Phosphate rock. If your garden soil is deficient in phosphorus, adding rock phosphate to the compost pile may accelerate its availability in the garden, far more effectively than adding phosphate to soil. If the vegetation in your vicinity comes from soils similarly deficient in phosphorus, adding phosphate rock will support a healthier decomposition ecology and improve the quality of your compost. Five to ten pounds of rock phosphate added to a cubic yard of uncomposted organic matter is about the right amount.
Rice hulls: See Buckwheat hulls.
Rock dust. All plant nutrients except nitrogen originally come from decomposing rock. Not all rocks contain equal concentrations and assortments of the elements plants use for nutrients. Consequently, not all soils lustily grow healthy plants. One very natural way to improve the over all fertility of soil is to spread and till in finely ground rock flour make from highly mineralized rocks.
This method is not a new idea. Limestone and dolomite--soft, easily powdered rocks--have been used for centuries to add calcium and magnesium. For over a century, rock phosphate and kainite--a soft, readily soluble naturally occurring rock rich in potassium, magnesium and sulfur--have been ground and used as fertilizer. Other natural rock sources like Jersey greensand have long been used in the eastern United States on some unusual potassium-deficient soils.
Lately it has become fashionable to remineralize the earth with heavy applications of rock flours. Unlike most fads and trends, this one is wise and should endure. The best rocks to use are finely ground "basic" igneous rocks like basalts. They are called basic as opposed to "acid" rocks because they are richer in calcium and magnesium with lesser quantities of potassium. When soil forms from these materials it tends to not be acid. Most basic igneous rocks also contain a wide range of trace mineral nutrients. I have observed marked improvements in plant growth by incorporating ordinary basalt dust that I personally shoveled from below a conveyor belt roller at a local quarry where crushed rock was being prepared for road building. Basalt dust was an unintentional byproduct.
Though highly mineralized rock dust may be a valuable soil amendment, its value must equal its cost. Application rates of one or two tons per acre are minimal. John Hamaker's The Survival of Civilization suggests eight to ten tons per acre the first application and then one or two tons every few years thereafter. This means the correct price for rock dust is similar to the price for agricultural lime; in my region that's about $60 to $80 a ton in sacks. Local farmers pay about $40 a ton in bulk, including spreading on your field by the seller. A fifty-pound sack of rock dust should retail for about $2. These days it probably costs several times that price, tending to keep rock dust a novelty item.
The activities of fungi and bacteria are the most potent forces making nutrients available to plants. As useful as tilling rock powders into soil may be, the intense biological activity of the compost pile accelerates their availability. And the presence of these minerals might well make a compost pile containing nutrient-deficient vegetation work faster and become better fertilizer. Were the right types of rock dust available and cheap, I'd make it about 5 percent by volume of my heap, and equal that with rich soil.
