Building a subsoil plough 1

May 9, 2014 in DIY, Fabrication, Homesteading, Mini-farming by Claude

Last year I set out to build a small subsoiler for my Kubota tractor. The idea was to build a subsoil plough, aka. a Keyline plough or a Yeomans’ plough to improve the soil on our property. Subsoil ploughs, if used in the right fashion can be used to build soil rapidly. About two thirds of the field we have is composed of very heavy clay and one third of more loamy clay. When I say heavy clay, I mean the kind of stuff you could just dig up, and burn into bricks or pottery. I’ve actually broken a good quality heirloom spade in this clay. It’s basically subsoil, not soil. Building soil is really necessary on that area if we want to successfully grow anything there.

heavy clay

Really heavy clay on the western half of the field

Subsoilers are used to brake up hardpan created by repeated ploughing and discing and compaction caused by heavy equipment. All these conditions make it difficult for plant roots to penetrate the soil and this inhibits plant growth. Subsoiling loosens up and aerates the soil and also improves drainage. Unlike normal ploughs, subsoilers do not turn the soil, so they can be used on pastures or with no-till farming methods. The idea is that the plant roots can easily penetrate the loosened soil and proliferate while the aeration also supports beneficial microbes in the soil. When the plants are grazed or mowed or harvested, as the case may be, their roots die back and add carbon to the ground. This has multiple benefits as it improves the nutrition of the soil, prevents compaction and supports soil microbiology. It literally grows the soil by storing biomass in it and, as an aside, it is one of the most efficient ways of sequestering atmospheric carbon and putting it to good use. P.A. Yeomans developed a design method in the 1950s for using subsoilers in a certain fashion to accentuate this effect and especially to improve ski hydration. Instead of ploughing straight lines, in Yeomans’ Keyline design, the subsoiler is used along the contour of the land – actually slightly off contour in order to very cleverly and simply hydrate dryer areas and to drain water away from areas that are too wet. I’m only scratching the surface of the topic here because this post is really supposed to be about building the subsoiler. You can fined out more about Keyline design from the Yeomans Plough page or Keyline Design with Darren Doherty, one of the leading experts in the field.

subsoiled meadow

Subsoiled meadow in the spring. Note how the soil surface is lifted up either side of the plough line.

While there are subsoilers for sale for larger tractors, nobody around here sells any implements for small tractors like a Kubota B6000. Subsoilers for small tractors can be had online, but they are usually somewhat too simply build, with straight shanks and no coulters, safety pins or modularity. Even though this was going to be only my second welding project, I figured I can’t be that hard to build one of those form scratch.

The basic inspiration for the plough was the Yeomans Keyline plough, although it obviously had to be smaller. From my research online I found out that it takes about 11 horsepower to pull one subsoiler tine. My Kubota B6000 has nominally 13 horsepower, but given its age it is likely a couple of those horses have either died or escaped, so I’m right on the edge with subsoiling, power-wise. On the plus side, the B6000 has four-wheel drive, so it can pull hard and steady so there was a chance of success. Furthermore, adding a coulter in front of the tine reduces the horsepower requirement considerably because the plough can then glide more smoothly and doesn’t have to rip through roots to move forward.

Subsoiler on a Kubota B6000

DIY Subsoiler on a Kubota B6000 tractor

So, the design requirements were to move the tine far enough back to allow for a coulter and to be rigid enough to take the forces applied to it. I didn’t feel like figuring out the forces involved, so I just went for a solid construction, leaning to over-specified rather than too weak. One other feature I wanted to include was a shear-pin. This is a simple piece of steel bar that is designed to fail if too much force is applied. The point of this is that if the plough its a rock under the ground, the pin shears and the tine comes loose, rather than halting the tractor abruptly and me biting the steering wheel. Since the lower bar of the three-point hitch is held by 12 mm screws, I decided to go with a 10 mm shear-pin, so that it is definitely the weakest point in the system (even though there are many screws holding the hitch, not just one, and screws are harder than mild steel, but anyway).

I built the frame out of 50×80 mm square tubing with some lateral reinforcements out of thinner square tubing that I salvaged from some old table legs had been town out at work. I had the idea to use the square tubing itself to fix the top link of the three point hitch, rather than weld on separate pieces. I just sliced the square pipe in on two corners from the top and bent that side into the pipe and welded it fast. Works, but I probably wouldn’t do it again, because the welding in this tight space is tricky. I like the look of the design though.

The finished DIY subsoil plough

The finished DIY subsoil plough

The tine itself is fabricated from 10×100 mm flat bar with a foot welded on at an angle. As a cutting edge I bought a tooth of a tiller that is made from harder steel. It is screwed on with a 10 mm screw. I drilled three sets of holes into the bar to give it three depth settings (plus the vertical range if the three point hitch obviously). Each set of holes consists of one 19.5 mm hole that takes a 19 mm pin to serve as the main fastening and as the pivoting point when the shear-pin breaks, and the other hole, lower down is for the 10 mm shear pin. I drilled the same pattern into two 6×100 mm plates that I welded vertically to the end of the frame for the tine to sit in-between. Finally, I added half a square pipe with a bolt welded on to serve as a holder for an arm to hold the coulter. The coulter itself comes later…

The first test run was successful only in so far as the plough is solid. However, for one, the tine is a bit too long, so that the top link needs retracting all the way while driving, but most importantly, I underestimated the power it takes to pull that tine through the hard clay. It works kind of OK in the loamier soil, but there it rips out the roots of the clover on the surface and quite rapidly builds a big pile of roots in front of the tine. Clearly it needs a coulter to slice the roots before the tine comes along. For the hard clay, the foot of the tine is at too steep an angle and probably too large as well. All it does at this point is pull groves on the surface, and as soon as it digs deeper than about 10 cm, it anchors the tractor in place with all four wheels spinning. Even though the B6000 is built like a tank, it lacks mass as much as it lacks power. I think in this case, the light weight of the tractor is a limiting factor.

Back to the drawing board…

On the plus side, version 1 of the subsoiler does come in handy as a digging tool. I have used it to help dig holes for planting trees by driving the plough into the ground and then lifting it up in place without moving forward. Since the toe is so steep it digs in really fast and the hydraulics of the little Kubota is strong enough to lift out quite large chunks of clay, so that helps a lot to get the initial work done when digging holes with a shovel.

Stay tuned for version 2! I took the picture above of the Keylined meadow a few days ago, so, spoiler-alert, it works out in the end : )


DIY subsoiler. Note the top link attachment.

DIY subsoiler. Note the top link attachment.

Attachment of the subsoiler tine to the frame showing the main pin and the shear-pin.

Attachment of the subsoiler tine to the frame showing the main pin and the shear-pin.