Is Rock Dust the Answer?

What is rock dust and can it help things grow?

 

Keith Gray, chairman of the Rotorua branch of the Soil & Health Association during the 1980s (Organic NZ Jan/Feb 2016), favoured rock dust enormously. He touted it as a fertiliser, but rock dust lacks sufficient nitrogen, potassium and phosphorus to qualify as fertiliser. Its proponents say however, that it remineralises eroded and leached soils, thus improving soil fertility. The experience of the Sustainable Ecological Earth Regeneration Centre (SEER) in Scotland, and research carried out by James Cook University in Australia (https://www.health-report.co.uk/mineral_info1.htm), suggests rock dust creates better soil structure, drainage, and better moisture holding capacity.

Remineralising soil with rock dust mimics the earth’s own processes. During ice ages glaciers pulverise boulders into particles. Carried over land until the glaciers melt, the mineral and micronutrient-rich particles are rushed down rivers to form alluvial deposits, or picked up by wind and deposited across the land.

The last ice age occurred 10,000–12,000 years ago so optimally mineralised soils are now uncommon. One answer has been to replicate the glacial process by crushing rocks and spreading the resultant powder (also called rock flour or dust, or stone meal) over soil to reintroduce these components.

Dr Julius Hensel, a German agricultural chemist in the late 1800s, is generally credited with discovering the value of rock dust. In 1892 he published a book on the topic: Bread from Stones (available free at soilandhealth.org/wp-content/uploads/01aglibrary/010173.hensel.pdf).

Originally a flour miller, Hensel one day spotted stones in the flour as he milled his grain. Milling them into fine grit, he spread them over his garden and was astonished by energised growth in his vegetables. He experimented, grinding more stones and throwing the meal around his fruit trees, and was rewarded by high quality worm-free apples from trees which had never before produced good fruit. The vegetables grown even in poor quality soil upon which he’d sprinkled rock dust were free of pests and diseases. He determined that stone meal alone was required for optimum soil and plant health.

This didn’t sit well with the followers of the theories of an earlier German agricultural chemist, Liebig, who had promoted a phosphorus-potassium-nitrogen based fertiliser. These proponents had money behind them, and stood to make a great deal more if their soil fertility theories were promulgated. They collaborated with authorities to have Hensel’s stone meal denounced as valueless.

Modern compound fertilisers (used in mainstream, non-organic farming) are descended from Liebig’s nitrogen, phosphorus and potassium (NPK) theories, often with other elements added to the main three.

In the introduction to Hensel’s book Bread from Stones, Dr Raymond Bernard, a health and metaphysical author of over 40 titles including The Hollow Earth, wrote: ‘The first person ever to fight the chemical industries, Hensel claimed the use of chemicals in agriculture poisoned the soil, destroying beneficial soil bacteria, earthworms and humus. It created unhealthy, unbalanced, mineral-deficient plants, lacking resistance to disease and insect pests, leading to spraying to preserve the defective specimens. It led to diseases among animals, and men who fed on these abnormal plants and their products, and it led to tremendous expense for the farmer, because chemical fertilisers were quickly washed from the soil by rainfall, requiring constant replacement. Powdered rocks being far less soluble, were not so easily washed from the soil, and released minerals for many years.’

Rock dust proponents claim that, in general, organic fertilisers provide nitrogen and phosphorus, but usually lack copper, manganese and zinc. Rock dusts from basalt and granite contain calcium, iron, magnesium, silica, phosphorus and potassium, along with trace elements and micronutrients. Calcium and silicon provide protection against disease by building fortified cell walls, which helps make plants drought- and frost-resistant.

Agricultural scientists tend towards the view that rock dust may contain these elements and nutrients, but they may not be plant-available, and, your soil may not be deficient in them.

“I’ve used rock dust since hearing how it saved Bavarian forests from acid rain damage in the 1970s,” says Steve Hart, soil fertility consultant and ecology architect from Environmental Fertilisers. “It’s difficult to quantify the positive results, but it affects vegetables by extending leaf life and reduces early conversion to seed. In fruit it reduces brown rot and other diseases.”

Michel D’Hondt, of D’Hondt & Sons (which sells soil amendments and fermented food products), says: “I have used it in seedling mix and bokashi compost with good results but it depends on the quality of the rock dust. We use it to increase Brix levels in our produce.”

Soil dynamics specialist at Chaos Springs, Steve Erickson, says: “The one big difference we saw was in the strawberries. The fruit kept coming nearly all summer. It was amazing. I plan to add more to our pastoral system and other garden areas in the next year.”

Dr Charles Merfield, agricultural scientist and head of the BHU Future Farming Centre, offers another view: “Some rocks do contain useful amounts of nutrients – rock phosphate for instance, but rock-dust rocks are not usually considered normal sources of nutrients. All soil is rock dust with added biology. The soil nutrient arena is a mare’s nest of the genuine, and snake oil.”

Director of the Centre for Sustainable Agricultural Technologies, Dr Tim Jenkins, agrees with Merfield. “I have seen no convincing evidence, or plausible theory on how rock dusts would work over and above what available nutrients they may contain. Most rock types are very slow to release nutrients. It is well established that crushed sandstone, although reasonably rich in potassium, does not release potassium on an agriculturally meaningful timescale at the recommended rates.”

Agricultural specialists, in the absence of quantifiable proof, tend to stick with the position that improving soil health is most cheaply and effectively managed by increasing organic matter input, reducing tillage and increasing crop diversity.

Applied in autumn, rock dust assimilates over winter and is more plant-available for spring growth. Many add it to compost. While rock dust doesn’t wash away easily, the minerals it supplies are taken up as plant nutrients and therefore deplete somewhat with harvest.

Rock dusts can be spread by hand, in mechanical spreaders or pumped in suspensions. It’s available from many gardening suppliers in convenient packaging with instructions, generally as a mix with organic amendments, but quarried basalt dust is a waste product which the adventurous might attempt to procure themselves.

 

First published in Organic NZ – July/Aug 2016 – Vol.75 No.4