[2] U. S. Geol. Survey, Professional Paper No. 14, p. 108. WASHINGTON. CLARKE. Silica SiO₂ 57.78 59.89 Alumina Al₂O₃ 15.67 15.45 Peroxid of Iron Fe₂O₃ 3.31 2.64 Protoxid of Iron FeO 3.84 3.53 Magnesia MgO 3.81 4.37 Lime CaO 5.18 4.91 Soda Na₂O 3.88 3.56 Potash K₂O 3.13 2.81 Water, basic H₂O⁺ 1.42 1.52 Water, acid H₂O⁻ .36 .40 Ferric Sulphid FeS₂ 1.03 .60 Phosphoric acid P₂O₅ .37 .22 Manganese Protoxid MnO .22 .10 The salient point which at once attracts attention in these tables is the great predominance of the oxid of silicon—silica, silicic acid, quartz, etc.,—over all other substances. While quartz occurs alone in enormous masses, as will be shown later, probably the greater proportion is found in combination with other oxids, notably those of aluminum, calcium, iron, magnesium, and the alkali metals potassium and sodium. Chlorin and fluorin, however, do not occur as oxids.[3] [3] A trifling amount of chlorin is found oxidized in the form of sodium perchlorate, in the nitre deposits of Chile. _The Chemical Elements Important to Agriculture._—Of the numerous elements known to chemists, only eighteen require mention in connection with either soil formation or plant growth; and of these only thirteen or fourteen participate in normal plant growth. They are the following: METALLIC ELEMENTS. NON-METALLIC ELEMENTS. Potassium Carbon Sodium Hydrogen Calcium Oxygen Magnesium Nitrogen Iron Phosphorus Manganese Sulphur Aluminum Chlorin Titanium Fluorin Iodin Silicon. Of this list, titanium, though a very constant ingredient of soils in the form of titanic dioxid, is not known as performing any important function in soils, and is not, so far as known at present, ever taken up by plants. Aluminum, in the form of its compounds with oxygen and silicon, is a very prominent and physically very important soil ingredient, but does not, apparently, perform any direct function in plant nutrition, and is absent from their ash, except in the case of some of the lower plants (horsetails and ferns).