By mid-September across much of Australia, small farms and lifestyle blocks are turning ground for summer crops, vegetable beds and new pasture. The weather is warming, the soil still holds winter moisture, and anything sown into lumpy, trash-covered ground will germinate unevenly. A disc harrow is the implement most small tractors use to turn rough ground into a workable seedbed, and the way it does that is less obvious than a set of spinning blades suggests.
A disc cuts because it is angled, not because it is driven
The discs on a harrow are not powered. Each gang is a row of dished steel discs on a shared shaft, and the whole gang rolls freely as the tractor pulls it forward. Nothing turns them except contact with the ground.
What makes them work is the angle. Each gang is set slightly across the direction of travel, so every disc rolls forward while also sliding sideways through the soil. That sideways slide is what slices clods, cuts through old stubble and lifts a thin layer of earth off to one side.
Increase the gang angle and each disc takes a wider sideways bite, working more soil and pulling harder on the tractor. Reduce it and the discs mostly roll, doing little more than pressing the surface. Most of what an operator adjusts on a harrow comes back to this one setting.
What the notches on a notched disc harrow actually change
A smooth disc meets the ground along a continuous curved edge, so its weight is spread along that whole edge. On hard or dry ground that spread-out pressure is often too low to break through, and the disc rides on top, polishing the surface instead of cutting it.
A notched disc concentrates the same weight onto the points between the scallops. Each point presses into the ground with far more force per square centimetre, which lets it penetrate soil that would carry a smooth disc. The notches also catch old stalks and trash and pull them down against the soil, cutting material that a smooth edge would push ahead in a growing pile.
There is a trade-off in the finish. Notched discs leave a rougher surface and throw soil more aggressively, which is why some harrows carry notched discs on the front gang to cut and break the ground, and smooth discs behind to level and firm the result.
The dish in each disc decides how much soil turns
Discs are concave, and that dish is what lifts and turns the soil after the edge has cut it. A deeply dished disc turns soil further and buries more surface residue, while a shallow dish mostly cuts and shifts the ground sideways.
On most small harrows the front gang throws soil outward and the rear gang throws it back in. Set correctly, the two cancel out and leave the ground roughly level. Set unevenly, they leave a ridge down the centre of each pass or a furrow at its edges, which then has to be worked again.
A small tractor sets the angle before it sets the depth
A harrow built for tractors in the 20 to 30 horsepower range is deliberately light, because a heavy one would be more than a compact tractor can lift on its three point linkage. That lightness means the discs rely on angle rather than mass to stay in the ground.
When the harrow skates over hard ground, increasing the gang angle usually helps more than adding pressure through the linkage. Some operators add weight to the frame where the design allows it, but that should stay within what the tractor can safely lift and carry on a slope.
Turning is where small harrows are most often damaged. Swinging the tractor round at the headland with the gangs still in the ground loads every disc sideways, which bends shafts and cracks bearings. Lifting the harrow before each turn, and lowering it fully before leaving the seat to make an adjustment, protects both the implement and the operator’s hands.
Overworking is the quickest way to spoil a seedbed
Each pass breaks clods smaller, and it is tempting to keep going until the soil looks like flour. That fine surface seals under the first heavy rain, forming a crust that seedlings struggle to push through and water runs straight off.
Repeated passes at the same depth also create a compacted layer just below where the discs run, often called a disc pan. Roots and water stop at that layer, and the problem only shows months later when crops wilt early on hot days. Two or three passes at slightly different depths usually leave a better seedbed than five at the same setting.
Sorting tillage tools by what they do to a clod
Implements are usually listed by working width, which says how quickly ground is covered and nothing about what the soil looks like afterwards. Grouping them by their effect on a single clod explains where each belongs in the sequence.
- Disc plough: large discs, inverts soil, primary tillage
- Offset disc harrow: heavy gangs, cuts rough ground and orchards
- Tandem disc harrow: cuts and levels, secondary tillage
- Tine cultivator: stirs soil, leaves structure intact
- Rotary hoe: powered blades, very fine tilth
A disc plough belongs at the start, when grass or hard ground has to be broken open. A harrow follows, cutting the turned ground into a seedbed, and a tine cultivator suits the final lighter work where soil structure matters more than a smooth appearance.
Soil moisture on the day matters more than the number of passes
Discs cut cleanly through soil that is moist but crumbly. Too dry, and hard clods simply roll ahead of the gangs. Too wet, and the discs smear the soil into slick bands that dry into bricks
A simple test is to squeeze a handful of soil from working depth. If it crumbles when pressed, conditions suit harrowing; if it forms a ribbon, waiting a few more drying days will save several extra passes later. In spring that window often opens a few days after a good fall of rain, depending on the soil type, once the surface has dried but the soil beneath is still moist.
Anyone comparing a disc harrow for sale for a small tractor should check the gang angle adjustment, whether the front discs are notched, and how the gangs are mounted and greased before looking at width. TMG Industrial lists its compact harrows by the three point linkage category and horsepower range they suit, which is the first match to make.