In tobacco regions tobacco refuse is a valuable source of potash, the
stems are about five per cent. potash.

LIME

_Lime_ is generally supplied to the soil in the form of quicklime made
by burning lime stone or shells. Other forms are gypsum or land
plaster, gas lime (a refuse from gas works) and marl. Most soils
contain sufficient lime for the food requirements of most plants. Some
soils, however, are deficient in lime and some crops, particularly the
legumes, are benefitted by direct feeding with lime.

Lime is valuable for its effect on the soil properties which
constitute fertility.

Physically lime acts on the texture of the soil making clay soils
mealy and crumbly, and causing the lighter soils to adhere or stick
together more closely.

Chemically, lime decomposes minerals containing potash and other plant
foods, thus rendering them available for the use of plants. It also
aids the decay of organic matter and sweetens sour soils.

Biologically lime aids the process of nitrification.

The action of lime is greatest in its caustic or unslacked form.

Too much or too frequent liming may injure the soil. It should be
carefully tried in a small way, and its action noted, before using it
extensively.

A common way of using lime is to place twenty to forty bushels on an
acre in heaps of three to five bushels, covering them with soil until
the lime slacks to a fine powder. The lime is then spread and harrowed
in. Lime tends to hasten the decay of humus. It should not be applied
oftener than once in four or five years.

_Gypsum_, a sulphate of lime, is similar to lime in its action on the
soil. Its most important effect is the setting free of potash from its
compounds.

_Gas lime_ should be used with great care as it contains substances
that are poisonous to plant roots. It is best to let it lie exposed to
the weather several months before using.

_Marl_ is simply soil containing an amount of lime varying from five
to fifty per cent. It has value in the vicinity of marl beds but does
not pay to haul very far.

CHAPTER XXII

COMMERCIAL FERTILIZERS--CONTINUED

MIXED FERTILIZERS

_What they are._

There are a large number of business concerns in the country which buy
the raw materials described in Chapter XXI, mix them in various
proportions, and sell the product as mixed or manufactured
fertilizers. If these mixtures contain the three important plant
foods, nitrogen, phosphoric acid and p