The mass of microorganisms in a soil usually far exceeds that of all other inhabitants combined, despite their individual smallness. It's their enormousness numbers that makes the difference. A single gram of fertile topsoil may contain 1000-4000 million bacteria, and up to 100 meters of fungal mycelium. With such numbers, it is not surprising that they have a large effect on the growing medium and on the plants growing in it. Microorganisms are found in greatest numbers where food supply is most abundant ---in the top 5 cm or so of soils, and around plant's roots, and in and around dead organic matter. There are MANY MORE IN A POTTING MIX composed largely of organic materials than in one composed mainly of sand perlite and soil. - Millet (413-)

When you are trying to build soil fertility buy adding organics to the soil is there a point of dimenished returns as you move down into the soil?

I am trying to build the soil up to a minimum depth of 12". Is the top two or three inches the predominately active region or does it extend deeper into the soil?

Microorganisms will go, with in reason, where there are sources of food, oxygen, moisture, and a livable temperature. I would not think that 12 inches would be a problem. - Millet (412-)