Minerals are required by plants as part of their food, to form their structure. The firmness of straw for example, is due to the presence in it of silica, the principal constituent of sand and flints. Potassa, soda, lime, magnesia, and phosphoric acid are contained in plants, in different proportions. All of these they must obtain from the soil. The alkalies above named appear to be essential to the proper development of the higher vegetable forms. Some plants require them in one mode of combination, and some in another; and thus the soil that is very good for one, may be quite unfit for others. Firs and pines find enough to support them in barren, sandy soil. The proportion of silicate of potash, (necessary for the firmness of wheat straw), does not vary perceptibly in the soil of grain fields, because what is removed by the reaper, is again replaced by decaying straw. But this is not the case with meadow-land. Hence you would never find a luxuriant crop of grass on sandy and limestone soils which contain little potash, evidently because one of the constituents indispensable to the growth of the plants is wanting. If a meadow be well manured, we remove, with the increased crop of grass, a greater quantity of potash than can, by a repetition of the same manure, be restored to it. So grass-land manured with gypsum soon ceases to feel its agency. But if the meadow be strewed from time to time with wood ashes, or soap-boilers' lye made from wood ashes, then the grass thrives as luxuriantly as before. The ashes are only a means of restoring the necessary potash for the grass stalks. So oats, barley, and rye may be made for once to grow upon a sandy heath, by mixing with the scanty soil the ashes of the heath-plants that grow upon it. Those ashes contain soda and potash, conveyed to the growing furze or gorse by rain-water. The soil of one district consists of sandstone; certain trees find in it a quantity of alkaline earths sufficient for their own sustenance. When felled, and burnt and sprinkled upon the soil, oats will grow and thrive that without such aid would not vegetate.