Section Iii.—Relative Value of Different Vegetable Manures.

There are two principles on which the relative value of different vegetable substances, as manures, may be stated to depend—first, on the relative quantity and kind of inorganic matter they contain; and second, on the relative proportions of nitrogen present in each.

1. Valued according to the quantity of inorganic matter they contain—the worth of the several kinds of straw and hay would be represented by the following numbers:—

Wheat straw, 70 to 360 Oat straw, 100 to 180 Hay, 100 to 200 Barley straw, 100 to 120 Pea straw, 100 Bean straw, 60 to 80 Rye straw, 50 to 70 Dry potato tops, 100 Dry turnip tops, 260 Rape, cake, 120

that is, a ton weight of each of these substances, when made into manure—provided nothing is washed out by the rains—will return to the soil the above quantities of inorganic matter in pounds. Generally, perhaps, these numbers will give the reader an idea of the relative permanent effect of these different kinds of vegetable matter when laid upon the soil. But, by a reference to the facts stated in pp. 58 to 64, in regard to the quality of the inorganic matter contained in plants, he will satisfy himself, that the effect of these manures on particular crops is not to be judged of solely by the absolute quantity of earthy and saline matter they contain;—that which the turnip-top, for example, or the bean-stalk, returns to the soil, may not be exactly what will best promote the growth of wheat.

2. On the other hand, if the fertilizing value of vegetable substances is to be calculated by the relative quantities of nitrogen they severally contain, we should place them in the following order:—the number opposite to each substance representing that weight of it in pounds, which would produce the same effect as 100 pounds of farm-yard manure, consisting of the mixed droppings and litter of cattle.

Equivalent quantities in pounds. Farm-yard manure, 100 Wheat straw, 80 to 170 Oat straw, 150 Barley straw, 180 Buckwheat, 85 Pea straw, 45 Wheat chaff, 50 Green grass, 80 Potato tops, 75 Fresh sea-weed, 80 Rape dust, 8 Fir saw-dust, 250 Oak saw-dust, 180 Coal soot, 30

This table again presents the same substances in a somewhat different order of value; shewing, for example, not only that such substances as rape dust and soot should produce a much more remarkable effect upon vegetation, than the same weight even of farm-yard manure, but also that certain dry vegetables, such as chaff and pea straw, will yield, when not unduly fermented, a more enriching manure than barley, oat, or wheat straw. It agrees, also, with the known effect of green manuring upon the land, since 80 pounds of meadow-grass ploughed in, will be equal in virtue to 100 of farm-yard manure.

Some writers ascribe the entire action of these measures to the nitrogen they contain. This, however, is taking a one-side view of their real natural operation. The nitrogen, during their decay, is liberated chiefly in the form of ammonia—an evanescent substance, producing an immediate effect in hastening or carrying further forward the growth of the plant, but not remaining permanently in the soil. The reader, therefore, will form an opinion consistent alike with theory and with practice, if he conclude—

1. That the immediate effect of a vegetable manure, in hastening the growth of plants, is dependent, in a great degree, upon the quantity of nitrogen it contains and gives off during its decay in the soil.

2. That the permanent effect and value of manures is to be estimated chiefly by the quantity and quality of the inorganic matter they contain—of the ash they leave when burned.

The effect of the nitrogen may be nearly expended in a single season—that of the earthy and saline matter may not be exhausted for several years.

Nor is the carbon of vegetable substances without its important uses to vegetation. From the statements contained in the earlier chapters of the present work, it may be inferred that, however much influence we may allow to the nitrogen and to the earthy matter of plants in aiding the growth of future races—the soundest view of these important natural operations is that which considers each element present in decaying plants to be capable of ministering food to such as are still alive,—though we may not be able as yet, either to estimate the precise importance of each element to any particular kind of crop, or exactly to adjust their relative quantities in our manures, so as to promote the growth of such a crop in the greatest possible degree.