A point of considerable interest is the influence exerted by manures on the composition of crops. It has been assumed in the preceding pages that the composition of crops of the same plant is uniform; but this is not strictly the case, as it has been proved that not merely the manure and soil have an appreciable influence on the crop's composition, but so also has the climate.
Absorption of Plant-food.
The laws regulating the absorption of plant-food are most interesting, although, unfortunately, very imperfectly understood as yet. The fertilising ingredients are capable of considerable movement in the plant, and are only absorbed up to a certain period of growth. This in many plants is reached when they flower. After this period they are no longer capable of absorbing any more food. The popular belief that plants in ripening exhaust the soil of its fertilising matters is consequently a fallacy.
Fertilising Ingredients lodge in the Seed.
The tendency of fertilising matters is to move upward in the plant as it matures, and finally to become lodged in the seed. It is for this reason that the cereals prove such an exhaustive crop. That nature, however, can in certain cases be very economical of her food-supplies, is strikingly illustrated by the fact that much of the fertilising matter contained in the mature leaves in autumn passes back into the tree before the leaves fall from it.
Forms in which Nitrogen exists in Plants.
The form in which nitrogen is present in the plant is chiefly as albuminoids. As, however, albuminoids belong to that class of bodies known as colloids, which cannot easily pass through porous membranes like those forming the walls of plant-cells, they are changed during certain periods of the plant's growth into amides, which are crystalloids, and consequently able to move freely about in the plant. Amides are most abundant in young plants during the period of their most active growth, and as the plant ripens the amides seem to be largely converted into albuminoids.
While the subject is not very clearly understood, it would seem to be pretty conclusively proved that there is a direct relation between the amount of the phosphoric acid and of the nitrogen absorbed.
Bearing of above Facts on Agricultural Practice.
The bearing of these facts upon practice is obvious. In the first place, they show how important it is that plants should be well fed when they are young, and that in the practice of green manuring it is best to plough in the crop when it is in flower, as no additional benefit is gained by allowing it to ripen, seeing that no further absorption of fertilising ingredients takes place after the period of flowering.