Under this name very different substances are grouped together, with the common characteristic of deriving from the grinding of rocks. Their agronomic use responds to different objectives, depending on the peculiar characteristics of each product.
I would like to point out only two types of rock flour that are particularly useful in composting: bentonite and phosphorite.
Bentonite is a clay flour, coarser if it is industrial bentonite, finer if it is oenological bentonite, used as a clarifier for wine. The second type of bentonite is easily available in winemaking product stores, but it is much more expensive than the first. The addition of clay is particularly suitable for improving the structure of light and sandy soils, its use in composting responds to various important needs: it is able to retain the mineral forms of nitrogen, in particular ammonia, preventing its loss by leaching or volatilization; binds bad odors that may form during fermentation; retains water, keeping the inside of the compost moist; clay particles, together with humic compounds, form particularly stable substances called clay-humic complexes, which have optimal physical and structural characteristics
for the soil and have a great capacity to retain nutritional elements for plants. Bentonite can be added at a dose of 5 kg per cubic meter of compost.
Phosphorites are sedimentary rocks, present in particular in North Africa, deriving from the deposit of marine organisms. Their interest lies in their high phosphorus content; this is present as insoluble tricalcium phosphate which, if added during the heap preparation phase, is solubilized by the organic acids produced by the microorganisms, which finally make it available for the plants. The use of phosphorites therefore allows the fertilizing power of the compost to be increased.
The use of basalt is included in part A in Annex II to the reg. EEC n.2092/91 as a product for fertilization and soil amendment.
The most widespread commercial products are composed of powders deriving from orthoclase, porphyry, lava, granite and basalt.