Tractor Ballasting and Four-Furrow Plough Decision
A 75 kW engine tractor with 65% drawbar efficiency pulls a 3-furrow mouldboard plough with 0.35 m share width at 0.25 m depth and 6 km/h. The soil draft is 55 kN/m². Rear tyres are 18.4-38 at 1.8 bar, and wheel slip is 22% in the field; the tractor carries no extra ballast. The farmer wants to buy a 4-furrow plough of the same share size to reduce tillage time, and a neighbour advises increasing tyre pressure to 2.2 bar to ‘cut through the soil’. The tractor is also used for road transport 15 km to another field. Field capacity of the 3-furrow outfit is about 0.35 ha/h at 70% field efficiency. The farmer has 40 ha to plough before wheat sowing and 10 days available, working 8 h/day.
1. Calculate the draft and drawbar power required for the 3-furrow and 4-furrow ploughs at 6 km/h, and compare them with the tractor’s available drawbar power.
Read the worked answer
Speed = 6 km/h = 6,000 / 3,600 = 1.667 m/s.
3-furrow width = 3 × 0.35 = 1.05 m. Draft = width × depth × soil resistance = 1.05 × 0.25 × 55,000 = 14,438 N ≈ 14.4 kN. Drawbar power = 14.4 × 1.667 ≈ 24.1 kW.
4-furrow width = 4 × 0.35 = 1.40 m. Draft = 1.40 × 0.25 × 55,000 = 19,250 N ≈ 19.3 kN. Drawbar power = 19.3 × 1.667 ≈ 32.1 kW.
Available drawbar power = 75 kW × 0.65 = 48.8 kW.
Both implements are within engine power. The 4-furrow plough needs about 32 kW, leaving margin. The limiting factor is not engine power but traction: at 22% slip the tractor is already losing traction with the 3-furrow plough.
2. Diagnose the 22% wheel slip. Evaluate the neighbour’s tyre-pressure advice and recommend ballasting and tyre-pressure settings for field and road use.
Read the worked answer
A wheel tractor on agricultural soil should normally operate at about 8–12% slip for efficient drawbar work; 22% is excessive. It means the tyres are spinning faster than the forward speed, wasting fuel, increasing tyre wear, and polishing the soil surface. The cause is insufficient weight or too-high tyre pressure for the draft load, not a lack of engine power.
The neighbour’s advice is wrong. Raising tyre pressure from 1.8 to 2.2 bar reduces the tyre footprint, increases ground pressure, and usually increases slip and soil compaction. It does not improve traction in this situation.
For field work, reduce rear tyre pressure to roughly 0.8–1.0 bar if the tyre load allows it, and add ballast in steps while measuring slip. Add enough ballast to bring slip into the 10–15% range under the heaviest expected pull, without exceeding the tyre or axle load rating. For the 15 km road journey, reinflate to the road pressure specified for the axle load, because low pressure at road speed can damage the tyre. A quick check is to measure slip with and without ballast on the same strip and choose the lower-slip setting.
3. Decide whether buying the 4-furrow plough will meet the 40 ha in 10 days, and whether it is technically sound with the current tractor.
Read the worked answer
Current output: 0.35 ha/h × 8 h/day × 10 days = 28 ha. That is short of 40 ha by 12 ha.
A 4-furrow plough increases width from 1.05 to 1.40 m, a ratio of 1.33. If field efficiency and slip stayed the same, output would rise to about 0.35 × 1.33 = 0.47 ha/h, giving 0.47 × 80 = 37.6 ha in 10 days. That is still slightly short of 40 ha and assumes no extra slip.
With 22% slip, the 4-furrow plough will likely increase slip further because draft rises from 14.4 to 19.3 kN. The tractor may not achieve 0.47 ha/h without ballasting and pressure correction. Therefore, buying the plough alone is not a reliable fix. The better decision is to correct ballast and tyre pressure first, then either hire a contractor for the remaining area, work a second shift, or use a lighter secondary tillage pass on part of the field. Buy the 4-furrow plough only if ballasted field tests show slip stays below about 15% and the axle/tyre limits are respected.