oneously they are, as furnishing by themselves
sufficient data upon which to base the practice of manuring_. A
consideration which is of much greater importance is the capacity that
different crops possess for assimilating the various manurial
ingredients from the soil. Considered from the point of view of absolute
amount, there is in most soils an abundant supply of plant-food; but of
this amount only a small proportion is available. Further, the amount of
this available plant-food will vary with different crops--one crop being
able to grow where another crop would starve. As illustrative of this,
in the Norfolk experiments it was found that the turnip was able to
assimilate potash from a soil on which the swede was practically
starved. It is on this fact more than any other that the principles of
manuring are based. Several explanations of the different capacities
crops possess of assimilating their food may be put forward. And we may
here point out that crops belonging to the same class exhibit, on the
whole, a certain amount of similarity in their manurial requirements.
Thus, for example, we may say that _gramineous crops_ so far resemble
one another in possessing _small capacity for assimilating nitrogen_,
_root crops for assimilating phosphoric acid_, and _leguminous crops for
assimilating potash_, and that, consequently, these crops are generally
most benefited by the application, respectively, of nitrogen, phosphoric
acid, and potash. But while a certain general resemblance exists, crops
belonging to the same class differ in many cases very considerably, as
we shall immediately see.

_Difference in Root Systems of different Crops._

One explanation of the different capacity possessed by different crops
for absorbing plant-food from the soil is to be found in the difference
of their root systems. Every agriculturist knows that crops in this
respect differ very widely. Crops having deep roots will naturally have
a larger surface of soil from which to draw their food-supplies than
crops having shallower roots. Such crops as red clover, wheat, and
mangels are able to draw their food-supplies from the subsoil to an
extent not possessed by shallower-rooted crops, such as barley, turnips,
and grass. Crops having surface-roots, on the other hand, have often
greater capacity for assimilating nitrogen,--this ingredient, as has
already been pointed out, being chiefly located in the surface-soil. The
tendency of growing