The answer is very little or even more likely, nothing. The question, and I'll let you pick the right words, comes as people complain about the horrible soil they are trying to garden in.

Quite honestly, in this area we are blessed with a rather silty loam soil that is ideal for most gardening activities. A few spots may have a bit more clay in it than we wish. These spots often are more poorly drained and have less air space.

If you have a clay-based soil, a bit of organic material will improve drainage and air-holding capacity of the soil. The solution is usually reached over time with manure, grass clippings, mulches, compost or even some peat moss.

If you add peat or any compost you need to mix it thoroughly into the soil. Many years ago, the advice was to make a wonderful hole for each new transplant. That idea raises the question of what happens to the roots when they grow past the edge of the wonderful hole you prepared. Do they expand as desired or remain in the small, wonderful hole?

Adding mushroom soil is often viewed as a cure for all problems. Mushroom soil is a great form of compost if it has been properly aged. Fresh from the mushroom house it is toxic to almost all plants because of the high level of salts (think fertilizer) it contains.

If you have any question about a product you may add, I offer a quick test. Plant a tomato plant in the product. If the tomato plant dies quickly you have a problem. If it lasts close to a week you have a safe product.

Another scheme is to add a few loads of topsoil on what you have. That's about the same as an arranged marriage. Over time they may function as one, but it will take time for moisture and air movement within the soil to operate between the two.

Constructing a raised bed might be one time when adding soil is appropriate. If your raised bed includes enough volume to satisfy most of the root expansion, it is a great way to garden. In new subdivisions where they strip the topsoil before building, they return a thin layer and sell the rest. You will have the same problems as stated above, but at least you will have something to work with.

I mentioned air-holding capacity of the soil a number of times. I think I read someplace that up to 25% of the soil volume should be air space. A like amount of volume is water.

Most of the garden flowers, vegetables, shrubs and trees we try to grow have very low nutrient requirements. Almost always these are naturally available in the soil. This is especially true if your lot is a former farm field.

There are a few exceptions where additional nutrients are needed. Quite honestly, the only two I can think of right now are corn and potatoes.

When it comes to soil, we are rather blessed. Too much attention, just like too much fertilizer and too much water, will cause you to wonder what went wrong in your garden.