es the crop must be
earthed up, care being exercised not to earth up too much, for, taking
six inches as the best average depth, the crop will be diminished by an
increase beyond this depth. One urgent reason for early work between the
rows is that a prosperous crop will soon put a stop to it. The moment it
becomes likely that the shaws will be bruised by traffic between the
rows they must be left to finish their course in their own way, because
the formation of tubers below will be in the ratio of the healthy growth
above ground. The Potato may be said to be manufactured out of sunshine
and alkaline salts. The green leaves constitute the machinery of the
manufacture, for which the solar light from above, and the potash,
phosphate of lime, phosphate of magnesia, and phosphoric acid from below
are the raw materials.

==Change of Ground and Seed.==--In common with all other crops, the Potato
needs as often as possible a fresh soil, and a renewal of seed from some
distant source. The need for a change of soil is made apparent by an
analysis of the root, which contains large proportions of potash,
phosphorus, and sulphur, with smaller proportions of magnesia and lime,
without which the plant cannot prosper. A succession of heavy crops of
Potatoes on the same land may be said to take from the soil its
available potash and phosphates, and this crop will not, like some
others, take soda instead of potash when the last-named alkali runs
short. Here then is a chemical reason for change of soil. Another reason
is found in the history of the species of fungi that prey on the Potato
when its growth is checked by heavy rains and a low temperature. These
leave their spores in the soil, like wolves hiding in ambush, to
destroy the next crop. They are powerless to attack any other crop;
therefore a suitable rotation gives them time to die out and leave the
land clean as regards the =Phytophthora= and other parasites that
destroy Potato crops. The necessity for an occasional change of seed
rests on old experience, and should scarcely need enforcing. One word
may be said here by way of explanation, and it is this: the seed house
that aims to put a good article in the market adopts measures which
altogether differ from those followed by the majority of persons who
have not been trained to the business. It is a common experience to find
that those who save their own seed from year to year have as a result a
constantly declining strain, so