Tractor and Moldboard Plow Matching for 50 ha of Sugarcane Within a 20-Day Window
A 50 ha sugarcane farm in Negros Occidental plans primary tillage with a 3-bottom moldboard plow, each bottom 0.35 m wide, working 0.20 m deep at 5 km/h. Soil resistance is estimated at 50 kPa. A dealer offers a 30 kW (40 hp) two-wheel-drive tractor with 13.6-28 rear tires, ballasted only with water. Field efficiency for the operation is 0.80 and the tillage window is 20 days at 8 working hours per day. There is no spare unit, and neighboring farms plant on the same schedule so no contractor is available. On the first day the operator reports the engine pulling down to 1,600 rpm, wheel slip of 25% measured by counting rear revolutions, and the plow riding out of the ground in the hardpan strips. The farm wants to know whether to sign for this tractor or step up to a larger unit.
1. Compute the draft, drawbar power and required engine power for the plow. Is the 30 kW tractor adequate? Show the efficiency chain you assume.
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Draft. Total cutting width = 3 × 0.35 = 1.05 m; depth 0.20 m, so the soil area loaded = 1.05 × 0.20 = 0.21 m². At a specific resistance of 50 kPa, draft F = 50,000 Pa × 0.21 m² = 10,500 N = 10.5 kN.
Drawbar power. 5 km/h = 1.389 m/s. P_db = 10.5 kN × 1.389 m/s = 14.6 kW.
Efficiency chain. Transmission and final drive about 0.92; tractive efficiency at 10–15% slip about 0.70 on a firm clay loam; combined overall ≈ 0.64. Therefore engine power required = 14.6 / 0.64 = 22.8 kW.
Reserve. A 20% reserve for hardpan strips, moisture variation and normal engine derating gives 27.4 kW. The 30 kW (40 hp) unit is therefore adequate on paper but with under 10% margin, which is too thin for a hardpan field where the resistance can double in strips.
Judgment. Sized correctly for an average field, undersized for this one. Either step up to 45–50 kW with a 4-bottom plow (draft would rise to about 14 kN and power to about 30 kW, still within a 45 kW engine with reserve) or keep the 30 kW tractor and accept a shallower first pass. Do not simply add ballast to the 30 kW unit - it cannot convert weight into power it does not have.
2. Compute the effective field capacity and the hours needed. Then recompute for the slip actually observed. What does this mean for the 20-day window, and what decision do you recommend?
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Theoretical capacity. Width × speed / 10 = 1.05 × 5 / 10 = 0.525 ha/h.
Effective capacity. 0.525 × 0.80 field efficiency = 0.42 ha/h. Time for 50 ha = 50 / 0.42 = 119 h.
Window. 20 days × 8 h = 160 h. The plan uses 119/160 = 74% of the window, leaving only 41 h (about 5 days) for rain, breakdowns, operator rest and turning losses on wet spots. That is thin but workable.
With the observed 25% slip. Effective travel speed falls to 5 × (1 − 0.25) = 3.75 km/h. Capacity = 1.05 × 3.75 / 10 × 0.80 = 0.315 ha/h. Time = 50 / 0.315 = 159 h - essentially the entire 20-day window.
What that means. There is now zero contingency. A single day of rain or one hydraulic failure means the last fields are planted late, and late planting in sugarcane costs tonnage far more than the price difference between a 30 kW and a 45 kW tractor. The engine also cannot hold rated rpm at 25% slip, so the real capacity is probably worse than 0.315 ha/h.
Recommendation. Do not commit to the 30 kW unit for the full 50 ha on this schedule. First quantify the hardpan (penetrometer transect) and run a chisel/subsoiler pass where it is confirmed - that removes the cause of the high draft. Then either take the 45–50 kW tractor with a 4-bottom plow, or sign the 30 kW unit only for the light fields and reduce depth to 0.15 m on the first pass, which cuts draft proportionally to about 7.9 kN (about 11 kW drawbar) at the cost of a second pass.
3. The plow rides out of the ground in hardpan strips and slip is 25%. Diagnose the causes and give the correction steps, distinguishing what ballast can fix from what it cannot.
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What is happening. The plow rides out because the hardpan resists penetration more than the tractor's weight can hold the shares down - the draft control or depth wheel is being overpowered. Slip at 25% is above the 10–15% target for 2WD tillage and confirms the tractor is traction-limited, not power-limited. Excess slip wastes fuel, polishes tires and lowers capacity exactly as computed earlier.
Causes to check. (1) Insufficient ballast - water-only ballast in the rear tires is usually well below the manufacturer's maximum for tillage; add cast or wheel weights to bring slip into the 10–15% band. (2) Tire inflation set for road transport rather than field work - lower to the field pressure recommended for the load, within the tire's limits. (3) Depth control set to position rather than draft/load control on a hardpan. (4) Shares blunt or the plow out of line, which raises draft. (5) The hardpan itself.
Correction sequence. Calibrate the slip measurement (count revolutions over a measured distance with a loaded implement, not free rolling); correct inflation; add ballast in steps and re-measure slip; set draft control; reduce depth to 0.15 m and check whether the shares stay in the ground; sharpen or replace shares. Then subsoil the hardpan strips with a chisel plow before moldboard work.
What ballast cannot fix. It will not make a 30 kW engine pull a plow that needs 30 kW of drawbar at depth in hardpan, and it will not break a compacted layer. Adding ballast beyond the manufacturer's limit simply overloads the tires and driveline. If slip remains high after correct ballasting and inflation, the machine is undersized for the job.