In 2005 we became interested in the remineralisation of the soil, primarily to invigorate the vineyards we had planted in Cortém. The theory is basically that through millennia of rain, water soluble minerals and trace elements have become washed out of the soil. The solution is to apply very fine rock dust, preferably Basalt, which contains silicates and elements essential for plant metabolism as well as many trace elements. The finer the dust the more easily bacteria and acids can work on the stone releasing the minerals in soluble form. The recommended amount is 4 tons per hectare every 5 years.


Finding Rock Dust

Agricultural Gardening The Silver Coast is not a volcanic region, so I was in despair about finding Basalt rock dust near Cortém Winery. But it seems that the Silver Coast/Oeste region has everything and very soon we found the rock dust in a quarry near us where they were making Basalt gravel (Brita) for road making. The quarry is on the rim of an ancient volcanic crater near in the Serra de Todo o Mundo, (a lovely name!), about 12km south of Caldas da Rainha near Painho. We went there and of course they had coarse Basalt dust which came from the stone crusher and was discarded. We took samples and had them analysed at the geological Lab LNEG in Lisbon. you can see the results below.
Agricultural Gardening

These tables are taken from the mineral analysis of the rock dust samples used for Basalmin®, made by LNEG (Laboratorio Nacional de Energia e Geologia I.P.) in Lisbon.

The Full analysis report is available on request.

Agricultural Gardening

There are a lot of silicates and essential elements as well as trace elements. So we bought 20 tons of this dust at 3€ a ton and had it transported to Cortém. The transport cost also 60€ so total cost was 120€ for 20 tons.

We have been back to the quarry several times to visit and got to know the owners. The name of the quarry is Jobasaltos. (The Jobasalt website is not online at this time due to maintenance but I was there yesterday and they were very busy.)

Here are their contact details again:-

Pedreira de Basalto
Casais Gaiola
2500-045 A-dos Francos
TEL: 26274420

As you can see from their website they now sell very fine rock dust for agriculture. Interestingly most to the dust goes to Spain. As of yet there is very little use of it by Portuguese agriculture. We think this is a wasted opportunity.

Unfortunately the dust is very heavy, hard and abrasive as we learnt very soon, we put it in our fertilizer spreader which broke completely after half an hour. All the metal parts had worn away! We then applied the rest of the dust by hand with shovel from a trailer. An alternative way would be by big bag hopper with opening at the bottom.

Agricultural Gardening So the real question is did it make the wines taste better? We do not know of course but over the years our wines won many medals and had that special Cortém taste. Anyway we will be applying Rock dust again 15 years after the first application, and now every 5 years.

General comments about rock dust

Rock dust can be used in gardens and compost where bacterial and fungal activity is very high. it reducers soil acidity. Using the 5 tons/ hectare suggestion above one gets 0.5Kg m2 so 20kg will do for 40m2 of garden.

There are many interesting references to remineralization on the internet and I have done a Powerpoint about rock dust which I presented to the geological society of Portugal some years ago and can be downloaded here.

Together with Jobasalt we were thinking of making this dust available in smaller quantities in bags for gardeners etc. It is a very interesting subject but has its fair share of quirky followers due the fact that remineralization has not been generally accepted and used by agriculture. This may have been due more to local availability as basalt is not found everywhere. But it is available locally here in our region it is surprising that more local farmers are not using it. In the archive section there are various texts and links about rock dust.