What Is in Black Dirt?
The decomposing plant material that gives black dirt its rich, dark color contains a feast of chemical nutrients. The form in which those nutrients are stored may not be the best form for your plants' needs, however, and other characteristics of black dirt may make it a less-than-ideal medium for most garden plants.
Soil Color Variations
In general, the more organic matter a particular soil contains, the darker its color is. So soil that is truly black probably contains a significant amount of organic material. Soil colors also can provide clues to the drainage capability of a particular soil; brown soils tend to drain well while black or gray soils tend to hold onto water and stay saturated once they are wet. Because many garden plants grow best in well-drained soil, consistently saturated soil can be problematic for them.
Black dirt's organic matter consists primarily of decomposing plant material. From 60 to 90 percent of that material is made up of water, and the remainder is a mixture of chemical components that are important nutrients for plants. Those nutrients include carbon, oxygen, hydrogen, sulfur, potassium, nitrogen, phosphorus, calcium and magnesium. Five percent or less of many areas' typical soils is organic matter, but 50 percent or more of black dirt, which often comes from boggy or swampy areas, may be organic matter.
Minerals, Air and Water
Like all soils, black dirt also contains several components other than organic matter. The minerals in soil are usually classified as sand, silt or clay. Sand is made up of tiny mineral fragments that don't stick together well, but the clay's minerals bond together, making clay dense. The density of silt's minerals is somewhere between the density of the minerals in sand and clay. All soils also hold air and water in the spaces between their other components; a soil's ability to hold air and water is determined by the proportions of its other components.
Ideal Soil Composition
An effective soil composition for the broadest spectrum of plant growth is 45 percent minerals, 25 percent water, 25 percent air and 5 percent organic matter. Black dirt's much higher proportion of organic matter gives it an abundance of plant nutrients. Not all of those nutrients may be chemically available to plants, however, and because organic matter can retain enormous amounts of water, black dirt's drainage may not be optimal for most plants.