I have a 72 inch tiller for my 43 hp tractor. My guess is it tills about nine inches. Thats enough to till in the weeds and provide a nice smooth planting medium. I wouldn't want to go any deeper because I don't want to disrupt the subsoil too much. If you go too deep you disturb the structure of the subsoil and the capillary action that brings moisture up from the subsoil.

There is a lot of microbe activity in the upper six inches of soil where organic material benefits from tilling in oxygen. Go deeper and you risk disturbing the benefits of an undisturbed subsoil. If organic material is too deep there is limited oxygen and you can get sour soil.

Zeuspaul

I thought the whole point of DD was to go deep - do it once and you're good to go. In that case 9" seems just a bit shallow. It also sounds like a lotta-lotta work.
Mixing matter in as you go sounds like a good plan - it will take more than you think. I grew some cukes one year using a trench with w bale straw and manure. 2 seasons later not much of a trace left.

Good luck whatever you decide to do-growing food is an important thing to do in my opinion!

Rent an excavator, dig your soil out 3' deep, mix your native soil with Great amounts of Pumice,Decomposed Granite, Grass and Leaf Compost,and well composted steer manure...put it back in the hole, from then on add fresh compost yearly and whatever specific to plantings amendments as necessary....and then you may enjoy the fruits of your magnificent bounty. , 2.. never walk on your garden,make paths,and you may be as fortunate as me and have virtually zero weeds,it is such a rarity for me to find a weed in my garden,mostly I have "volunteer plants that I transplant to their own bed to see what kind of blessing they will contain...

{{gwi:30700}}

I believe in no-till gardening, so I completely disagree with double digging. If you have to do it, the tractor will do the best job. Doing it by hand will both break your back and do a poorer job.

Oh my! digging down 3'????
I have totally clay soil and I do raised beds about 8-10 inches. Buy some yummy soil for the first year, add manure (free from freecycle and craigslist) and home made compost and don't have to do anything else!
I had to dig up a raised bed this year because gophers got through the wire after 9 years and found lovely soil due to years of compost and manure!
Less is better!

I never thought of an excavator as a garden tool before - wow!

Double digging isn't done with a tiller. As I understand it, to double-dig, you first remove the topsoil, then you loosen the subsoil, then replace the topsoil.

When I do it, I always try to add some amendments to the top of the subsoil; humic shale ore, Sulfur, and Phosphorus, for example. I don't put compost down that deep, but I have resorted to burying sod on top of the subsoil as a way to get rid of it.

I use a broadfork designed by Eliot Coleman to loosen the subsoil. Its tines are about 10" long, and I only use it to loosen the subsoil, not turn the subsoil. I dig a narrow trench in the subsoil and then work back from there. I don't step on either the subsoil or the topsoil, I stand on a board when I'm working on the soil.

As I understand it, the concept is to try to preserve the soil strata. I'll let the real soil scientists explain why.

Once I have double-dug a bed, I stop tilling it completely. Soil amendments like compost get scratched into the top layer of soil of my double-dug beds only to increase the interface between the organic material and the soil.

I don't think what the OP has in mind is truly double-digging. I think the OP is trying for deep tilling. I know that hardpan was becoming an issue in Illinois farmland. If there's hardpan that needs to be broken, I think there's specialized plows for that. Lloyd might know more.

Thanks for all the responses. I was actually shooting for a traditional deep/double-dug bed but hoping to accomplish it with the help of machinery. I would think going down 3' would upset the subsoil and fungi balance. I also don't know how I'd mix the soil with the amendments outside of the bed. How about I propose a new plan and you folks tell me what you think.

1) Mark bed boundaries.

2) Sod/Grass - Do I need to remove the sod or can I simply till it under? If I have to, I'll probably just rent a sod cutter to remove the grass completely. (That route would also drop the level 3-4 inches and allow the tiller to go deeper)

3) Till - Use a PTO mounted tiller to till down 8-9 inches.

4) Manure - I have a virtually UNLIMITED supply of aged horse manure so I figure after the first tiller pass, I could put down at least a foot of manure and then run over it with the tiller to mix it into the soil.

5) Irrigation - Run drip/soaker hoses down the bed and back, connect it to the nifty watering timer I found at the local Habitat for Humanity Re-Store. (It's even got a wireless moisture sensor to prevent overwatering)

6 Plant and topdress with manure and/or mulch for weed suppression.

I'm still unclear about how to use green manure or cover crops. Our miniature sheep could benefit from growing some alfalfa but I don't understand how/when to do this. So what do you think of the plan? I really want to try intensive farming methods like deep beds to increase my yields and this seems like a decent compromise. Especially since I plan on adding another 100' bed every year.

Sod/Grass - Do I need to remove the sod or can I simply till it under?

If it's bermuda grass you have to kill it. If it is invasive grass you don't want it. If not you can till it in and benefit by keeping the topsoil and organic matter.

Manure - I have a virtually UNLIMITED supply of aged horse manure so I figure after the first tiller pass, I could put down at least a foot of manure and then run over it with the tiller to mix it into the soil.

Thats too much manure. I would till in about three inches.

I would think going down 3' would upset the subsoil and fungi balance.

I think it depends on the type of subsoil and soil structure you have. I graded sandy loam topsoil to a depth of about three feet. That garden did not do anywhere near as well as the garden with the same soil left in place which is about eighteen inches of sandy loam over clay.

When I prepare a bed I use the rippers on my boxblade. They go down about twelve inches and loosen the soil but they don't turn it over. Then I use the PTO roto tiller to churn the upper surface to mix in the weeds. Then if I have it on hand I mix in compost or manure with the PTO roto tiller.

Zeuspaul

I don't have bermuda grass but I'm still scared of tilling it in. Won't I be struggling with the grass coming back up? Or is it not a concern if I topdress with enough manure and/or mulch? Again, I want to do this right so if that means renting a sod cutter to remove it then so be it. I can keep the sod, compost it, and then add it back to the garden next year. If I had been thinking I would've smothered last fall but I simply don't have time now. I need these beds for this year. And I'm not touching Roundup ever again.

I'd like to buy a used boxblade/box scraper anyway. I need it to groom the gravel drive and it sounds like it would work well to loosen the soil without adversely disturbing it too much.

So it sounds like the new plan would be to use a boxblade to loosen deep, PTO till the top 6" or so, add a few inches of aged manure, and then PTO till the manure in. It won't exactly be a true double dug / deep bed but it'll be pretty close. Still need to figure out what to do with that grass though...

Sounds good. I would go ahead and remove the grass. If you have aged manure you are in good shape. Setting the sod aside and using it later is a good plan.

Not all box blades are created equal. I have a nice set of rippers on mine. If I tilt the box up the rippers point down and I get about twelve inches of depth. Using just the rippers the bed looks pretty good but that may depend on soil type.

I can also work manure into the soil with the rippers. If I didn't have the roto tiller I would be fine with just the box blade. However one pass with the roto tiller and the beds look like velvet.

If double digging means moving topsoil below 12 inches I think it is a bad idea. You don't want organic matter that deep because it wants oxygen or it becomes anaerobic.

Using my tractor and box blade I wouldn't have any need for a sod cutter. I would scrape the grass off with the box blade. I have top and tilt which makes it a lot easier as you can adjust as you go. Sometimes working backwards and using the box blade like a dozer works better. You have to play with it. It may be best to rip three inches first and then scrape the grass.

Scrapping the sod off a 6 ft x 100 ft bed might take me a half hour. Ripping would take about ten minutes. An hour to spread the manure with the bucket. The rip and break for lunch. Change to the rototiller after lunch would take about a half hour and then five minutes to till.

Zeuspaul

Thanks for the feedback Zeuspaul. I could really use a box scraper on the property anyway to help maintain a large gravel area and drive. And if I'm hearing you clearly, the boxblade could accomplish everything I need done to the beds too. How cleanly can you scrape the sod off with the blade? I really wanted to loosen up the soil 12" deep but don't see how the scarifying teeth can get that deep with the sides. Can you explain more about the "top and tilt"? Heck, I could just rent a box blade for the weekend and do the beds and the gravel. I'd rather not have to rent a sod cutter, box blade, and PTO tiller so even if I could get away with the last two I'd be happy, just one and I'd be thrilled.

I should have said hydraulic top and tilt.

The utube link below shows manual top and tilt and using the top link to angle the rippers down. I use a more severe angle which digs the rippers deeper yet.

40 sec: Note the curved cutting blade. You want one for forward and one for reverse.

4 min: Manually adjusting the top link to angle rippers down. I have hydraulic top link so I don't have to get off the tractor.

5 min: Manually adjusting tilt link. I have hydraulic here too.

I have a third hydraulic cylinder on the rippers which swings them up and out of the way which doesn't seem to be an option in the movie.

The most valuable hydraulic link is the top link because slight variations in the blade make a big difference in the way the blade cuts.

Everything could be done manually except a few operations with the top link which require changing the angle as you are grading.

Zeuspaul

Here is a link that might be useful: Boxblade

The box blade really is the easy solution you're looking for. That is the only tool professional landscapers use to prepare soil for gardens. They disturb as little as possible so as to leave the soil firm underfoot.

I think digging deep is a mistake on several levels. If you have unlimited manure, use it as a mulch on top of the soil. For example if you planted a row of peach trees, mulch a row of manure to both sides about 3 feet out and 1-3 feet high. You'll never ever have to water those trees and the soil will be the best in the county...without digging or tilling. If weeds were to sprout in the manure/compost, they pull out or turn over very easily.

Figured I'd report back on my little project. I couldn't find anybody who rented a PTO tiller large enough but I found a local farmer who let me borrow his 60's Rotavator Gem. This thing is old school but its a monster and certainly did the job. Self powered, independent tines, and over 600 lbs! Cheap is good but free is better!

Went 2" deep the first pass to tear up the sod and then a few more inches each subsequent pass. REALLY should have lasagna'd the whole area last fall. Max depth on the tiller is 9" and I mixed in as much of that aged horse manure I could haul (about 12 yards). The soil is soooo nice... loose and fertile. Weather and life have prevented transplanting and sowing but I'm hoping we can start tomorrow. Can't wait to see what we can grow this first year!

Here is a link that might be useful: {{gwi:271448}}