric acid
in an acid phosphate. Some of the effectiveness may be due to the lime,
although very little of it is in forms regarded as valuable for the
correction of soil acidity. There is evidence that basic slag favors
clover. It has not been found feasible to ship this material many
hundreds of miles inland from the seaboard to compete with acid
phosphate, but it is an excellent source of phosphoric acid for soils
that are not rich in lime.

Muriate of Potash.--The mines of Stassfurt, Germany, contain an
inexhaustible supply of potash in various compounds. Muriate of potash
is prepared from the crude salts, and the commercial product on our
markets has the appearance of a coarse and discolored salt. It is
handled in large bags, and inclines to become moist by absorption of
water from the air. It contains some common salt. The content of actual
potash is about 50 per cent. The potash is readily available, but the
loss from leaching out of the soil is very small. Muriate of potash is
our cheapest source of potash, and should be used for all staple crops
except tobacco, sugar beets, and, possibly, the potato. Tests even on
heavy soils fail to show any injury to the quality of the potato, and
on light soil the muriate may always be used.

Sulphate of Potash.--Some sulphate of potash is imported into this
country. Its content of potash may vary 1 or 2 per cent below or above
50. Its physical condition favors mixing more than does the muriate. It
usually costs several dollars a ton more than the muriate, and the fact
that it is known to favor quality in tobacco, and is popularly supposed
to do so in the potato, creates demand at the higher price. It is
soluble in water, and quickly available. As a rule, it has no higher
agricultural value than the muriate.

Kainit.--Unlike muriate and sulphate of potash, kainit is a crude
product of the German mines, having received no treatment to remove
impurities. It contains 12 to 13 per cent of potash, and is rated as a
sulpha