April 10, 2017

Double Digging for Rainwater Collection

Last summer was a doozy in terms of too much heat and too little rain. If you're a regular reader, than you know we're working on additional measures to harvest more rainwater including a larger collection tank and garden swales. One day Geoff Lawton had a link in his newsletter to this page (second video down), and it got me thinking about double digging.

Double digging is a biodynamic (French intensive) soil preparation technique. In permaculture terms this could be classified as "earthworks," i.e. the arranging of the earth to effectively capture and utilize water. What I'm actually creating are hugelkultur swales.

Why not just make regular hugelkultur beds by building them on top of the ground?  Let's answer that by starting with a look at the soil in my garden.

Shovel included for size comparison. I tried to get a cat to volunteer for
that, but for some reason none of them were interested in posing in a ditch.

The soil in my garden is classified as cecil sandy loam. It, along with the other cecil soil types, are typical for my part of the country. This soil type has a light brown sandy loam topsoil and a red clay subsoil. Diagrammed, it looks like this ⬇

The degree of topsoil darkness is an indicator of the amount
 organic matter present. The red in my subsoil is from iron.

In the garden, my topsoil is typically about four to six inches in depth. Being mostly sand, it doesn't retain moisture very well. Water drains quickly and the soil dries out quickly. It has just enough silt and clay so that it dries rock-hard during drought-like conditions.

When it rains, the topsoil quickly becomes saturated and water drains down to the clay subsoil. The clay absorbs some of the rainwater, but because my garden is on a gentle slope, the rest is drained down to the bottom of the garden.

During prolonged periods of rain it's a big muddy puddle down there and it retains water for a long time. We discovered this by accident our first winter here. We wanted to plant an almond tree at the bottom of the garden and hit water while digging the hole. This wasn't a tree that liked having wet feet, so we planted it elsewhere. At the time I didn't understand what was going on, but after researching and observing, I see the patterns and the problems and want to take positive action to solve them.

The flip side is that the top of the garden dries out very quickly. It seemed to me that double-dug beds would be a good way to harvest more rainwater to benefit the summer garden.

Here's what I've been doing. It starts by hand digging a trench about two shovelfuls deep. The first shoveling removes the topsoil, which is set to the side. The second shoveling removes a layer of clay, which goes into a separate pile to be used elsewhere.

Unlike my swale, which was filled only with logs, branches, bark, and corn cobs (no soil), this trench is filled with layers of rotted wood and bark, biomass, compost, and topsoil. I'm also adding dolomitic limestone and soft rock phosphorous because my soil is poor in those two minerals.

Warm weather planting is upon us, so that's the next step.

Now when it rains my double-dug bed should catch and retain some of the subsoil runoff.

Will it work? Shortly after I took the photo at the top of this blog post it rained. One and a half inches later, my little trench had collected and was holding four inches of rain runoff.

Of course, once the trench is filled in it won't hold this much, but between that trench and the organic matter I'm adding, this should do much to help my poor garden during dry spells.

One last comment, this plan won't work in all types of soil. The particulars of my cecil soil generously facilitate it. Plus, in addition to rainwater catchment, double digging adds much needed organic matter to my soil in a way that should keep it productive for many years to come. Additional soil building and nutrients will be added as topdressed compost and mulch, where rain can deliver them to the roots as nature intended. No more hitting the clay and washing down the hill! Also, I have to admit for some renewed hope for root crops as well. Things like carrots and mangels were never able to penetrate that heavy clay subsoil and have been stunted for me. I'm counting more room for good root growth as another plus.

Speaking of soil types, this might be the time to do a quick plug for my little eBook, How-To Home Soil Tests: 10 DIY tests for texture, pH, drainage, earthworms & more. Click the link or the book cover to learn what it contains and where to find it. These are the same tests I used to learn about my own soil! All of them are easy, inexpensive ways for learning more about your own soil, including tests and charts for everything listed in the subtitle, plus how to read your soil color and weeds.

I've only just begun this project and don't anticipate having all my rows double dug before it's time to plant this year. It will be something I'll just keep working on for awhile to come.


Mike Yukon said...

Thank you, that was very interesting and useful for people who have a garden and wonder why they have to irrigate so often.

Leigh said...

Thanks Mike, I sincerely hope it is useful to some gardeners out there. We all have different situations when it comes to gardening, which really makes it challenging sometimes, to find answers to problems and questions.

Renee Nefe said...

looks like a sunken garden vs a raised garden. looks like a great plan. Hope you get great yields. My apple tree has budded out, but I need to have it treated...it's sick. :( I should also get out there and prep the bed for this year.

Leigh said...

It's only sunken under ground, LOL. I could even build a raised bed directly about them.

Dani said...

We have the same type of soil - thin layer of top soil above red rock filled clay.

I did what you did in my one shadecloth veggie patch and, together with the mulch, the harvest this year was outstanding :D I know you'll see the difference too...

Leigh said...

Dani, that's an excellent idea. I find that all of my raised beds dry out quickest, so maybe this would help for them too.

DFW said...

Thanks for the info Leigh. We have some areas that need to have water catchments & others that we need to drain better.

Jason and Michelle said...

Our soil here is sandy, it doesn't retain water at all. I compost my grass clippings, chicken coop bedding/pooh, and leaves. I am hoping eventually, I will have some decent soil.

Leigh said...

Dan was just working on drainage around the house yesterday. Ideally we should be able to redirect water from places where it needs drainage to spots where water is needed!

Leigh said...

I'm not sure which is harder to work with -
sand or clay. Compost is definitely the answer for both! You probably mulch too, which I find builds soil as well.

Rain said...

Hi Leigh :) Thanks for the details in your post. I have no clue what the soil is like here, but we have a soil testing kit that I'll use in a month or so when the snow melts and the ground is workable.

I've read on some blogs that people refuse to double dig, but I saw a video on it, and I will be double digging the area where I plan my pumpkin patch. I'm not into big tillers at all, because I do care about the earth's environment under there and I don't want to disturb things too much.

Leigh said...

Even permaculture acknowledges and uses earthworks to build terraces, swales, and ponds. Dan's and my introduction to permaculture was with Sepp Holzer's book and we were astonished at how he uses heavy machinery to rearrange large areas in the first stages of implementing a design. I consider my double-dug beds and swales to be small scale earthworks. :)

Learning about your soil will be fun and truly key to how you work with it.

Sam I Am...... said...

Leigh, You are so knowledgeable and I know it has come from many hours spent researching and working outdoors. Thank you for sharing it with us. I think that is probably similar to the soil we have here in AR but we also have Bermuda Grass which has left me no choice but to garden in containers for now.

Leigh said...

We have Bermuda grass too! It's the "wiregrass" I complain about so often. sigh

The Web Soil Survey website is an excellent place to pinpoint your soil and learn more about it. You can even search by your address. It really helped me to understand everything I'd been observing, and that was the first step in knowing what to do to improve it.

Chris said...

Good interventions. Ideally you would grow more topsoil, by growing cover crops and slashing them back into the ground. Which then adds more humus to soak in the water.

However, that takes several seasons, to build enough spongy humus topsoil. In the meantime, you can do strategic interventions like this. It will be great to see the results. :)

Leigh said...

After last summer and running out of rainwater and losing so much of the garden, I just had to do something. Building soil from the top down that is deep enough to hold moisture would take many seasons! We've improved our soil considerably, but still have a long way to go. Would love to eventually connect some swales to these.

Harry Flashman said...

Off Topic. Got your comment about "Chloe". Sorry about that. I replied and gave some background on that particular individual. Again, sorry he is bothering you.

Leigh said...

No apology necessary! Some people are angry and jealous about everything and everybody and this is what they do. Trolls are just a fact of internet living.

jewlz said...

Looks brilliant- hope it solves the problem for you!

Leigh said...

Me too. It's tough having to ration water during a long hot dry spell; to have to pick and choose which plants get it. It may take me awhile to get all my beds converted, but it should be a long-lasting solution for happier and more productive plants.

Su Ba said...

I don't have clay, nor your soil type, but based upon my own experience and experiments on my lava-soil farm, I'd say you have a great idea going there!

My "top soil" originally consisted of hydrophobic degraded organic material mixed with volcanic dust. Like your sand, it drains rapidly, but is very reluctant to get wet in the first place. Below it (1"-10") is cracked pahoehoe lava which acts as your clay in that it does not allow the passage of much water, thus rainwater flows downhill atop the lava. When I first started developing my gardens I was puzzled that almost all the land looked like it was in some arid region rather than a place that got 60"-80" of annual rain. Plus after a rain I had boggy ponds in the lower areas.

In some areas I created trenches in the cracked lava and made beds just as you describe. Wow, they worked! Not super the first year, but every year they have been getting better. Every year I redig and incorporate more organic material because the original compost, weeds, and manure decompose causing the bed surface to sink. These beds do successfully capture water resulting in less flooding lower down. My bogs no longer appear after a rain and the garden beds don't dry out as quickly. Oh, I still have to water because of our drought cycles here, but on moderate years I've managed to get through a growing cycle with very little irrigation.

I hope this scheme works out for you too!

As for double digging, tilling, permaculture, etc......as they say in permaculture, it depends. Every situation is different. On my own 20 acres I have areas that tilling is not a good solution, but then there are sections that are far more improved by repetitive digging.

Leigh said...

Su Ba, that is so interesting about your soil and your similar solution! I've been doing quite a bit of research on soil and find it to be a fascinating subject. Sepp Holzer creates "humus pits" at the bottom of some of his slopes to catch rain runoff plus whatever they wash down. He digs them out periodically too, and uses it for fertilizer.

It seems the no-till method works best in vegetable gardening as raised beds. You can just make your own soil and not have to deal with the soil that's naturally there. My biggest frustration is our "wiregrass," which grows up through raised beds and acts like a vine while taking over and choking everything else out. I get a lot of suggestions for mulch, which does slow it down, but it doesn't stop it.

Farmer Barb said...

I was once counseled by a guy from Maine. He suggested that I only worry about gardening the soil in a 2' x 2' hole. Make up a 100' grid and dig a series of holes. Garden in them this year. Next year move the grid over two feet. Keep doing it 2 feet over until eventually all the soil will be enriched 2' down and it wouldn't kill me to try to dig up the whole thing. Now, I have an excavator. I am clearing a 100 sqft. section adjacent to last year's pumpkin patch. I use the rocks elsewhere and fill in the difference with barn leavings. I hope to be able to send all the spring overflow in my beds down there. My clay here is pretty unforgiving, too. The reality of New England is the rocks. What you dig up this year will only be replaced by the frost heaving from next year.

I look forward to seeing what you are able to grow this year!

Leigh said...

Hmm, that way you would eventually create two foot high quality soil over several years. I like that idea. Maybe if I were younger I'd do that too! My kind of soil compacts all to easily, so I'm kinda hoping these will be permanent beds. It's usually the wiregrass that takes over and defeats me there, but maybe one of these years I'll defeat it.

Jeff Anthony said...

Hi Leigh! I came across this post and it was an aha moment for me. I'm in middle Georgia, and have the exact same soil as you do. Well, not exact but pretty darn close. Two feet of sandy topsoil, then red clay and then... SANDSTONE... No wonder they named this county Rockdale. :) Anyway I have a few questions. I assume your beds run perpendicular to the slope? I have a literal square acre with 200' long, 30" wide raised beds, but before I knew any better, I designed it going not perpendicular, but not parallel, either. The obvious answer to my dilemma is to rotate the entire "square" acre and start over... uh, no. So to illustrate, the top end of my beds are draining fairly well, but the bottoms are wet and the walkway/trenches fill up. I use a walk behind with a rotary plow to make the beds/trench-walkways. It's looking more likely I need to make a swale bed on the contour above the garden, and plant figs or something. If you were me, would you completely rotate (and remake about 45 beds...), or do you have any suggestions? I'm not lazy, just getting older and my back isn't what it used to be. Thanks for the info and the great blog! Jeff

Leigh said...

Hi Jeff. Welcome! Thank you for taking the time to comment. I love getting to know others who have similar goals.

It definitely sounds like we have the same soil. Cecil sandy loam extends from Alabama to Maryland; very common here in the southeast. To answer your question, my beds run kinda perpendicular to the slope. We made them to terrace down the most obvious slope but later realized that the ground also slopes gently to the side. I don't know if I explained that very well, but what happens is exactly as you describe - one end of the bed drains quickly while the other end contains a huge puddle for several more days. I entertained redigging and rearranging all the beds for half a nanosecond, but that was it. It would just be too much work. What I did was to dig out the highest end a little more in hopes of leveling the bottom of the bed. So looking down into the freshly dug swale one end is deeper than the other. On the other hand, by the time I fill it with all my hugelkultur materials I don't know if it will make that much difference, since I'm not tilling them every year. My goals are to build the soil and retain moisture, and I'm finding that my hugelkulture swale beds are a good step toward that. I water these beds less frequently than the others.

I looked to see if you have a blog but your web page came up not found. It would be interesting to see your garden.