Rock dust is increasingly used in organic farming. It consists of finely processed mineral powders that protect vegetable plants and fruit trees from pests and fungal diseases. These powders are not pollutants, as they are entirely natural materials. There are no toxic effects on crops since their mode of action is mechanical. Several types of rock dust exist, mainly distinguished by the nature of the sedimentary rock they originate from. Rock dusts commonly used in agriculture include kaolin, zeolite, and bentonite. In recent years, farmers, particularly in organic olive and grape cultivation, have successfully employed these rock dusts.
In this article, we’ll outline the distinctive features of the best rock dusts available in the market.
Using Rock Dust in Organic Farming
According to the regulations outlined in Annex 2 of D.M. del 18/07/2018 n. 6793, rock dust falls under the category of permitted products in organic farming. They are considered “enhancers”, strengthening the natural defenses of plants. Specifically, these stone powders are “products obtained as is from the mechanical milling of various types of rocks, whose original composition must be specified”. Let’s delve into the specifics of the most commonly used rock dusts in agriculture: kaolin, zeolite, and bentonite.
Kaolin is a rock dust revolutionizing natural olive cultivation. It’s a natural sediment formed through meteoric degradation of stones due to water action. With its unmistakable white color and fine texture after extraction and processing, it is sprayed on the tree canopy to combat the main olive orchard pest, the olive fruit fly. Olive trees treated with kaolin develop white leaves. This chromatic alteration discourages the olive fly from being attracted to the drupes. Moreover, kaolin creates a protective mechanical effect, forming a protective layer that prevents pests from laying eggs.
Advantages and Disadvantages
In addition to its direct impact on the olive fly, kaolin shields the plant from excessive sun exposure, preventing dangerous sunburn. It is used starting from early summer, and its only drawback is that it can wash off easily from plants after heavy rains, necessitating reapplication. Nonetheless, kaolin powder is cost-effective (find it here).
Zeolites are volcanic minerals extracted from sedimentary lava deposits. Various types of zeolites exist in nature, but those widely used in agriculture belong to the chabasite and cubana series. Both are available in specialized stores (find them here), and no authorization is needed for their purchase. Zeolite is available in granular and powder forms.
Granular zeolite is excellent for improving soil characteristics. It corrects acidic soils and enhances the soil’s water-holding capacity while absorbing nutrients from organic fertilizers. Additionally, it purifies the soil from heavy metals.
Powdered zeolite, also known as micronized, can be mixed with water and serves a dual purpose: as a natural fungicide and pesticide. When sprayed on the stems and leaves of vegetables and fruit trees, it forms a thin protective layer capable of safeguarding plants from pests and fungal pathogens. This rock dust can also be mixed with copper-based products to enhance their effectiveness against powdery mildew, tomato downy mildew, and peach leaf curl. It’s also proving effective against the dreaded Asian stink bug.
Bentonite is a volcanic rock dust primarily used in winemaking and viticulture. It is mainly employed in wine production processes for clarification and is approved for use in organic winemaking (find it here). Several types of bentonite exist, each suitable for specific grape varieties. Its primary use is in white wine production.
In viticulture, bentonite is used as an anticryptogamic agent to protect grape clusters before harvesting. It is highly effective against gray mold, a disease that can compromise the quality of the grapes. The advantage of this rock dust is that it rinses off grapes easily, preserving their flavor. Another common application is in trunk paste preparation, a protective substance applied to the main stems of fruit trees to safeguard them from wood diseases and pests.
Applying Rock Dust to Crops
To use rock dust in agriculture, it must be finely processed to achieve an infinitesimal particle size. This enables rock dust to dissolve effectively in water and be sprayed without issues using agricultural spraying equipment. The choice of equipment depends on the size of the garden, orchard, olive grove, or vineyard. It ranges from simple shoulder pumps, manual or electric (such as these) to more professional atomizers that attach to tractors and are powered by their engines. In both cases, it’s crucial to thoroughly clean the equipment, especially the nozzles, after using rock dust. Residues can quickly accumulate and clog the equipment, necessitating expensive replacements.