In looking at the amounts of potash, we are at once struck by their great discrepancy. Such a crop as mangels removes more than six times as much potash from the soil as the cereals. Turnips also make large demands on this ingredient, removing over four times as much as the cereals. Leguminous crops, such as red clover and beans, remove about twice as much.
Capacity of Crops for assimilating Manures.
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.
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 shallow-rooted crops will therefore be towards impoverishing the surface-soil; whereas the occasional growth of a deep-rooted crop brings the plant-food in the subsoil into requisition. In this connection it may be well to draw attention to the singular capacity possessed by certain crops for absorbing nitrogen. Of these the case of clover is the most striking, and has long puzzled agriculturists. The discovery, which has been repeatedly referred to in these pages, that the leguminous order of crops, to which clover belongs, have the power of absorbing the free nitrogen of the air through the agency of micro-organic life in the plant and in the soil, has furnished an explanation of this long-debated problem.
Period of Growth.
A further reason is the difference in the period of a crop's growth. A crop which grows quickly, and consequently occupies the ground during a comparatively short period, will naturally require a richer soil, and therefore a more liberal treatment with manure, than one whose growth is more gradual.
Another consideration is the season of the year during which active growth of the crops takes place. For example, in the case of the wheat crop, active growth takes place in spring and ceases early in the summer. Since, however, nitrification goes on right through the summer, and nitrates are most abundant in the soil in late summer and autumn, such a crop as wheat is ill suited to obtain any benefit from this bountiful provision of nature, and is consequently particularly benefited by the application of nitrogenous manures. Root crops, on the other hand, sown in summer, continue their active growth into autumn, and are thus enabled to utilise the nitrates formed in the process of nitrification. The custom of sowing a quickly growing green crop, such as rye, mustard, rape, &c., after a wheat crop, is a practice which aims at conserving the nitrates and preventing their loss by autumn and winter rains. The name "catch crop" has been applied to such a crop. By ploughing under the green crop, the nitrogen removed from the soil in the form of easily soluble nitrates is restored in an insoluble organic form, and the soil is at the same time enriched by the addition of much valuable organic matter.[243]
It is chiefly the above facts that form the scientific basis of the long-pursued practice of the rotation of crops.
Variation in Composition of Crops.