Turning sub soil into top soil is usually done by nature but it can also be done by those needing a useable garden soil. Due to dying plant and animal matter, the top soil has a high concentration of nutrients, making it ideal for planting. and the next layer is the sub soil. The subsoil is composed of clay and sand and is usually a very hard, compact layer of soil. There is very little decayed plant or animal matter of any kind that reaches down into this layer of soil. This tends to be a very poor soil for planting because there is no nutrition for plants to grow from. So what can be done to make sub soil more like top soil?
To begin with, the soil is going to be hard packed and because of this, roots will not be able to penetrate it. Even if they could, the soil would not drain well enough for the roots to survive long and the water would build up and drown the plants. Therefore, the soil must be dug up and sand, gravel and any other porous material needs to be added to give the soil a loose texture to it. Sand by itself is not good enough to loosen the soil because it will tend to pack back down around the roots. The soil must be dug at least one foot deep. The deeper the better, to help create a top soil depth similar to a more natural level. What is being created is a soil that is similar to what it would be like if nature had the time to do it.
Compost needs to be layered over the top of the soil six inches to one foot in depth. This needs to be completely mixed in with the gravel and porous material below it. This represents years of what nature would have to do to help put dead plant, insect and animal matter into the earth. This material should be a mixture of grass clippings, wood chips, hay, straw, leaves and any other materials available at the time. The more of an assortment in the composted material, the more nutrients will be added to the soil.
For this kind of soil, a good green manure crop should be planted on it first to help increase the soil's level of nutrients. The cover crop reaches into the soil and helps loosen it even more, as well as it will add necessary nitrogen. Cover crops are usually used at the end of planting season to help the soil rest through winter months and it also helps to protect it against the elements that could damage the soil by hard rains or high heat. However, in this situation, it is helping to turn an otherwise poor soil condition into a soil rich in nutrients.
Even with doing all this, remember that soil can be depleted of life-growing nutrients. Organic matter needs to be continually added back into the soil to replenish what plants remove. Loosening soil, using compost and green manure crops are excellent ways to help keep soil from being depleted.