50 
many soils contain so small a store that they quickly become so 
far exhausted as to cease to yield profitable returns. Of these 
four substances it is, generally speaking, the phosphoric acid that 
begins to fail first, for most soils contain much more nitrogen and 
potash than they do phosphoric acid ; then the nitrogen begins to 
fail, and finally the potash and lime. Some soils may also per- 
haps fail in magnesia, though I have not yet had such brought 
under my notice. But the four substances—nitrogen, potash, 
phosphoric acid, and lime—are the four plant foods about which. 
we have chiefly to concern ourselves in manuring. But we may 
reduce the list still further, for we cannot give phosphoric acid 
to the soil without at the same time giving lime, for any substances 
we may use for supplying phosphoric acid to the soil invariably 
contain the phosphoric acid combined with lime. Thus, then, 
our list is reduced to three—namely, nitrogen, phosphoric acid, 
and potash ; and these are the three plant foods to which we may 
practically confine our attention in manuring. 
But how are we to know that we shall have to apply any or 
all of these plant foods to our particular soil? Our soil may be 
naturally rich enough in these plant foods, or at least in one or 
two of them. We may find this out in most cases by means of a 
chemical analysis of the soil ; there are cases in which a chemical 
analysis does not give all the information one wants, but it will 
at least show us if the soil is poor in plant foods. If the chemical 
analysis show us that the soil is poor in any particular plant food 
then we may make up our minds at once that we shall have to 
supply that plant food. But there is another way of finding out 
which of the three plant foods a soil requires to have given 
to it. I refer to the system of trial or test plots in the field, 
which during the last few years I have so often advocated. 
Suppose we have a series of plots laid out according to the 
following plan :— 
Plot 1. Nitrogen, phosphoric acid, and potash; light dressing. 
Plot 2. Do.; medium dressing. 
Plot 3. No manure. 
Plot 4. Nitrogen, phosphoric, and potash; heavy dressing. 
Plot 5. Same as Plot 2, but without nitrogen. 
Plot 6. Do., but without phosphoric acid. 
Plot 7. Do., but without potash. 
Plot 8. No manure. : 
% Plot 3 receives no manure; Plot 2, alongside of it, receives a 
mixture of the three plant foods; this mixture it is customary to 
call a complete manure. Now, if Plot 2 gives us a better yield 
than Plot 8, we shall know that the soil needs to have some 
