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I'm about to connect two existing beds with a new raised bed that'll have a roughly 10' x 6' footprint. I'm ready to kill & cover the grass completely with cardboard from Amazon.com boxes, and I have eight big trash bags full of shredded paper to pile on top. The landscaping contractors who mow the neighborhood grass supply me with all the grass clippings my heart desires, so the shredded paper will get enough nitrogen-rich goodness on top to avoid robbing nitrogen from the finished bed. Lastly, I've got about 5 cubic yards of finished compost lined up to top it all off.
Then it occurred to me that this is going to be 100% organic material. I don't want my nice new raised bed to sink down and flatten out after the organic matter decomposes. I've always read here that healthy soil needs a mineral component, which I've neglected in this plan so far.
So here's where I'd love your opinions. Do you think I should address this by mixing gravel in with the rest of my raw materials? Coarse sand? Topsoil? Wait 'til the leaves fall and dump a bunch on top before winter? How about using deep-rooted cover crops, letting them do the work of mining minerals from the nasty clay soil that lies 3" under my lawn? Perhaps some combination of the above? None of the above?
I would skip 'killing the grass' before you put the cardboard down. Just the cardboard is adequate. I would mix in at least a 50% component of topsoil or sand, otherwise you'll be facing LOTS of shrinkage & exposed roots. Be careful about the high N amendments - especially grass clippings, because of ammonium toxicity, and make sure the clippings aren't from lawns recently treated with broad leaf (or other) herbicides.
My suggestion is to use a pick or roto-tiller to break up a few inches, maybe even 6 inches, of that (nasty solid) (ultra-valuable dispersed) clay that you have. It is clay that holds and buffers mineral nutrients, and is a necessary part of balanced soil. Mix this clay in with your sources of compost.
Then also mix in some fine and coarse sand. Maybe crushed rock. I wish I knew what kind of sand or crushed rock is best! Good loam has "some of everything" - organics, clay, silt & sand.
I just discovered, and love, "coir" - coconut-husk-fibre. Like peat moss, but cheaper and easier to re-hydrate.
I would to try out crushed pumice, crushed fluffy lava or crushed compressed volcanic ash (tuff). That should add both granulaoty/grainage/aeration and also water-retention. Maybe even minerals!
One thing I found in my all-clay yard with mostly-shallow slopes. Any excavation below the grade of the existing clay turns into a permanent puddle that will drown and kill roots.
From the deepest point of your excavation, you must also dig a narrow trench (downhill all the way) to the lowest point where you want runoff to run to. It's easy to grade it perfectly: first do a bad job, then when it rains heavily, it will tell you where the high spots are!
In my small yard, any width of trench is sufficient, even one no wider than the blade on my mattock (2"). A hoe-width seems like more than enough!.
It seems that I don't even need to fill it with gravel wrapped in filter cloth, excepot to prevent tripping over it.
Well, the bed just went in. Is there anything more exciting to a gardener than a brand new area to work with? I feel like a kid at Christmas! I'll post pictures later. Right now it's in the mid-80s out there and I'm in no mood to pass out.
I started with ten big trash bags full of shredded office paper to form the first grass-killing layer on top of the lawn. I soaked everything with the hose to keep it from blowing away (and to encourage the worms to visit). That layer ends up measuring ~2 inches thick, pressed flat.
Next came a little over 3 cubic yards of a 50/50 mix of sandy topsoil and finished compost. That layer's about 2 feet thick in the middle of the bed, and slopes down to about 4 inches thick at the edges. The total footprint ended up being about 12 feet by 6 feet, connecting the two pre-existing circular islands I mentioned at the top of the thread.
I'm inside cooling off now, but my next task will be to scatter some cover crop seed ... most likely this mix: http://www.territorialseed.com/product/8892/70 . Time will tell whether I opt for traditional suburban plantings with mulch around them, or whether I go a bit nuts and try something more adventurous. I'll have all winter to figure it out, and the cover crop will keep the soil vibrant and aerated until I make up my mind. The important thing is that the soil will be covered, and that the microorganisms inside will have goodies to eat. For more, take a look at these links:
After the cover crop has had a few weeks to get established, I'll spread one 50 lb. bag of alfalfa pellets (to add nitrogen), one 40 lb. bag of rock dust (for trace minerals), the last half of a 40 lb. bag of kelp meal (for micronutrients), at least one 5 gal. bucket of spent coffee grounds (aka "wormnip"), and possibly even some homemade biochar. That'll all get gently watered in, and the bed will then be tucked away for the season. My goal is to make this an almost zero maintenance project, needing minimum fussing/watering/weeding by Yours Truly.
Congratulation on your brand-new 60 square feet of soil! Sandy topsoil? You lucky dog!
I've seen some cover crops like fall rye suggest 1.6-2.5 pounds per 1,000 square feet, which I make to be 6.6-10 grams / square yard or 0.7-1.1 gram / square foot. For 60 SqFt, that's 42-67 grams or 1.5 - 2.4 ounces.
The cover crop mix suggested 3-5 pounds / 1000 SqFt, so for 60 SqFt, around 4 ounces.
Of course different mixes and different applications will suggest different rates, and you may want to seed more or less thickly. Maybe soil building wnats more seeds per square foot?
On the other hand, I see that one of your linked charts suggests 1 0z/100 SqFt, much less than I saw elsewhere. So take my numbers with a grain of salt!
I happen to have a few pounds of Fall Rye / Winter Rye, and a "Soil Builder Cover Crop Mix", from this year's distribution. Much more than I have any rational use for, but they were only 69 cents or $1.20 per pound..
If you would like 4-8 ounces of each, or more, and some innoculent, let me know your address. Maybe plant a "comparison" square foot of each and then share the expeience. (I see that Peacefull Valley is backordered on fall Rye.)
I envy your no-doubt well-draining soil!
I was also looking at some "Tillage Radishes" that promise to drill down through heavy soil deep enough to reach China (not really), but at least to go deep and leave big honking holes behind.
Cover Crop Solutions, LLC http://www.TillageRadish.com
I don't think I'll try to contact a local dealer and explain why I only want a few ounces insted of pallets of 50# bags. Instead I think I'll try to trade at DG for some Daikon radishes or maybe oilseed radishes. (I see a pound of Daikon seed at PV for $5.70, but it's too much work to discover thier shipping and handling charges.)
3 months after your original post I'm checking in on it cuz I've just gotten myself "all informed" about minerals. More like aware. You have added 40 pounds of rock dust so you are already aware of the need for minerals. Does your inquiry mean that you don't know if that is enough? Or the right kind? I want to know too. I am bumping your post to see if anyone else has input.
I shopped around online & found the best prices were for big bags of 40-50 lbs each. I live in a new development that had all of its forest topsoil scraped away, so I figured I'd buy "a bag o' this and a bag o' that" until my budget said to stop. After a couple of 5-gal buckets' worth of mixed azomite/greensand/rock dust gets spread on the new bed, I'll use some more around my edibles.
Anything left over goes into my compost bin, so the trace minerals get distributed along with the finished compost.
Needless to say, the cover crop & weeds grew like triffids. Rather than uproot everything & toss it, I took a spade, carved out sod-like chunks, and simply flipped each one over (for free organic matter). It looks like hell at the moment, but I plan to cover the bed with shredded leaves I saved from last fall.
On a positive note, four peppers & four tomatoes are happily growing on one edge.