已知它存在于沙子、淤泥和粘土中，但它存在于所有土壤中吗?< / p >

tl;dr

硅无处不在，在一切事物中。石英砂中的含量接近100%，泥炭或石灰石(以及衍生土壤)中的含量不到1%。找到完全纯净的土壤(比如少于0.1%)是一厢情愿的想法。< / p >

Silicon dioxide (or silica, or silicon oxide, it's all the same thing) can exist in two "forms":

1. As the compound, most commonly known as quartz, but also all kinds of other materials such as chert, agate, jasper, opal, etc.
2. As the chemical component in other materials (think oxygen in water). Most of the hard and brittle solid materials you know have silica as a chemical component in them. This includes most rocks, clay minerals, concrete, bricks, ceramics, etc.

Silica the compound (as quartz and others) is one of the most abundant materials on the surface. Sand (the white-yellow stuff) is commonly 99% quartz. Silt and clay are actually definitions of grain size, not chemical composition. Most soils that are silt or clay size also have abundant quartz in it. Quartz is a component of dust. This stuff gets everywhere.

For soils that don't have quartz proper in them, most will have silica as a chemical component. Clay minerals, pyroxenes, amphiboles, micas, feldspars, and all silicate minerals (which are the most common rock forming minerals on Earth).

It is easier to talk about the rocks and soils that don't have any silica in them.

1. Peat and other organic soils (coal?). These form by decay of plant material, and should not have any silica in them. However, since silica is a component of dust, and dust accumulates when these soils form, there will be some amount of silica. The amount can vary from a few percent to as little as a tenth of a percent, but it will be there.
2. Evaporites. Things like salt, gypsum, etc. These are chemically precipitated rocks which should not have silica. Then again, dust. There will be some amount.
3. Carbonates. Limestone is calcium carbonate. Limestones are rarely pure though, and the most common contaminant is - you guessed it - silica. Because of dust. Or just silicate sediments floating in the sea when the limestones form.