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. CHAPTER IX.