Sand allows a lot of aeration. Silt allows very little. Humus is in between.

This is a good question. There is a lot of information regarding these. I will give just a brief description. The most important material is humus. Humus is basically decayed organic material. This is what is vital to plant health. Vital. An example of humus is what happens when your compost completely breaks down. It is the decayed parts of leaves and plants and insects and dead animals, etc. which is brought about by all the microbe activity. This is where plants draw their nutrients from the ground. Humus has wonderful properties. It has a high carbon content which is important. It will hold moisture and prevent leaching of nutrients, yet it also provides drainage and air to the soil. Humus is crumbly and has that sweet earthy smell. It allows the soil to breath. There is no better fertilizer really...it is all natural. Add humus to clay soils to bring life to the soil and help to break down the clay. Clays tend to not allow water to drain well. Sand, (silica), really has no nutritional content for plants. Add humus to sandy soils to make sandy soils more productive. Sand often lets nutrients leach through. Never add sand to clay soils. You can add clay to sandy soils in order to help retain nutrients. As a sidenote, very fine grade sand is sometimes used as a barrier to termites...it cuts them up. Silt is a sediment. It has a particle size coarser than clay but finer than sand. It is in-between. The term loam is a good combination of clay, silt and sand.