Soil is just a combination of particle sizes, Sand, Silt , and Clay. Depending on where you are the % of each changes. Certainly you don't really want just sand or just clay, which you probably don't have even if you think you do. Top soil has the addition of organic matter. Think of topsoil as chocolate milk and subsoil as white milk, organic matter as chocolate syrup. You don't make chocolate milk by taking white milk(subsoil) and adding chocolate milk(topsoil), you add the syrup(organic matter). So don't add topsoil (good just as it is) and dilute it in subsoil. Add organic matter of any sort you have cheap. Manure, peat, compost, mushroom soil(aged) etc. and till that in. People also mistakingly add sand to clay thinking it will make the soil better. It does not untill you hit a threshold that is huge on the sand part. Think of clay as a glass of water, very small bits. Now add marbles(sand). The porosity and drainage doesn't change untill the marbles fill the water and touch each other creating pore space. The sand would have to be added until it touched and there was not enough clay to fill all the space around it. Also sand is inert. it has no Cation Exchange Capacity(abiltity to provide important elements to plant roots) and clay has a large C.E.C.

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