ATTRA Question of the Week

N.J.
Oregon

Answer: I am pleased to provide you with information on amending soils with limestone and granite dust.

Agricultural limes are chemical compounds commonly used to decrease soil acidity and as a source of calcium as a plant nutrient. They are made up of carbonates, oxides, or hydroxides of calcium and magnesium. Ground limestone is calcium carbonate and is the most common and widely used liming material. It is made up of the minerals calcite, which is mostly calcium carbonate, and dolomite, which is primarily calcium-magnesium carbonate. Limestone is referred to as calcitic when little or no dolomite is present. As the magnesium increases, it becomes graded into a dolomitic limestone. Calcium oxide, also called burned lime or quicklime, and calcium hydroxide, commonly referred to as hydrated lime, are two other forms of lime.

Calcium and magnesium are both provided to the soil and plants from dolomitic limestones and the oxides or hydroxides made from them. Dolomite or dolomictic limestones are best used when available magnesium is low. Calacitic limestone should be used where sufficient levels of magnesium are present in the soil to avoid the buildup of excessive magnesium.

Rock and mineral powders, such as granite dust, may be rich in both macronutrients and trace elements. They require ample amounts of soil organic matter to hold and buffer them, a balanced pH to make them available, and enough space for mineralizing microorganisms. Applying copious amounts of unanalyzed rock dust to sterile soils with low levels of organic matter can create more serious soil issues.

Granite dust as a macronutrient is often sold as a "slowly available" potash source for organic production. Total potash contents in granite dust typically vary from 1 to 5%, depending on overall mineral composition of the rock, but granite is mostly feldspar, a mineral with low solubility. Therefore, little potash fertility is derived from this material.

Plants require trace elements in very small quantities and imbalances or overdoses can be toxic. Raw mineral soil amendments, such as granite dust, generally provide sufficient trace elements in correctly balanced amounts. However, they need to be mineralized by soil microbes so that they are available to plants.

References:

Brady, Nyle and Ray Weil. 1999. The Nature and Properties of Soils (12th Ed.). Upper Saddle River, NJ; Prentice-Hall, Inc.

Gilman, Steve. 2002. Organic Soil Fertility Management. White River Junction, VT; Chelsea Green Publishing.

Sullivan, Preston. 2001. Alternative Soil Amendments. ATTRA Publication. Butte, MT; NCAT.