You may or may not have heard of the touted benefits of rock dust. This 'miracle' soil amender has been promoted by many people but there's not a significant amount of scientific research to back it up. There is some credible research however, so I thought I'd share with you the benefits of rock dust.
Soil comes from rock. Rocks slowly break down over time, producing gradually smaller rocks until there is nothing but dust left and it is this dust that makes up our soil. These rocks contain minerals and nutrients that plants use to grow and thus so does the resultant soil. Some of these nutrients however, get washed from the top layer of soil, usually into soil that is too deep in the ground for many plant roots to reach. When we harvest edible plants and ornamental crops (flowers etc.) we also take away a lot of the nutrients that were in the soil and unless we return an equivalent amount of organic matter to the soil, these two processes can significantly reduce the quality of our soil. Other processes such as erosion, also contribute to the loss of nutrients.
Our response to this loss of nutrients has always been to replenish them through the application of fertilisers, whether they be organic or more recently, synthetic fertilisers. Rock dust is another organic option that you can choose from. Given that soil is comprised of rock dust, adding rock dust to replace lost nutrients makes sense. There is also some scientific evidence to suggest that rock dust, particularly when combined with organic matter/fertiliser, is highly beneficial to soil. One reason that the combination of rock dust and organic fertiliser is so helpful is that the organic fertiliser usually contains readily available nutrients whereas the nutrients in rock dust may need to be converted to a form that is usable by plants. Microorganisms are generally responsible for performing this conversion. So the organic fertiliser provides 'instant' nutrients and attracts soil microorganisms that convert the nutrients from the rock dust into a usable form, giving a 'triple hit' sort of effect.
How Does Rock Dust Help?
- Rock dust by itself and in combination with organic fertilisers can neutralise soil pH
- Rock dust in combination with organic fertilisers increases soil enzymatic activity (soil enzymes are important because they help release nutrients and convert them to forms that can be uptaken by plants)
- Rock dust by itself and in combination with organic fertilisers can increase the amount of chlorophyll present in plant leaves and can also increase the rate of photosynthesis (the process that plants use to produce sugar, which they use for energy) - the combination is more effective than either rock dust or organic fertiliser alone and rock dust is more effective than organic fertilisers
- The amount of chlorophyll and rate of photosynthesis are accepted measures of plant health
- Rock dust alone and in combination with organic fertilisers has been shown to protect plants from diseases - the combination works better than rock dust alone
- Rock dust alone and in combination with organic fertilisers has also been shown to reduce the severity of diseases that plants do contract - again the combination works best
Interestingly, in one study I read, organic fertilisers by themselves did little to protect plants from disease or to reduce the impact of disease. Neither rock dust nor organic fertilisers nor a combination of the two seem to affect plant height though they may have a small effect on the size of plant stems and trunks. Rock dust and a combination of rock dust and organic fertiliser may slightly increase the biomass of plants.
So rock dust is an effective soil amendment though it's probably not as good as some people claim. It also works best when combined with an organic fertiliser. And of course its efficacy will depend on the health of your soil to begin with - if your soil is already very healthy and full of available nutrients, rock dust may not be so effective.
A Note on Various Rock Dusts
Rock dust comes in different sizes. Or rather, the size of the rock dust particles varies. This does not affect how effective the rock dust is, just how quickly nutrients are released. The smaller the particles, the more quickly nutrients can be released from the particles so fine rock dust is good if you need to add nutrients quickly but coarser rock dust is more suitable if you want nutrients to be released over a longer period of time.