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| In my area the dirt is largley gravel (sand and rock) I a m building raised beds for my garden. I was wondering if sifted gravel would serve the same purpose as rock dust would in a garden. At the end of the day isn't sandy gravel broke down rocks anyway? |
Follow-Up Postings:
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| Sifted gravel isn't going to hold moisture as sand will. There is, sand C-33 (concrete size) and finer grades, manufactured sand (from quarry fines), gravel dust (same but almost powder)), and then the various sized crushed stones. Each of these has different drain rates. But to answer your question I'd say yes. No organics out of the quarry or in true sand. Help that soil out. |
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- Posted by jonhughes So.Oregon (jonehughes@hotmail.com) on Thu, Nov 18, 10 at 1:22
| My take on this: Decomposed granite is a rock powder and is an excellent amendment for clay soils. All rock powders are great sources of minerals and micronutrients. All growing soils need them. As the microbes and macrobes like earthworms, digest the insoluble minerals, they break down into the various soluble micronutrients that all forms of plants need. For example, limestone rocks are rich in calcium. Granite rocks are rich in potassium, etc. Seaweeds of course are the king of micronutrient fertilizers and soil amendments. There can be up to 70+ trace elements in seaweed. In locations where seaweed is not readily available; rock powders is one answer to the problem. Research continues to reveal that insoluble tiny particles like rock powder minerals, can be easily digested or absorbed via microbial activity, over time, into the anatomy of growing plants, thriving in the presence of rich organic compost, and other forms of powerful biostimulants like aerobic compost teas. Obviously a heavy clay soil is very difficult to initially dig, whereas a sandy loam is much easier. Heavy clay soils can be improved considerably by adding a lot of sawdust and mason's sand. DO NOT use beach or river sand! Mason's sand, being crushed granite, has sharp edges and actually loosens the soil.
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| Adding rock dusts to soils will do little unless sufficient quantities of organic matter are also present since the soil microbes that will change the dusts into nutrients the plants growing there can use need organic matter to live and function. Ove the years I have seen the levels of Phosphorus and Potash go from Below Optimum to Above Optimum without adding anything other than compost and other forms of organic matter. I have seen the soil pH go from 5.7 to 7.2 without spreading lime. Concentrate on getting the humus (residual level of organic matter in the soil) increased and don't worry about the rest. |
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- Posted by eaglesgarden 6b - se PA (My Page) on Thu, Nov 18, 10 at 7:40
| jonhughes, I have to confess that your garden is one of the most magnificently beautiful gardens I've seen. Out of curiosity, are you in the concrete business? You seem to have all the tell-tale signs in your garden. Everywhere I look I see cinder block walls, and crushed stone. Well done, and thank you for sharing your pictures. |
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- Posted by gardenguyfrompa (My Page) on Thu, Nov 18, 10 at 9:41
| I will simplify my question. I am doing a huge raised bed garden in the spring. I will have 14 4x10 raised beds. My options to fill them are limited. I can get 3 year old cow manure, I can get sifted top soil, I can produce my own dirt, my property is basically on a gravel pit, my dirt is lots of rocks and sandy dirt which I can sift myself, and I also will have a descent amount of compost, but only enough compost to maybe add a thin layer to each bed. What should I use out of these choices? |
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- Posted by jonhughes So.Oregon (jonehughes@hotmail.com) on Thu, Nov 18, 10 at 13:43
| Well Personally I would take everything you can get your paws on on mix it all up,then just get on a compost making binge and continue top dressing the beds.All of my beds are 3 to 4 feet deep with the same mixture throughout.
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Here is a link that might be useful: Raised Beds by Jon (see all 9 short videos ;-)
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| g-guy, I'm in the camp of all the topsoil I can get (or make), the cow manure is vital in limitation as is organic anything. That being said with 14 beds you may get premium soil in some and a less grade in others. Especially as a start. I am very fortunate to have access to large amount of old leaf compost even though it still needs work. I have made beds of nothing but this and it works but I'd rather have good screened soil as supporting element. If I planted corn in potting soil it would fall over in a thunderstorm. In dirt with plenty of potting soil/compost and 20% sand it will stand. Jon has an amazing bed setup that I also envy but I expect he changed through experience what he originally started with. |
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| The current issue of Organic Gardening magazine has an article about a soil building technique used in Australia and New Zealand that is similar to Pat Lanzas Lasagna method. |
Here is a link that might be useful: Lasagna Gardening 101
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- Posted by piranhafem (My Page) on Fri, Nov 19, 10 at 12:13
| This thread was very timely for me. I went to a Master Gardener's open house and the man in the "Ask the Expert" booth advised that I improve drainage and start to amend my heavy clay soils by digging some deep holes in my tree wells and backfilling with sand. When I asked him what kind of sand, he told me any kind would do, that I could just go out to the desert and haul loads of sand from the wash. Now I'm going to go with mason's sand instead. Thanks John Hughes! (I'm already mulching heavily with composted wood chips so hopefully that will do it.) --Maureen |
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| It is unfortunate that they myth that adding some sand to clay will improve drainage is still bandied about, especially by someone that claims to be a Master Gardener who should know better. To make enough difference in the drainage of clay soils you need to add somewhere between 45 and 75 percent sand and that is a lot, tons of sand, according to places such as Cornell, Purdue, Michigan State, The University of Washington, etc. What will make a difference in the drainage of clay soils is organic matter, but here you need only get about 8 percent into that soil to see a difference and that is still a lot of organic matter. |
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| Although I do definitely agree with you, Kimm, that mixing sand with clay is awfully risky business, from Maureen's post it sounds to me like the "man in the booth" was describing something more along the lines of creating a sump rather than blending soil types. Those can be a pretty cheap and relatively effective way to install supplemental drainage. The idea is that water should weep into the hole (or preferably more than one hole) laterally more quickly than it would move down into the heavy native soil. You'll end up with a perched water table at the bottom of the sump but eventually the capillary action of the more finely textured native soil, along with gravity, will pull the water downward and out of the sump. Since you will have that perched water, you want the bottom of the sump to be at least three or feet deep. Sand works okay but I prefer the method in which you would dig a hole about 6 inches in diameter and drop in a 4 inch PVC pipe with holes drilled in it:  Then you cap the pipe and backfill around it with gravel Don't wrap the pipe with a filter sock. It's commonly done, but the socks commonly clog with silt and clay particles, rendering the sump ineffective. One nice thing about the pipe is that you can look down the tube for standing water to see if the root ball is saturated. If it is you can pump it out. Another option that isn't quite as good as the pipe but is better than sand would be to backfill the sump with pea gravel. Either way, rather than starting your sump at the bottom of the planting pit, it works best if it goes all the way to the surface like this:  |
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- Posted by piranhafem (My Page) on Sat, Nov 20, 10 at 21:16
| Yes Garg, my Master Gardener was going for a sump, to get the water to penetrate deeper. My mulberry trees are suffering from salt burn. The clay soil is so dense and compacted, I can't get water to penetrate more than about 1.5 feet. It just runs off laterally, or stands and evaporates, even with heavy mulching. He suggested holes backfilled with sand to get the water to penetrate more deeply. I have pea gravel -- maybe I should use that instead of the mason's sand? I was under the impression that the mason's sand would add nutrients, but if the pea gravel will work significantly better to get down to the root ball, I'll go with pea gravel. Any idea how many of these sumps I should dig per tree, based on the diameter of the drip line? thanks! --Maureen |
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| I would go with the gravel since it will drain better and nutrients are easy to come by, whether in the form of fertilizer, compost or a combination. The mason's sand probably wouldn't have to much nutrition to offer anyway. Two sumps is usually pretty safe. Sometimes in really heavy soils in a high rain fall area, you might want to go three. But, if you are in a relatively low rainfall area and your trees can stand a little extra water, one could do the trick. but if the pea gravel will work significantly better to get down to the root ball Remember, you're not just going to the root ball, you're trying to take water past it; just like in the picture up there. |
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| GGFP - How quickly and readily the minerals locked in rock dust become available depends on the size of the particles, and the smaller the particles the larger the fraction of nutrients immediately available and the faster currently unavailable nutrients DO become available. Also, it would do no good to add rock dust unless the nutrients therein contained were deficient in your soil. There is an optimum level of o/a fertility, to which everything soluble in the soil solution contributes, and an optimum ratio of nutrients (to each other). Exceeding either the optimum fertility level (EC/TDS) or conditions where the ratio of nutrients is skewed are both counterproductive. A sump will not work in clay soil unless the water has somewhere to go - somewhere to drain to ...... unless you mechanically remove it via a pump. If you look at the picture provided upthread, it isn't difficult to imagine water moving laterally from the surrounding soil to fill the sump hole. Then what? You have a hole full of water and the few gallons in the hole has had absolutely no measurable influence on the surrounding water table - nothing gained for the effort. Even pumping it out by hand a dozen times will affect no, or very little change in the surrounding water table, which you DO need to consider. Add a trench or a tile so the water can be moved to a location where it will flow away; or rig a pump/float set-up, and you have something that will affect a change. Al |
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| A sump will not work in clay soil unless the water has somewhere to go - somewhere to drain to ...... unless you mechanically remove it via a pump. That's simply not true in many cases unless you include straight down through the clay in your "somewhere to go" statement. Water moves just fine through clay, just very slowly. It will move downward through the natural channels and pore spaces between the clay particles that are formed by the (clay)+Ca+(clay) and (clay)+Mg+(clay) bonds. Here's how it works: First, you have the capillary action of the clay pulling water downward. The same force that can pull on water so hard that it will suspend it in a perched table when clay is over sand, due to the differences in sizes of the void spaces, is the same force that will pull water down and out of the sump, into the clay below. Second, gravity is also on your side. The heavier the water column, the more gravity will work for you to pull the water down into the clay. That's one of the reasons why I prefer the gravel rather than sand, and the pipe rather than the gravel. The gravel and pipe both allow more water per cubic centimeter of space than the sand, which means a heavier head of water in the same size column (or another way to say it is a shorter column will provide the same amount of weight). Also, the gravel has even less capillary action than sand and won't be working against the forces pulling the water down. That is to say, the water column (perched water table) won't have to be as tall for gravity to help push the water down. The pipe will provide the heaviest head of water and capillary action will be, for our purposes, virtually zero. If you look at the picture provided upthread, it isn't difficult to imagine water moving laterally from the surrounding soil to fill the sump hole.......you have a hole full of water and the few gallons in the hole has had absolutely no measurable influence on the surrounding water table Well, I like "imagination time" as much as the next guy but it's a pretty big leap to assume that everyone everywhere has a high water table. I'm not even going to explain further. I'll just let you think that one through on your own. Will drainage happen slowly? Sure, but it will happen. The success depends on a number of things. Exactly how heavy is the clay? What is the silt percent? How much rainfall do you receive? Is irrigation monitored properly, etc... |
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- Posted by piranhafem (My Page) on Sun, Nov 21, 10 at 13:51
| Yikes, you guys are talking way over my head. I'm a pretty smart person but this is all new to me. Al, the sumps aren't meant to pull water into them, they are meant to help water penetrate more deeply and move more easily through and past the root zone of the trees. I live in Marana, AZ where rainfall is sparse. Mulberries are water pigs, and if anything, my mulberries need more water, they just need it to penetrate deeper and move more quickly, to flush the salts. Gargwarb, I hear you that the PVC would be better, but I don't think I'm up for the time and work involved. (I'm dreading just digging the holes!) And since I have the pea gravel handy, I'm going that direction. Thank you so much for the advice, I really appreciate it. --Maureen |
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| Your welcome. |
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| Great discussian. Good ideas were presented that might be helpful in future problem solving. |
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