Marsha Silverstein, age 11, of Methuen, Mass., for her question:

Why are some soils red?

Soil is a powdery mixture of the minerals found in the rocky crust of the Earth. These minerals vary from place to place and for this reason the grass bearing, forest growing, crop yielding soils also vary from place to place. The most plentiful elements in the ground are oxygen, silicon and aluminum. Taking the crust as a whole, the fourth most abundant element is iron.

The ground, of course, tends to be moist, especially when it happens to be covered with layers of rich, loamy soil. And the metallic element iron tends to rust in air and moisture. It loses its hard and solid qualities along with its dark coloring and decays into soft crumbs and flaky fragments of rusty red. Reddish soils, as a rule, are tinted with powdery particles of rusted iron.