[PaulF]

Maybe I meant optimum organic levels in the soil. I came across this when I lost my lazy and checked for myself.

Ideal Organic Matter Percentage
The University of Missouri Extension suggests that organic matter make up at least 2 percent to 3 percent of the soil for growing lawns. For gardens, growing flowers and in landscapes, a slightly greater proportion of organic matter, or about 4 percent to 6 percent of the soil, is preferable. The percentage of organic matter that occurs naturally in soil varies greatly, according to the University of Florida, from 1 percent to more than 90 percent in muck soils. Organic matter itself is composed of living biomass like microorganisms, dead tissue or partly decomposed materials and stable, fully decomposed humus.