s
soil. This can be done at the expenditure of a little time and trouble,
and should be carried out in the following way. The field on which it is
desired to carry out the experiments should be divided into the
requisite number of experiment plots. These, which may be the tenth,
twentieth, or fortieth of an acre in extent, should be, if possible, on
a level piece of ground--all of them equally free from the shelter of
hedge or tree, and otherwise subjected to the same conditions. The
nature of the soil of the different plots, as well as its past
treatment, should be similar. It is desirable, in order to minimise
experimental error as much as possible, to carry out the experiments in
duplicate, or even triplicate. In the first place, there should be what
is called a _nothing_ plot--_i.e._, a plot receiving no manure. The
produce obtained from this plot, compared with the produce obtained from
the other manured plots, will thus furnish data for estimating the
respective amounts of increase obtained by different manures. One very
simple kind of experiment is what is called the "seven-plot" test. It
consists in testing the results obtained by using nitrogenous,
phosphatic, and potash manures alone and in different combinations. Thus
the plots would be manured respectively as follows:--

No. No.
1. Nothing plot. | 5. Nitrogen and phosphates.
2. Nitrogen. | 6. Nitrogen and potash.
3. Phosphates. | 7. Phosphates and potash.
4. Potash.

The subjects of other experiments might be such as the respective values
of nitrogen in the different forms of sulphate of ammonia and nitrate of
soda; phosphoric acid as superphosphate, and in an undissolved form as
Thomas-slag; the relative importance of artificial and farmyard manure;
the effect of manures applied at different times, as well as the effect
of different quantities of the same manure; the most economical manures
for different kinds of crops; and numerous other interesting problems
connected with the practical application of manures.

In carrying out these experiments, care should be taken not to have the
experimental plots _immediately_ adjoining one another, as the manure
applied to the one plot may, by soaking through the soil, affect the
result on the adjoining plot. Especial note ought to be taken of the
weather during the progress of the experiment. In order to make such
experiments as valuable as possible, they ought to be conti