Soils and fertilizers . r Ume or other base that it contains. The quan-tity of potash or other plant-food material in a rock willdepend on the proportion of minerals containing those sub-stances that compose the rock. 8. Important minerals. — There are a few minerals thatit will be well to mention: (1) because they or their productsoccur in very large quantity in soil and influence its physicalproperties; (2) because of the plant-food material thatthey contain. Quartz and feldspar are examples of the classfirst mentioned. Quartz is found in almost all soils, andmay form from 85 to 99 per cent
Soils and fertilizers . r Ume or other base that it contains. The quan-tity of potash or other plant-food material in a rock willdepend on the proportion of minerals containing those sub-stances that compose the rock. 8. Important minerals. — There are a few minerals thatit will be well to mention: (1) because they or their productsoccur in very large quantity in soil and influence its physicalproperties; (2) because of the plant-food material thatthey contain. Quartz and feldspar are examples of the classfirst mentioned. Quartz is found in almost all soils, andmay form from 85 to 99 per cent of their composition. Itis particularly prevalent in sandy soils. It usually occursas a large grain, called sand, is hard and insoluble and con-tributes no plant-food material. A soil with a great dealof quartz is usually a light, easily worked soil. On weathering feldspar contributes to soils a mass ofvery finely divided matter known as clay, the smallest ofthe soil particles. It, therefore, forms part of the clay in. Plate II. Soil Formation. — Heat, cold, and frost have been largelyinstrumental in fracturing the rocks in the upper figure, and in produc-ing the rock dfebris and soil in the lower. Note that vegetation hasalready well started on the slope. SOIL AS A MEDIUM FOB PLANT GROWTH 7 soils and adds to their plasticity, and in addition, this veryfine material is an absorbent, holding the soluble plant-foodmaterials of fertilizers in a form that prevents them fromleaching from the soil, and yet gives them up to plants rathereasily. As examples of the second class we again have the feld-spars as they furnish lime, magnesia and potash; calcite,which contains lime; hematite, which consists largely ofiron; dolomite, which contains both lime and magnesia;apatite, which furnishes phosphoric acid and lime, and gyp-sum, which is a combination of lime and sulfur. These minerals and the plant-food materials containedin them may be reviewed in tabular form thus: Mineral Plant-fo
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