ry elements that we find in stable manures. They may
carry nitrogen, phosphoric acid, or potash,--any one or two or the
three,--and the three are the constituents that usually are lacking in
available forms in our soils. Examples of the best modern skill in
farming may be found in the rational selection and use of commercial
fertilizers.

Tillage.--Man's ability to assist nature in the work of production
finds a notable illustration in the matter of tillage. Its purpose is
to provide right physical condition of the soil for the particular
class of plants that should be produced, while destroying the
competition of other plants that are for the time only weeds. Most
soils become too compact when left unstirred. The air cannot enter
freely, plant-roots cannot extend in every direction for food, the
water from rains cannot enter easily, there is escape of the moisture
in the ground, and weathering of the soil proceeds too slowly. The
methods used in plowing, harrowing, and later cultivations fix the
productive power of a soil for the season in large measure.

Control of Soil Moisture.--The water in the soil is a consideration
that has priority over plant-food in the case of agricultural land. The
natural strength of the soil is sufficient to give some return to the
farmer in crops if the moisture content is right throughout the season.
The plant cannot feed unless water is present; the process of growth
ceases in the absence of moisture. One purpose of plowing is to
separate the particles of soil to a good depth so that water-holding
capacity may be increased. When the soil is compact, it will absorb and
hold only a very limited amount of moisture. We harrow deeply to
complete the work of the plow, and the roller is used to destroy all
cavities of undue size that would admit air too freely and thus rob the
land of its water. Later cultivations may be given to continue the
effect of the plow in preventing the soil from becoming too compact,
but usually should be required only to make a loose mulch that will
hold moisture in the ground, and to destroy the weeds that would
compete with the planted crop for water, food, and sunshine.

CHAPTER II

THE NEED OF LIME

The Unproductive Farm.--When a soil expert visits an unproductive farm
to determine its needs, he gives his chief attention to four possible
factors in his problem: lack of drainage, of lime, of organic matter,
and of available plant-food. His first con