Rockdust - has anyone used it?

[Welsh Wizard]

Seems like a load of marketing BS to me. Rockdust? Isnt that sand? Come on people, just build a decent soil the slow way by adding organic composts and mulches. Why truck heavy mine tailings down from Scotland, creating loads of CO2 when the solution to a good soil is within your backyard compost heap?

Just goes to show you can fool some of the people some of the time....

[King Carrot]

I've been interested in this topic, but not used it yet.

It seems pretty logical to me. All the trace nutrients and minerals come from rock originally. We tend to exhaust these and just give the plants the basic main nutrients that are required for growth and forget about everything else.

As well as healthy robust plants, giving the plants minerals and vitamins, should mean they're a lot better for us. Combined with 100's of years of selective breeding for size and uniformity and not nutritional value, the lack of minerals in the soil means "empty" food and could explain why we're prone to various diseases .

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Hammer nail head I suspect,

Unless of course your an expert at making your own high quality compost balanced with npk and trace elements and the manure you use comes from animals fed with highly nutritious feed (racehorses for instance).

[Alison]

The woman on the plot next to me used it last autumn round her fruit trees and swears blind this is why one of her plum trees is covered in fruit and the one where she didn't apply it is barren. No idea if she's right but she's very happy and if I'm very lucky she might let me have some of the plums when they ripen!

[Dynamo]

Just goes to show you can fool some of the people some of the time....

I don't think it shows anything of the sort. I've read many reports on the stuff online over the last couple of days, many of which were independant, and also comments on various forums, and they all seem to be favourable. Maybe the foolish ones are those that are too quick to dismiss it.

[Welsh Wizard]

The use of rockdust and composted materials as soil fertility amendments

Campbell, Nicola S. (2009) The use of rockdust and composted materials as soil fertility amendments. PhD thesis, University of Glasgow.


This thesis aims to investigate the use of two materials: rockdust and greenwaste compost for use as soil fertility amendments. A field trial was conducted over three years to investigate the impact these materials had on plant yield, plant nutrient content, soil chemistry and soil microbial communities in direct comparison with chemical fertilizer and farmyard manure. There were annual applications of compost, manure and chemical fertilizer in spring with one rockdust application in the autumn prior to the first year of the trial. Two harvests were carried out each year in summer and autumn to determine differences in plant yield. The soil was analysed prior to applications to determine the baseline chemical status then was analysed at two more points through the trial. Results from the field trial showed clear effects of organic amendments on plant yield that were attributed to nitrogen addition by compost and manure. NPK chemical fertilizer produced a yield effect at an earlier point in the trial compared to manure and compost. This, and the chemical analysis of the materials showed that the organic materials required a period of mineralization of organic-N in order to replenish soil available nitrogen. The results from the field trial also showed differences in plant nutrient content (as a measure of plant quality). The organic treatments were shown to generally produce higher plant nutrient content than the NPK fertilizer showing that the increased yields of the inorganic fertilizer treatment impacted negatively on the nutrient content. No yield effects due to rockdust addition were apparent after 3 years of the field trial. In addition, rockdust did not impact on plant nutrient content nor did it affect the soil chemistry despite 3 years of weathering that was considered sufficient time to release nutrients to the soil. Samples of soil were taken in the summer after the final year of the field trial to determine long-term changes in the soil microbial communities between the treatments. Results showed that there were fewer long-term changes than were initially expected in soil microbial communites at the end of a 3 year trial of these materials. Short-term greenhouse pot trials were also conducted using 3 different test crops to investigate the use of greenwaste compost and various types of rockdust for use as growth media in comparison with a peat based control. Results showed that while greenwaste compost supported plant growth as well as the peat based media, no rockdust type increased plant growth beyond the yield in a greenwaste compost control. The short-term supply of nitrogen in greenwaste compost was as good as that of the peat based control and while the phosphate supplied by the peat based control was shown to give excessively high plant P content. Results from the pot trials showed that greenwaste compost could wholly or partially replace peat in plant growth media without negatively impacting on plant yield or quality. The likely effects of various types of rockdust on soil chemistry were investigated by carrying out nutrient extractions using increasing extractant ‘strength’. Nutrient extractions showed that a high degree of rock weathering was required to release small quantities of trace elements from rockdust samples. Sodium, calcium and to a lesser degree potassium, magnesium, iron and phosphate were supplied in greatest quantities from most rockdust samples with basic rocktypes releasing highest quantities of nutrients. As a result of the work carried out in this thesis, it is concluded that composted greenwaste could be a valuable addition to agricultural soil and that it could replace peat in some plant growth media. There are some implications to the use of composted materials – transport and application costs and the potential addition of potentially toxic elements to soil; however the potential nutrient addition and improved plant quality could make it an attractive fertility amendment in some organic farming techniques. It was concluded that rockdust was not shown to influence plant yield or quality in the agricultural setting of the field trial, nor was it shown to be a useful addition to plant growth media. Therefore rockdust could not be proven to be a useful soil fertility amendment.

What exactly were the claimed benefits of rock dust again? The science seems not to support any benefit over compost.

[Welsh Wizard]

I will go as far as to say: Rockdust:The soil additive for the gullible. Unproven, expensive, no better than compost, environmentally unfriendly , but its good to get rid of an undesirable byproduct of mining activity by selling it to silly gardeners to spread on their gardens isnt it?

[King Carrot]

Are you quite sure your "scientist" actually knows one end of a carrot from the other, after all it is only a thesis by a student

Maybe she's even being funded through uni by the "Anti Rockdust Lobby"

I'm really not seeing any issue with using this stuff as a one off addition when starting the veg garden, or adding it to poor soil or even beds made with bagged compost to improve the trace elements in the ground

[Welsh Wizard]

Why? What benefit can be proven to come from the use of rock dust? The positive trials I have seen have been very poorly designed studies carried out by suppliers of rock dust without adequate controls vs regular greenwaste compost or NPK fertilizers and thus are scientifically of very little value for the competent gardener who will doubtless use either compost or NPK to help fertility. (I give you the trial here http://www.fcrd.com.au/Reports/FCRD-...8%20Report.pdf as an example of this.) On the other hand the University of Glasgow paper above had a very robust design and is a fantastic scientific study in my view.

[Welsh Wizard]

Seriously, go on and read the thesis rockdust fans: it blows away the idea that rockdust is anything other than hype. You can download it free here: http://theses.gla.ac.uk/617/01/2009campbellphd.pdf

Disclaimer: I have no personal connection to Dr Campbell. I am just fed up of all the superstition and myths pedalled by gardeners. I am trying to drill down to the science to back up the wonder claims made for gardening products and methods and largely finding that they are a load of old cobblers. Most things added to soils make little difference compared to manure or compost alone. Locally produced manure and compost are cheap and environmentally sustainable, while even locally produced rockdust is very energy intensive to pulverize and crush and thus totally unsustainable.

[King Carrot]

I just think the above stuff is all a bit pointless as I'm aware that rock dust on its own will grow little or nothing, Plants basically use NPK to grow, when used alongside NPK fertilizer (which is incomplete as a fertilizer) rock dust, seaweed/kelp dust will provide many many trace elements missing in fertilizers that plants require to be healthy, the rock dust looks like it would release its trace elements over a longer period than seaweed for example, whats the prob getting these trace elements from rock dust if you can not get seaweed dust more readily?

As far as I'm concerned rock dust would be only about adding trace elements alongside NPK/compost/manure to ensure that my vegetables have total access to a complete spectrum of elements-which gives me healthy plants and vitamin rich veg, for the record I use seaweed dust mixed in with my fertilizer mix but I would use rock dust if seaweed dust were not available.

I'm going to stick another picture of me npk, limes and seaweed dust, pride and vitamin rich joy:




Goodnight

[Welsh Wizard]

Nice work there King Carrot, makes me jealous of that gorgeous vegetable! No doubt a lot of hard work put in there!

Most soil as I understand it is fine in terms of micro nutrients: it is rare for a plant to suffer from a micronutrient deficiency.

The theory behind micronutrient fertilization from rock dust seems good but in reality when tested scientifically in the above PhD it simply isnt true:

"All rockdust samples released negligible quantities of nutrients in short-term weathering equivalent extractions and showed that there could be release of calcium, phosphate, magnesium, iron, manganese and potassium in the longer term. The highest nutrient release was from the most basic rock type (Orrock quarry basalt) and the finest material (Duntilland quarry fines). However, the trace elements provided by all of the samples were very low (below detectable levels in many extractions). Hence, rockdust is an unlikely trace element fertilizer since nutrients added in the short term are low.

[Welsh Wizard]

In fact looking at section 6.4.3.1

"Rockdust addition only had an effect on iron and lead concentrations in soil in the January 2005 soil extractions. The analysis of this data showed that the addition of rockdust actually lowered the concentration of these metals compared to no rockdust addition. This was unexpected since rockdust was expected to increase
trace metal availability in soil following application."

So rock dust actually made the soil worse in terms of some elements!

[rana]

As a gardener with scientific leanings it is good to see other grapes out there are prepared to question marketing mumbo jumbo and spend time investigating the subject matter thoroughly before making appropriate conclusions (and throwing away their money).

It could be tempting to invest in Rockdust as a business (profiting from peoples gullibility) but not in the product itself. As mentioned earlier in the thread this business model is the gardening equivalent of selling vitamin pills.

Perhaps if the gulllible gardener feels better about their crop there is a psychological benefit.

[Welsh Wizard]

Rana, I think you are right: the stuff provides little benefit to the plants but a psychological benefit to the purchaser. It is psychology that powers snakeoil sales after all...

It is almost an example of the placebo effect in action: "I spread rock dust and look how good my plants have grown" ignoring the fact that most plants will grow perfectly well on their own in a bed with some compost or manure. The kind of gardener who spreads rockdust will likely have already built a good soil via addition of organic material and get good results anyway irrespective of the addition of mine tailings to the mixture.

One trial I looked at found that rockdust lowered wheat yields vs a control when spread on the field. Don't see them putting that on the rockdust advertising do you?

[rana]

Welsh Wizard, exactly. I was going to mention the placebo effect in my earlier post.

I also noticed that the FCRD report gave plenty of evidence in the form of stats, mentioning averages, standard errors, t test confidences (presumably to impress the punter) but none of the root data was included or the collection methods for that data, just stating random selection.