Instructive though these figures undoubtedly are, _they must not be regarded, as often erroneously 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._