Hydroponic, all-mineral growing in a drought

[DaveW]

Pumice, like most potting materials will vary according to it's source:-

"The chemical properties of pumice will vary from deposit to deposit, but essentially, pumice is primarily Silicon Dioxide (Amorphous Aluminum Silicate), some Aluminum Oxide, and trace amounts of other oxides."

http://www.hesspumice.com/pumice-pages/ ... fined.html" onclick="window.open(this.href);return false;

http://www.britannica.com/EBchecked/topic/483354/pumice" onclick="window.open(this.href);return false;

http://www.soylu.com.tr/eng/pumice_character.htm" onclick="window.open(this.href);return false;

"pumice is volcanic glass formed by the solidification of lava that is permeated with gas bubbles. Usually found at the surface of a lava flow, it is colourless or light grey and has the general appearance of a rock froth. The viscosity of the lava, the quantity of water vapour and gas, and the rate of cooling together determine the fineness of the vesicular substance. Large amounts of gas result in a finer-grained variety known as pumicite. The chemical composition is that of granite."

Basically then pumice is equivalent to pop-corned granite! Whether all cacti can extract any nutrients from it I do not know, but the so called "Lava Cactus" (Brachycereus nesioticus) of the Galapagos grows on lava flows therefore must be able to extract the nutrients it needs from them without any extra fertilisation except for anything obtained from rain water:-

"As its name suggests, the lava cactus occurs exclusively on barren lava fields, lying at sea level. Not only is it one of the few plants that survive in this extremely dry, challenging habitat, it is often one of the first plants to colonise a fresh lava flow."

https://www.inaturalist.org/taxa/lava_cactus" onclick="window.open(this.href);return false;

Therefore growing plants in potting soils using pumice may not be as inert and nutrient free for cacti as it they first seem since we know some cacti roots contain bacteria that can digest such rocks.

[BarryRice]

Hey Guys,

My pumice looks more or less like Steve's.

One difference between the soil mix that Steve uses, and that I use, is that he mixes his pumice with DG (decomposed granite), while I use crushed granite. I tried DG for a while, but around here the DG is so rich in clay that I'd have to dry filter it, then wet filter it, and I'd end up with only about 50% the original volume, and even then it had a lot of fines.

So... I switched to crushed granite, which has very little in the way of fines. Mixed with the pumice, it is a good mix for the plants in 3" (7.5 cm) pots or bigger, but for smaller plants in smaller pots, I've found it useful to add a bit of fine-grained stuff to the mix so they don't dry out.

Really, I don't feel like I'm striking out in new territory, I'm just synthesizing the methods that others use.

Cheers

Barry

[jp29]

Ah, the infirmities of old age! In my case impending immobility is my most limiting - and irritating - complaint. I suffer mightily from severe edema (swelling) of my lower legs - mitigated by the diuretic furosemide and an (ugh) salt free diet. As a result I cannot stand or sit for any length of time without experiencing a lot of discomfort (typing these posts is agonizing for me). I also have arthritic knees which means I can't walk very well - or far - without the aid of a walking stick. My goodness I am becoming a physical wreck! (it may happen to many of you as you reach old age which I define as eighty plus years).

All that leads me to the subject at hand: growing my cacti hydroponically in plastic pots using an inorganic medium/substrate (my preference being just pumice). That appeals to me for the following reasons:

1. I have done it before and it worked very well for me. My plants were healthy and robust.

2. I need my potted plants to be reasonably lightweight to accommodate my winter plant maintenance regimen a part of which involves staging my small growing Brazilians indoors on the south facing windowsills in my Scriptorium (I am a life-long Calligrapher) at night and during cold days (we do get some of those in Tucson) and transporting them outside (using the metal mesh trays I show in many of my pics) to the patio for the benefits of full sunshine and fresh air during the 60-70 deg F days of sunshine we frequently have during our glorious Tucson winters. Using straight pumice meets my light weight criterion.

3. I can't stand around very long at all preparing multi-component growing media or fertilizer - just using pumice and a liquid fertilizer really appeals to me.


4. I want my growing supplies to be few in number. One container each of pumice and liquid fertilizer and a supply of small plastic pots will suit me just fine.


5. I am becoming inceasingly lazy and want my cactus growing to be simple and uncomplicated.


That will do for now I think. I will probably come up with more reasons in which case I will generate further posts.

[jp29]

.......... One difference between the soil mix that Steve uses, and that I use, is that he mixes his pumice with DG (decomposed granite), while I use crushed granite. I tried DG for a while, but around here the DG is so rich in clay that I'd have to dry filter it, then wet filter it, and I'd end up with only about 50% the original volume, and even then it had a lot of fines ..........

That has also been my experience so far with DG, Barry. I purchased a bag at Lowes just to see what it looked like. It was far too heavy for me and did indeed contain a lot of dust/fines which looked suspiciously like very fine clay to me. I think it was actually just (overly) crushed unrefined granite. I will stick with just my "old style" pumice.

.......... Really, I don't feel like I'm striking out in new territory, I'm just synthesizing the methods that others use.

Maybe, Barry, but you have performed a great service for me by initiating this discussion and I thank you very much for doing that.

[jp29]

Previously I wrote:

.......... While we were waiting out the storm and engaging in casual conversation I mentioned my experience with pumice and my yearning for the early stuff. "Well, James" said Dan "what you don't know is that there is a small remnant pile of it hidden from view behind the mound of new stuff" .......... and as the rain let up he said to one of his workers "Thomas, go and fill a ten gallon container with that old pumice for James" (or words to that effect) ..........

Here is a close-up pic of the "early" (or "old") pumice I speak of


The pile of "new" <3/8" pretty uniform particle size pumice at Bach's Cactus Nursery


And finally a pic of the "remnant pile" of the "old" pumice I mention

It is the pale tan colored stuff mostly behind the pilings (and beneath the Cholla) at the rear of the "new" pumice pile. There doesn't look to be much, but the remnant pile is about one to two feet deep and several feet in length - enough to last me for my lifetime. This afternoon Dan gave me that remnant pile -- again, what a great friend!

I do need to clarify something: when I say that I use this pumice raw and unscreened, I mean that I don't screen out even the smallest granular pumice fines - but I do sift out any dust using a flour sifter. Inasmuch as I don't re-use my watering solution, I am actually growing my plants "semi-hydroponically" using this pumice as my inorganic growing medium.

[BarryRice]

Hey James

Pure pumice sounds perfect for you.

My main benefits of the inorganic approach are:

1) Rot is not an issue for me.

2) I do not have to repot every year or do, except to accommodate enlarging plants.

Cheers

Barry

[jp29]

Thank you, DaveW, for that very useful (typical from you) reference post.

[jp29]

.......... My main benefits of the inorganic approach are:

1) Rot is not an issue for me.

2) I do not have to repot every year or do, except to accommodate enlarging plants ..........

Thanks for those two reminders, Barry.

[DaveW]

I presume the difference between what you Americans call decomposed granite and crushed granite is that decomposed is natural scree from below granite cliffs which is just scooped up and graded, whereas crushed granite is stone waste from granite quarries that has been crushed and graded?

I have not heard of decomposed granite being available in the UK, in fact never heard of it until I joined this forum. However in the building trade we used to get what was called "quarter to dust" granite chips from the builders merchants which was used in concrete for paving slabs, or to make steps harder wearing and non slip. It consisted of everything that went through a quarter inch (6mm) sieve or riddle including the dust, but the dust was granite dust with no contamination with clay like Barry and James mention for decomposed granite.

http://shop.lordsbuildersmerchants.com/ ... -for-/584/" onclick="window.open(this.href);return false;

[jp29]

I presume the difference between what you Americans call decomposed granite and crushed granite is that decomposed is natural scree from below granite cliffs which is just scooped up and graded, whereas crushed granite is stone waste from granite quarries that has been crushed and graded?

I have not heard of decomposed granite being available in the UK, in fact never heard of it until I joined this forum ..........

I had heard of it, Dave, but hadn't researched it or given it much thought until recently. I don't usually use (or like to cite) Wikipedia as a reference, Dave, but in this instance I think it will suffice:

http://en.wikipedia.org/wiki/Decomposed_granite" onclick="window.open(this.href);return false;

Here is the DG I purchased at Lowe's and which I rejected for my use as being far too heavy and powdery with much (clay?) dust:

[jp29]

Previously I wrote:

.......... I do need to clarify something: when I say that I use this pumice raw and unscreened, I mean that I don't screen out even the smallest granular pumice fines - but I do sift out any dust using a flour sifter ..........

Here I am sifting out the dust from some of my dry, raw, unscreened, "old style" pumice using a five cup capacity trigger operated flour sifter:




The above pumice after sifting to remove dust

It is a little tedious and does run counter to my desire to keep things uncomplicated (and remember I am getting increasingly lazy these days). However, I can sift the small quantity of media I usually prepare in short order. Besides, it is a pretty relaxing enterprise. I usually do it sitting on a rocking chair assisted by my cat, Gus.

[BarryRice]

Hey DaveW

You have idea about the difference between DG and crushed granite. However, I think that DG is probably mined at open pit sites where suitable amounts of DG have accumulated.

Cheers

Barry

[DaveW]

Thanks James and Barry. The Wikipedia article seems to imply it is rather like "rotting" or fragmenting granite whereas the other is crushed hard stone? However James picture shows something like we used to get as crushed granite, so presumably the two are interchangeable?

It is hard to find any real information on the web other than the Wikipedia article James links to. I did find these quotes though:-

"Decomposed Granite Gravel is mined from an igneous intrusion in the Texas Hill Country near Austin. It is a dull gold in colour and consists of pieces from one inch in diameter to clay-sized particles. Decomposed Granite Gravel allows drainage, yet compacts and stays in place because the finer particles take a set."

The remark underlined supports the fact that the dust needs to be sifted out for our purposes since it sets hard unless separated by other ingredients in the potting soil.

"Naturally occurring Decomposed Granite starts out as solid granite rock is compressed over millions of years and broke down creating a natural gravel."

"Decomposed granite (DG) slopes are often difficult to revegetate after existing topsoil and vegetative cover have been removed. In this study, naturally vegetated soils and denuded DG cut slopes were evaluated to compare edaphic (soil-related) conditions that may differ between the vegetated and non-vegetated states. Potential plant nutrient deficiencies were evaluated by soil nutrient extraction, plant tissue analysis and by greenhouse bioassay methods. The DG cut slopes have approximately one-quarter to one-half the clay content, soil organic matter, water-holding capacity, and total and available nitrogen (N) of the adjacent natural soils. Soil extraction tests for phosphorus (P) availability gave variable results, but a growth response to P amendment was observed in a greenhouse bioassay. The native topsoil's and the DG cut slope materials had non-limiting levels of pH, exchangeable Ca, Mg and K, and total P. Cation exchange capacity and sulfate–sulfur (S) were low but similar between the two soil groups. Principle nutrient limitations are interpreted to be N, P, and possibly S and boron (B), in order of decreasing severity. Based on these results, we conclude that the removal of humified soil organic matter on the cut slopes reduces plant growth directly by lowering several of these essential nutrients, and indirectly by decreasing the soil's water-retention and infiltration capacity. Soil organic matter is viewed as a critical factor and major contrast between barren and revegetated DG materials."

It looks like unless you add some humus content to materials like granite or pumice you need to regularly fertilize since they are lacking in some elements the plants need.

"Decomposed granite consists of weathered granitic particles that range from gravel to silt size, and usually include some clay. The name of decomposed granite refers to the fact that the particles are the result of natural weathering, or decomposition, of granite rock."

We had a similar discussion on the BCSS Forum as to what is being sold as "Rock Dust" as a wonder fertiliser in the UK:-

http://www.bcss.org.uk/foruma/viewtopic ... st#p230207" onclick="window.open(this.href);return false;

http://www.organicgardeninfo.com/crushed-granite.html" onclick="window.open(this.href);return false;

I was amused by the remark in the above link:-

"Its important when adding minerals such as CG to your soil to add sufficient amounts of organic matter (compost) so the bacteria can convert these minerals to needed nutrients."

Isn't adding organic matter supplying the nutrients anyway and covering up the fact that the CG is little more than a soil conditioner adding drainage, which is of course why it is used as a potting soil ingredient for cacti, however those who use it apply fertilizers rather than incorporate humus?

[jp29]

.......... The Wikipedia article seems to imply it is rather like "rotting" or fragmenting granite whereas the other is crushed hard stone? However James picture shows something like we used to get as crushed granite, so presumably the two are interchangeable? ..........

I believe your supposition is correct, Dave.

.......... The remark underlined supports the fact that the dust needs to be sifted out for our purposes since it sets hard unless separated by other ingredients in the potting soil ...........

And tends to migrate to the bottom of containers where it can form a drainage impeding "sludge"

.......... It looks like unless you add some humus content to materials like granite or pumice you need to regularly fertilize since they are lacking in some elements the plants need ........... We had a similar discussion on the BCSS Forum as to what is being sold as "Rock Dust" as a wonder fertiliser in the UK .......... Isn't adding organic matter supplying the nutrients anyway and covering up the fact that the CG is little more than a soil conditioner adding drainage, which is of course why it is used as a potting soil ingredient for cacti, however those who use it apply fertilizers rather than incorporate humus?

Our private "Horticultural Triad" (see below) has bandied about -- and seriously discussed at length -- the above, and other related subjects, Dave.


........................................................................................................

Private "Horticultural Triad":

I have been extremely fortunate to have had Dan Bach as a very close friend for what is now some forty years. Dan has owned and operated a successful wholesale and retail Cactus Nursery here in Tucson since 1969. But he is not just a commercial grower, he is also a University educated horticulturalist (University of Arizona in the early 1960s) -- Baccalaureate degree in horticulture - Masters degree in plant physiology with work toward a PhD in that discipline. Upon graduation from the U of A he laid out, planted and managed two large commercial Pecan Orchards in the Tucson area and subsequently established his own cactus and desert plant nursery.

Dr. Jerald E. Wheeler -- PhD Plant Pathology (Purdue University and University of Arizona) was a classmate of Dan Bach and remains a close friend of his to this day. Jerald is also my good friend. He is a highly respected Plant Pathology consultant with a world-wide clientele which includes some very large and prominent commercial growers. Along with Dan, Jerald has constantly provided me with plant growing advice and guidance over the past several years.They are the knowledgeable academicians -- I am just a cactus enthusiast who has been growing them for a long time.