Improve Soil Fertility With Rock Dusts

The famously fertile farmlands of North America, Europe, and New Zealand owe much of the credit to the crushed gravels left by melting glaciers in the last Ice Age.  As early as the 18^th century, farmers understood that rock dusts replenished minerals in depleted soils, and left small rocks and crushed gravels in the ground where they would break down over time and replenish deficient trace minerals.

To be effective, rock dusts should be worked into the soil with compost, which will provide the necessary organic acids to speed up mineral release. When making a compost pile, add rock dusts on each successive layer so that the organic acids that are formed in the decomposition process will begin to break down the dusts. Granite dust is the exception, and should be applied directly to the soil with finished compost.

The most widely used and readily available rock dusts include :

Dolomitic Limestone
Dolomite is often referred to simply as lime, but it is actually much more valuable as, unlike crushed limestone which is pure calcium carbonate,  it contains both calcium carbonate and magnesium carbonate. It’s useful in sandy soils and to remedy magnesium deficient soils, but is most often used to raise the pH level

Green Sand
Green Sand is the crushed product of a sandy rock or sediment that contains a high amount of glauconite, a greenish black mineral formed in marine deposits.  Green Sand has been sold as an organic mineral fertilizer and soil conditioner for over 100 years. It contains potassium, phosphorous, magnesium, and iron, and is favored by growers because the gentle release of phosphorous and potassium won’t burn plants. Green Sand also has excellent water absorbency.

Rock Phosphate
Rock Phosphate is a very forgiving, slow release source of phosphorous that  is so mild that it almost cannot be over applied. Depending on where it was mined, rock phosphate can be contaminated with other undesirable minerals, and is best purchased from a reputable gardening company, which can vouch for the source.

Gypsum
Powdered or pelletized garden gypsum is particularly useful in breaking up heavy clay soils. The mineral is also high in calcium, which benefits plants.

Granite Dust
Granite dust is a great source of slow release potassium, and while they can vary some in composition, usually contain between 3-5% potassium. Granite dust is commonly sold by landscape supply companies, often referred to as granite screenings, and used by landscapers for setting and leveling stones in patios and pathways.

Crushed Basalt
Basalt rock is formed by volcanic activity, and is used in large quantities for road building. The dust from the preparation of basalt gravel, called ‘belt dust’, is readily available, and an excellent source of minerals including silica, which improves plant cell structure and calcium and phosphorous intake.