HORTICULTURAL SOILS. 
81 
How Plants are Dependent upon the Food Supply in 
the Soil. 
The invaluable investigations of Rothan^sted just referred 
to, and others of a similar kind, illustrate this fact among others 
—that the crop or particular plant we grow has to do not only 
with the supply of food in the soil as a whole, but also with each 
of its ingredients separately. The total productive power of a 
soil cannot exceed its power to supply to the growing plant each 
and all the necessary food constituents. Every plant we culti¬ 
vate must have a certain amount of each of the nutritive 
elements—potash, phosphoric acid, and nitrogen—or it cannot 
grow satisfactorily. Thus the plant cannot rise above the level 
of the lowest ingredient in the food supply. If each description 
of food comes up to the required standard, and other conditions 
of heat and moisture are favourable, a good result may be 
expected; but if any one element falls below this standard, the 
growth of the crop must suffer. 
We have seen in the various illustrations brought forward 
that the food supply available to plants varies greatly in different 
soils. Sometimes one constituent and sometimes several may 
be lacking. An horticultural soil may have a proper texture, 
with a suitable amount of moisture, and, in fact, a full supply 
of everything the plant needs, except phosphoric acid ; if so, it 
cannot yield a full crop. Add phosphate in an available form 
and the growing plants will be benefited. Another soil may be 
deficient in potash, another in lime, another in nitrogen, still 
another in two or three of these substances. This same variation, 
as shown in Table VII. ( b ), may run through inherent fertility 
of the soil and in the solubility of its constituents. Therefore 
an horticultural soil may be deficient in available mineral 
ingredients or in available nitrogen. Or it may be so compact 
that air and moisture cannot get into it to work over the crude 
material it contains, nor the plant roots make their way through 
to obtain the food that has been made soluble. Again, it may 
be so loose and non-retentive that the food constituents will 
escape by drainage. Or, on the other hand, it may be so dry 
that fertilisers will be useless, and plants wither for lack of 
moisture ; or so wet and cold as to prevent plant growth. In these 
several cases proper tillage operations will assist in amending the 
G 
