Rapidly-Cut Swales with Tractor Blade

Bill Wilson
August 4, 2015

Best of Blog, perennial agriculture, photos, projects, swale, videos, water harvesting

3 Miles of Swales Cut in 5-Hours

We are making early progress on our 320-acre Missouri permaculture-farm project (Jordan Rubin’s Heal the Planet Farm). Last fall, a local dozer operator was brought in to dig the first swales. It was a small dozer but it did a respectable job and did the work in a relatively short period of time compared to an excavator. See the blog post with video here.

Before we brought the dozer back in this spring, Kevin, a long-time farmer in the area who is also Jordan’s lead farmer on this project, suggested that we simply try the 135hp farm tractor with it’s 9-foot tilting scraper blade (it’s just over 6′ wide when fully angled at 45 degrees) to see what kind of swales it would cut. It certainly seemed worth trying though I’d never seen it done before.

Adam and I headed out early one morning with the laser level and marked off about a mile of swales with white-wire flags. When Kevin arrived later in the morning with the tractor all he had to do was adjust the angle of the blade, drop it down, and start running. He ran three passes on every swale we had marked and did it all in about 60 minutes!

We repeated this process over the next two days and in the end Kevin had cut about 3 miles of swales in about 5 hours including the time it took to put in the spillways.

This is huge… for with just a basic farm tractor and a simple blade the farm is now capable to holding an additional 100,000 gallons of water every time there is a good rain and the swales fill. And with all of the rain they have had this spring it probably comes to 1/2 million gallons of water that has now soaked into the landscape rather than running off into the ravines and creeks.

Is this really helpful? Because of the poor quality of the farm’s soils (low organic matter, compaction, clay) it’s ability to absorb water is very limited. The recent soil percolation tests we did showed that the soil can only absorb 1/3 – 1/2 inch of rainfall per hour. When 2 inches of rain pours down within a one-hour period (not uncommon here) that means that most of it runs off the property adding to flooding conditions down stream and leaving plants thirsty just a few days later. What if the farm had absorb all 2 inches! No flooding downstream. More water for plants. Better plant growth. Greater yields. It’s all very simple math really.

contour flags — With the first cut, the blade cut in an average of 6-8 inches, scraping the disturbed soil to the downhill side

Second Pass at Swale — With a second and third pass the wheels of the tractor dropped into the swale cut which dropped the blade down further as well, allowing the blade to take another 3-4 inches from the bottom of the swale with each additional pass.

Berm removal to make spillways — By pulling away the berm soon after being cut, the sod below is not damaged and now becomes an excellent spillway capable of handling a lot of water without eroding.

Swale 3 months later — Here is what the swales look like 4 months later.

4 Misssouri Swales — Field view of 4 of the freshly cut swales. An existing livestock pond on the right.

Swale Map for North 60 — Hear is a top-down view of the locations of the same 4 swales. All 7 of these swales cover about a 40-acre area that is 1/3 of a mile long. The white marks along the swale are where we placed the spillways.

Missouri Swales as they are installed — Here is a view of the entire 320 acres with the location of the swales and existing and proposed pond locations. This is just the beginning. The plan is to put in many more swales, ponds and also keyline the entire farm.

Perc Test — How do we know that the soil only absorbs 1/3 – 1/2 of an inch of rainwater per hour? We did multiple ‘perc’ (percolation) tests throughout the farm.

Soil Perc Test Marker — **How to perform a Perc Test?** Dig a 1-foot wide/deep hole. Fill it with water and let hole saturate for several hours.
Return and drive in a stake with 1″ markings. Fill the hole again with water close to the top to any of the lines.
*Return* in an hour and note how much the water level dropped. If it dropped 3.5 inches then this is your percolation rate… 3.5″ per hour.
This means that your soils will be able to absorb about 3.5 inches of rainwater each hour.
A good percolation rate is about 3-5 inches per hour.
Less than this and water could be running off in a hard rain.
More than this (usually in sandy soils) and there could be a significant loss in soluble nutrients and the inability of the soil to hold water for dryer times.
The best solution for slow or fast percolation rates is to build soil organic matter and soil life. The life in the soil needs the organic matter for food and shelter and as they process this organic matter they turn it into humates that act like gluey sponges that can hold water and nutrients for decades. Buy building these stable carbon substances we are literally pulling carbon from the atmosphere and storing it in the soil where it can grow healthier food. Boom!

Bill with Laser Level — Me (Bill Wilson) with laser level. Adam and I were able to mark out the swales quickly. One of us would take the first readings, putting in wire flags, and the other would come through and double check the first’s readings. Just like in carpentry “measure twice – cut once”.

Adam - Swale is 20 inches deep at this point — Adam Haugeberg, the farms full-time permaculture designer/farmer at the time, measures the depth of the deepest swales. The swales varied in depth of between 14-20 inches. Where the soil was softer, the blade dug in deeper. With this kind of blade there is not a way to pre-set the depth so the operator’s skill and ability is important.

I know this is not a common way to cut swales and the water sits towards the back of the cut instead of soaking into the berm, but for the amount of time it takes to put them in and for the amount of water the swales hold, it certainly seems like it might be a perfect option for many other permaculture design projects. It’s perfect for us because the swales also create the contour template from which our keyline plowing plan will work from. More on that later.

Give us a shout if you have questions.

If you have already experimented with a tractor blade or you end up trying what we’ve done, please share with us what sort of results you get. The more we learn from each other…the better.

Toward a greener world… Bill

P.S. Cody asked a great question in the comments below about mowing. If one does not have livestock to keep the vegetation down, and this is important to do, here is a sketch of how to approach maintenance. Remember, the scraper blade that cut the swales is 9′ wide but when angled as fully as possible (45 degrees) it cuts a swath just over 6′ wide, perfect for a 6′ wide brushmower.

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