The primary consideration as to what benefit it might add centers almost completely on its physical properties - mainly on its size relation to the other soil ingredients and what fraction of the whole it represents.
That's kind of like asking "When is a hammer a good choice on a construction project?"
Whether or not you agree with my answer sort of depends on whether or not you place some value on the claims that it is good source of micro-nutrients or that somehow there is a benefit associated with its claimed paramagnetism. I tend to discount both of those claims, leaving me again with only its physical properties as considerations in deciding it's value as an amendment to mineral soils. While it adds porosity in certain cases when its size is conducive to improving that property, so does organic material; and organic material has more micronutrients that are more readily available than those that would be derived from the sand. It has a little better water retention than mineral soils, but only on a size for size basis, and size is very important. I would again give the nod to OM over the lava sand in that department (water retention). You can see I'm not particularly excited about it from an analytical perspective. I'm thinking that somewhere it might shine is as a fraction of the mineral component of raised bed soils.
My impression is that sand tends to improve drainage and aeration in heavy clay soils, BUT there must be a goodly amount of organics (humus, compost, etc) present as well. Otherwise, the clay just glues the sand particles together into an even harder, denser mass.
Work it in well and mix it up.
I've been adding it to my raised beds lately, since adding as many bags of manure as I can afford only helps for a year or so. (I had soil so clayey that I needed a pick to break it up, and then it was gluey, almost like modeling clay.)
An advatage of sand is that it does not oxidise in a year or two, as compost and manure can. Wood shavings may last longer, but they seem to encourage a lot of white fungal hyphae.
Sand is forever.
My question is: what grade and _kind_ of sand is most helpful in gardening? For example, for improving drainage and aeration and ease of breaking up big clods with a cultivator instead of a pick ... in very heavy mostly-clay soil with some organics added.
I am guessing (pure ignorance) that finer-is-better, since I get more particles per dollar that way, and "play sand" (which seems pretty fine to my fingers) does help me some. Maybe I should ask "what is the most effective kind of sand for lightening soil PER DOLLAR".
Is there more advantage to coarse sand or crushed rock (say 1/2 mm and up) if price is no object? If they really touched each other, I would expect "bigger to drain faster", but I also don't think soil SHOULD have so much sand that each sand grain rests on anotehr (instead of being embedded in a matrix of silt and clay).
I found this table in Wikipedia for "US Grades" of sand
very fine sand 1⁄16 mm - ⅛ mm
fine sand ⅛ mm - ¼ mm
medium sand ¼ mm - ½ mm
coarse sand ½ mm - 1 mm
very coarse 1 mm - 2 mm
One only need fly or walk through areas where lava sand was the only addition to the vast territories down wind from the eruption to see the value. Where the fallout landed only on top of the soil it is clear as day and night what the value is. One can actually see the value by following a mountain stream observing the banks and closely related stream side differences. Since no one mixed any of this example into the soil the observation would not tells us if additional mixing into the soil would make any improvements or cause any harm.
I have used Ironite at the rate of forty pounds per one thousand square feet mixed in the first six inches for quite noticeable positive effects. My soil is sandy loam with an organic content above ten percent and amended to a PH of 6.8 to 7.2. with an 0.02 test variance factor.
The only thing I was largely interested in was PH. I used the simplest form of soil test which does not show more than a very few of the 60 + trace minerals found in products like Ironite or azromite. I let the plants tell me the rest of the story by their vigor and production. The PH was a crop related need. This would be to high, sweet or close to neutral for average garden and potted plants. My most recent main crop was Giant Pumpkins seeking maximum growth with strong healthy body. My best was just under 1000 lbs. The world record held in Ohio is 1725 pounds.
So this may end with a smile here is my largest shown carved as a night pumpkin.
For heavens sake...I kinda get the feeling that anything that might be said not in accordance with big brother Al who is apparently sitting at the right hand of God would be out of order here or any place else.
Sorry I am one of the stupid ones. I came here more than once. Every time for what ever reason I got my butt kicked by big brother Al. Here it seems to be his way or no way. It seems I am to stupid to remember that fact.
Al if you have to continuously explain what it is you are talking about maybe you should try to speak in terms that would be less likely to be misunderstood. Maybe you should explain also where all those flushed through fertilizers go and what they do to the waters of the world. If they were not in excess and of non-biodegradable material why would they need all the flushing through ? That would seem to waste both water and fertilizer. That would seem true in fields and pots alike.
I will try not to step again into your neck of the woods in the future. No acorns here for me anyway.