Introduction
Rock dust is finely ground mineral material used as a soil amendment. Depending on the source, it may supply certain nutrients or trace elements. However, rock dust for garden use is not a cure-all and does not replace compost, fertilizer, or a soil test.
If you are interested in rock dust gardening, first learn what your soil needs. University Extension guidance recommends adding mineral amendments when soil-test results and product information support their use.
What Is Rock Dust?
What is rock dust? It is rock crushed or milled into a fine powder. Natural weathering also helps explain how does a rock turn into soil. Over long periods, rock breaks into smaller mineral particles that become part of soil.
What is rock dust used for? A rock dust soil amendment may provide minerals when its composition matches a soil need. It is sometimes called rock powder fertilizer, but most products are not complete fertilizers.
The rock dust minerals depend on the parent material. Particle sizes also matter because finely ground material has more surface area exposed to weathering. Still, nutrient release depends on the mineral and soil conditions.
Learn more about choosing mineral amendments for your soil and the role of rock powders and mineral amendments.
Types of Rock Dust
Glacial rock dust comes from material ground by glaciers. Because a glacier can move across different rocks, glacial rock products may contain a mixture of minerals.
Examples include Soil Conscious Glacial Rock Dust, McKenzie Rock Flour, and Gaia Green Glacial Rock Dust.
Volcanic rock dust comes from volcanic rock. Basalt is one example. A basalt rock dust fertilizer may contain calcium, magnesium, iron, silicon, and other elements, but the exact analysis varies. When using volcanic rock dust for plants, check the product rather than assuming all basalt materials are alike.
What is azomite? AZOMITE is a mined mineral product from Utah. If you wonder how much azomite to add to soil, follow the current directions for micronized AZOMITE instead of using one rate for every garden.
How to Use Rock Dust in the Garden
Before using rock dust for plants, test your garden soil. A soil test can identify pH and nutrient levels and may prevent unnecessary applications. Browse soil testing options before adding rock minerals for garden use.
What does soil amendment mean? It is a material added to soil to change a physical or chemical property. Rock dust fertilizer may supply selected minerals, but it does not provide the organic matter found in compost.
If my garden soil is hard as a rock, adding mineral powder is not automatically the answer. For improving soil structure, organic matter is often more useful. Compost can improve aggregation and water management, including in some sandy soils.
For rock dust for gardening, use the application rate on the specific product. Some products can be mixed into garden soil or potting mixes. Others may be applied to the soil surface as directed. Avoid breathing any airborne dust for plants or other fine powder during application.
Read more about trace elements in soil and remineralizing garden soil. You can also compare trace mineral products.
Rock Dust Is Not Rock Phosphate
Rock phosphate is mainly a phosphorus source, so it is different from general rock dust for gardens. Oregon State Extension recommends applying rock phosphate according to soil phosphorus test results and recommends none when phosphorus is already high.
For details, read the soft rock phosphate guide and organic fertilizer guide.
Other nutrient needs may call for sulfur and magnesium fertilizers, a complete fertilizer such as Bio-Live, or other fertilizer and growing supplies. Products intended to support soil microbial activity should also be chosen for a documented need rather than assumed to work better when combined with rock dust.
Conclusion
Rock dust for gardens can have a place in a long term soil plan when the product supplies something your soil needs. The right choice depends on the source rock, product analysis, soil quality, crop, and test results.
Rock minerals for garden use should not replace compost or balanced fertility. Use small amounts only when appropriate, follow product directions, and do not assume adding more will improve plant growth.
For another perspective, read the University of Maine Extension guidance on rock dust.
8 comments
I’ve known about this for sometime but this explanation has been a great help. Thank you
Does anyone know of any peer reviewed scientific papers that have been published in regard to the effects of applying mineral dust on soil?
David, you can mix it into the soil and water it in. It is soluble so it will be fine to add it after you have planted.
I’ve been searching for solid information on rock dust. This is great info. Thank you for sharing!
Heavy metal contamination in azomite and glacial rock dust would it be harmful to fruit trees?