Presently, I have started an experiment with a possible new medium for the culture of container grown citrus. I came across the idea for this experiment from a web site that I found accidentally. For those interested, I have listed the web site at the end of this post.

The objectives of this trial are the same as I found on the web site. It is to determine the suitability of high quality, graded, pre-washed and sterilized scoria as a growing medium for container grown citrus, and to assess the physical and the growth properties during crop production of a citrus tree.

Scoria has a high bulk density of 1.5 g cm-3, therefore it does not float when placed in water. Bulk density is a measure of the weight of the media per unit volume and is
determined by dividing the dry weight of the media by the volume of the media. Under a microscope a scoria granule looks much like a sponge, with a multitude of open passages and tunnels throughout the granule, which I expect will act as a water and nutrient reservoir from which the tree's root system can draw on. The first step to prepare the product to be used as a medium for citrus culture, was to remove all particles and fines less than 3/8 inches by sifting, followed by three rinses with distilled water to assist with the removal of any powers remaining attached to the useable particles. Three-eights inch sized scoria was chosen because it has a 63+- percent pour space which should provide excellent root zone oxygen and gaseous exchange. The total porosity is the combined volume of the aqueous and the gaseous phases of the medium. The scoria can be sterilized in boiling water. Scoria has been found to be stable, and can be reused in plant production for multiple years. About a week ago, I transplanted a 9-1/4 inch tall Saint Dominic Sour Orange seedling tree using a 3-gallon air root pruning container, filled with the scoria. I intend to fertilize with a 250 PPM nutrient solution adjusted to a 6.5 PH. The web site that brought my attention to this experiment, re-uses their fertilizer solution over and over in a hydroponic culture system. As with all my other citrus trees, my fertilizing plan will use a freshly made nutrient solution each application. Scoria filled containers are heaver than containers using CHC. A three gallon container filled with 3/8" scoria at water filled porosity, which is the amount of water remaining in the media after the media has drained to container capacity, plus the weight of the 9-1/4 inch tree, has a total weight of 17.38-lbs. I expect good results, but..... time will tell. - Millet (361 ABO-)

http://www.nftgroup.co.nz/site/hydrotech/files/Articles/ScoriaGerberaReport.pdf

Very interesting Millet. I had thought of using scoria when I was researching using pumice. Pumice is similar but much less dense and floats in water. Both sound like an ideal potting medium for container citrus. Unfortunately I didn't get either. I couldn't find scoria available here, and pumice is prohibitively expensive. I settled on clay pebbles which have much the same properties as pumice and scoria, and while still expensive are much cheaper than pumice. I have potted some seedlings in a 50/50 mix of clay pebbles/compost. I will also try 100% clay pebbles at some point. I will report my findings here in the future.

I am not sure, but I think that scoria is lava stone, which you can buy in supermarket - it is used in propan butan grills, for example, pack of 3kg lava stone for grills - cca. 12 eur here in Slovakia.

@Millet it is always a pleasure to follow your experiments with new growing medium, and getting inspired by your work.

Radoslav wrote:

I am not sure, but I think that scoria is lava stone, which you can buy in supermarket - it is used in propan butan grills, for example, pack of 3kg lava stone for grills - cca. 12 eur here in Slovakia.

Scoria does look like lava stone used in grills and also Aquariums, whether it is the same thing or not, i do not no. Perhaps it is a selection of lave rocks with a certain pour space to allow the best oxygen/water ratio for the roots?

I have used lave rocks in my Aquarium in the past, and some plants growing on top on them would always thrive, but also break them down over time until nothing was left but fine dust.

In my area one can purchase scoria from landscape nurseries, either in bags, or in bulk truck loads. They load it using a front-end loader just like gravel or sand. It is very inexpensive. - Millet (365-- JUST ONE YEAR TO GO - ABO)

Very interesting, Millet. I look forward to your observations.

I found this for those wondering about the differences between scoria and other volcanic materials.

http://facweb.bhc.edu/academics/science/harwoodr/GEOL101/Labs/VolcanicMaterials/


Barbara

Millet, are root pruning containers sturdy? do they flex when you pick them up? the reason I am asking is because I would imagine the scoria has an abrasive surface and the act of moving the containers might move around the scoria rocks damaging the roots?

Ryan, good question. Root pruning containers are extremely sturdy. The most sturdy plastic container on the market. The three gallon ARPC container that I used has absolutely zero flexibility. Would make a good bomb shelter. - Millet (362-)

Barbara, thank you.
Quite useful and interesting info on volcanic materials.
Following the link to "Igneous Rock Identification"
http://facweb.bhc.edu/academics/science/harwoodr/GEOL101/Labs/Igneous/index.htm
we can find that scoria is mafic i.e. rich in iron (Fe) and magnesium (Mg),
and that pumice is rich in silica (SiO2).

Volcanic scoria is very porous due to its many cavities and channels, caused by the escaping hot gases during an eruption. This tunneling and cavitation are what holds irrigation water and nutrients after fertilizing. I wanted to be sure that the adhesive attraction of the scoria for the water was not stronger then the absorption by the tree's roots. I submerged the container and let it soak, then drain until no more liquid was coming out of the bottom. This put the water level in the scoria at container capacity. I then weighed the container at the end of the each day for three consecutive days to determine the tree's water use. I again soaked and drained the tree once more, and once again weighted the tree at the end of the day for another three days, but this time the container was covered with a Root Makers Root Cap. The Root Cap prevented any loss of water from evaporating from the surface of the scoria, giving the amount of water used only by the tree. The results are shown below.


Daily water used................Daily water used with Root Cap (tree's use)

Day 1 .82-lb. (372g)............................Day 1 .64-lb. (290g)
Day 2 .38-lb. (173g)............................Day 2 .24-lb. (109g)
Day 3 .27-lb. (123g)............................Day 3 .22-lb. (100g)

Total: 1.47-lb.(667g).....................................1.1-lb. (499g)


These wights show that the roots of the small 9-inch Saint Dominic Sour Orange tree has no trouble pulling water/fertilizer form scoria. - Millet (362 ABO-)

Noticed the beginnings of new growth throughout the tree today while I was fertilizing. - Millet (358 ABO-)

I would like to complement Millet on doing a real experiment on a growth media modification albeit with a small N. I recognize that forum members' research budgets are very limited on a lot of scores. That said, there is a LOT of scientific literature over the last 25 years, at least, on scoria (more commonly called tuff in the horticultural literature). Two examples are these links:
http://journal.ashspublications.org/content/117/3/415.full.pdf
http://journal.ashspublications.org/content/124/2/205.full.pdf

I will suggest that a little time on Google Scholar might help people on this forum use their budgets more effectively. That said, it is always fun for me to do an experiment to confirm the conclusions others have produced in different systems. I am sure Millet is enjoying this and I look forward to reports of the results as we go along! Thank you Millet!

Last January, I transplanted a small Sour Orange tree into a 100 percent scoria medium. At the same time I also transplanted two small Sour Orange trees of the same size into a mixed medium of 2 parts CHC, 1 part Perlite, 1 part Scoria. It has been three months since I begun the experiment. All three trees have been given the exact same treatment. Watering, fertilizing, flushing were done at the same time throughout the experiment. The cultivation has been identical. The tree in 100 percent scoria has not grown as well and the two trees in the mixed medium. The scoria tree has continually suffered from an iron deficiency (even though chelated iron has been applied with each fertilizer application. Another problem with scoria is the natural high pH level of the medium. On the plus side, scoria provides excellent root zone oxygen, and the scoria (lava stone) holds enough root zone water in itself to last 10 days. The two Sour Orange trees planted in the mixed medium, have been growing very well, with no evident deficiencies, and have out grown the scoria tree by 8 inches.

I still believe scoria has the possibility of being a great medium for container trees. I have read a lot of literature on scoria, so I anticipated, both the iron deficiency and the pH problem, before I started the experiment. My plan going forward using a 100 scoria medium is:

1. First to soak the scoria for several days in a Chelated Iron solution that has been pH adjusted.

2 Adjust the pH to 6.5 using Sulfuric acid with each watering and fertilizing.

3. Apply chelated Iron as required.

The high pH of scoria (7 to 10) depending on the particular batch, was of course a contributor to the iron deficiency. I anticipate that the second phase cultivation should solve the problems the scoria tree has experienced. Only time will tell.

I will report back at the end of July, after three months of cultivation.

Millet (272 BO-)

Thanks for the update.

You have a nice catch (role of pH):
Although scoria is mafic i.e. rich in iron (Fe) and magnesium (Mg)
and even you have applied chelated iron
the scoria tree showed iron deficiency.

"Nutrient availability in relation to soil pH":

Source: http://en.wikipedia.org/wiki/Soil_pH

Any sign of a manganese deficiency on the scoria tree?

Maybe possible abundance of Mg should be considered too?
Excess of magnesium (Mg) reduces the availability of potassium (K).
http://scialert.net/fulltext/?doi=rjphyto.2010.67.77&org=10