Rocky farmland is not a niche Korean agricultural problem — it is the dominant land management challenge across the country’s highland agricultural zones. Gangwon-do’s potato and vegetable belt sits on granite-derived soils that annually push new surface stones through frost-heave. North and South Gyeongsang’s apple and persimmon orchards occupy the slopes and valleys of the Taebaek and Sobaek mountain ranges, where granite outcrops and embedded boulders are a constant feature. Jeju Island’s entire agricultural surface is volcanic basalt — one of the hardest and most abrasive rock types in Korean agriculture.
For generations, Korean highland farmers managed these rocks with hand labour: picking by team, stacking windrows at field margins, and breaking large stones manually. As seasonal agricultural labour has become scarcer and more expensive throughout the 2010s and 2020s, mechanized stone clearance has moved from a premium option to a commercial necessity. This guide describes the five-step mechanical sequence that Korean highland operators use — from the initial surface rake through to final seedbed preparation — with the specific machines, tractor requirements, and field conditions for each step.
Why Rocky Farmland Is an Annual Problem, Not a One-Time Event
Korean highland farmers frequently ask why stone clearance must be repeated every spring, even on fields that were thoroughly cleared the previous season. The answer is frost-heave — a soil physics process driven by the freeze-thaw cycle of Korean highland winters.
When soil moisture freezes in winter, it expands volumetrically. Stones — which have lower thermal conductivity than surrounding soil — act as thermal barriers that prevent freezing uniformly around them. The differential expansion pushes the stone progressively upward over multiple freeze-thaw cycles. By spring, stones that were 15–20 cm below the surface in autumn may have migrated to within 3–5 cm of the surface or broken through entirely. In Gangwon-do highland zones at 500–800 m altitude, where winter temperatures regularly reach -15°C to -20°C and freeze-thaw cycles occur 30–50 times per season, annual stone surface emergence is a predictable, consistent agricultural management requirement — not an exceptional event.
On Jeju Island, the mechanism is somewhat different. Jeju basalt underlies a shallow soil profile in most agricultural zones. Agricultural activity — tillage, vehicle traffic, root penetration — progressively disturbs and breaks down the interface between the basalt substrate and the soil layer, creating ongoing surface accumulation of angular basalt fragments. Jeju farmers typically clear stone twice per season: once before spring planting and once after autumn harvest before winter cover.
The 5-Step Mechanical Clearing Sequence
The optimal sequence depends on the stone density and size distribution in your specific field. Below is the complete sequence — which steps are required depends on your starting conditions.
Surface Stone Raking — EP-EW-4000 Rock Rake
When to use: Fields with light to moderate surface stone where the largest individual stones are below approximately 30 Kg and there are no large embedded boulders. Annual frost-heave maintenance clearance on established production fields.
The EP-EW-4000 rock rake (75 HP minimum, 3.6 m working width, Cat. 2 hitch, 540 RPM PTO) sweeps the field surface at 3–6 km/h, gathering stones into neat windrows without crushing or collecting them. The spring-tooth design separates stones from the fine soil fraction with minimal soil disturbance — important for fields already in production where the seedbed structure from the previous season is worth preserving. At 75 HP minimum, the rock rake runs on the same tractor that will subsequently operate the potato furrower, planter, and mid-season cultivator — no dedicated stone clearance tractor is needed.
The windrows produced by the rock rake are then collected either by the CT-2100 rock picker (Step 3) or by a front-end loader and truck for fields where the volume warrants mechanical collection. On very lightly stoned fields, windrows can be pushed to the field margin manually or with a tractor-mounted blade, eliminating the need for a separate collection pass altogether.
Rock Crushing — THOR 2.4 or THOR 3.0 Stone Crusher
When to use: Fields with medium to heavy stone density including embedded boulders above 30 Kg, or new field development from rough mountain land. Also required before Step 3 when large stones exceed the rock picker’s 80 Kg weight limit.
The THOR 2.4 stone crusher (180 HP minimum, 2.4 m, 90+6 carbide teeth, 550 mm rotor) processes all embedded and surface stones up to 30 cm diameter in a single pass, simultaneously mulching brush and vegetation. The crushed output — uniformly sized aggregate from fine gravel to approximately 5–10 cm — falls back onto the field surface through the adjustable hydraulic output grid. For road construction applications, this aggregate is the road base material. For crop preparation applications requiring complete stone removal, this crushing pass enables Step 3 (rock picking) to proceed efficiently without the picker encountering stones above its handling limit.
On new land being converted from scrubland or former mountain forest, the THOR 2.4’s simultaneous crushing-and-mulching capability is particularly valuable: one pass processes the surface rock and incorporates the woody vegetation material. This replaces what would otherwise require separate root clearing, stone removal, and vegetation management operations.
For slopes above 20% — the common condition on Korean mountain farm access roads and orchard preparation sites — the Kit Drawbar pull-mode redistributes the machine’s 2,300 Kg weight from behind the rear axle (where it lifts the front wheels) to in front of the rear axle (where it stabilizes both axles on the slope). Korean mountain orchard operators in Cheongdo-gun, Yeongcheon, and the hillside farms of South Gyeongsang consistently cite this feature as the primary reason they selected the THOR 2.4 over competing products.
Stone Collection and Removal — CT-2100 Rock Picker
When to use: Any application where zero residual stone is required after clearance — ginseng, seed potato, certified potato seed crop, premium vegetable production, orchard drip irrigation installation. Follows Step 2 on heavy-stone fields; replaces Step 2 on lighter-stone fields.
The CT-2100 rock picker (110 HP minimum, 1.95 m working width, 2.5 m³ hydraulic-tip bunker) physically lifts stones from 5 cm diameter up to 80 Kg weight and loads them into the bunker. When the bunker is full — typically 4–5 tonnes of collected stone — it tips hydraulically to discharge into a waiting truck. The full-width rubber shield prevents stone projection, making the machine safe to operate near roads, planted crops, and greenhouse structures.
In the ginseng zones of South Chungcheong (Geumsan-gun, Yesan-gun, and surrounding municipalities), where ginseng root grading at harvest is directly affected by any stone contact during the 6-year growing cycle, the THOR 2.4 + CT-2100 two-machine sequence has become the professional standard for seedbed preparation. Ginseng farm operators who adopted this sequence report significant reductions in grade-loss rates at inspection — the combination reliably produces the stone-free standard that ginseng production demands.
Primary Tillage — PSW-3200 Rotavator
When to use: After stone clearance on any new or previously uncultivated land, and as annual spring primary tillage on established production fields before furrowing and planting.
After stone clearance, primary tillage creates the loose, aerated soil profile that planting machinery requires. The PSW-3200 Rotavator (140 HP minimum, 3.0–3.6 m adjustable working width) operates at 3–5 km/h with 540 or 1000 RPM PTO, incorporating crop residue and creating a uniform seedbed depth across the cleared field. At 1000 RPM PTO setting, the higher blade tip speed produces finer particle size suited to potato ridge formation and vegetable bed preparation. At 540 RPM, coarser incorporation is achieved at lower fuel cost — appropriate for initial tillage passes on rough new land.
The PSW-3200 B variant includes an integrated 2,000 Kg fertilizer bunker, combining primary tillage with base fertilizer incorporation in one pass. For highland potato production specifically, this reduces the total number of pre-planting field operations and the associated tractor traffic on the newly cleared and tilled soil — an important soil structure consideration on the fragile highland soils of Gangwon-do.
Planting or Infrastructure Installation
When complete: After Steps 1–4 in the appropriate combination, the cleared and tilled field is ready for crop planting, irrigation installation, orchard infrastructure, or road surface compaction, depending on the land use objective.
Step 5 is the land use outcome — not a specific Watanabe machine, but the agricultural operation that the clearing work was performed to enable. Different objectives require different specific preparations:
Potato and vegetable planting: After PSW-3200 rotavator tillage, the standard Watanabe potato sequence follows — furrower (EP-R-380/R-580), fertilizer applicator (EP-ADB-380/ADB-480), and planter (EP-PAI-2100 or EP-PANTHER). The stone clearance work at Steps 1–3 directly determines the quality of the seedbed into which the potato system plants — a stone-free, uniform soil profile is the foundation of efficient mechanical planting and clean mechanical harvest.
Orchard establishment: After THOR crushing and CT-2100 picking (Steps 2–3), drip irrigation line installation can proceed across the cleared and leveled orchard floor. Sapling planting follows the irrigation layout. Orchard operators in Cheongdo-gun consistently report that the post-THOR field condition — with crushed stone aggregate remaining but large boulder and embedded rock removed — is suitable for drip irrigation installation without further picking on moderate-stone fields.
Farm road construction: For road construction applications, Step 2 (THOR crushing) is the only required step. The crushed aggregate produced by the THOR is the road base — graded, compacted, and left in place. Steps 1 and 3 are not required, and Step 4 is not relevant for road surface construction. This is the most common standalone THOR application: one pass converts rocky mountain track into a stable all-weather agricultural road surface.
Which Steps Apply to Your Situation?
| Situation | Step 1 Rake |
Step 2 Crush |
Step 3 Pick |
Step 4 Till |
|---|---|---|---|---|
| Farm road construction (mountain) | Optional | Required | No | No |
| Annual potato field clearance (light stone) | Required | Optional | Required | Required |
| Ginseng seedbed (heavy stone) | Optional | Required | Required | Required |
| New orchard land preparation | Optional | Required | If needed | Optional |
| Annual Jeju basalt surface clearance | If light | Usually | Required | Required |
The Cost of Not Clearing — What Rocky Seedbeds Actually Cost
Korean highland farmers sometimes delay or reduce stone clearance to reduce spring operating costs — a decision that creates larger costs later in the season. The downstream costs of insufficient stone clearance on Korean agricultural soils fall into three categories:
Harvester damage. Potato, carrot, and onion mechanical harvesters in Korea routinely encounter stone-induced mechanical damage when operating on insufficiently cleared fields. Harvester share and web damage from stone contact results in repair costs that can exceed the seasonal cost of thorough stone clearance on a per-hectare basis. Harvester downtime during the time-critical harvest window compounds this cost significantly.
Crop quality losses. In ginseng production, stone contact during the 6-year growing cycle causes root bifurcation, bruising, and deformity that reduces harvest grade. Premium ginseng (1, 2, 3급) sells at multiples of standard grade; a crop with high stone-contact deformity falls disproportionately into lower grades, representing a revenue loss per kilogram that can be 3–5 times the cost of adequate stone clearance at planting. The same grade-loss mechanism applies to carrot and potato crops sold under fresh market quality specifications.
Irrigation system damage. Orchards and vegetable operations using drip irrigation systems suffer from stone puncture of drip lines and damage to emitters when stones in the soil profile shift under freeze-thaw cycles. Drip line replacement costs per kilometer in Korean orchard conditions are significant — annual stone management that stabilizes the upper soil profile reduces this cost.
Selecting the Right Machine Scale for Your Annual Clearing Area
The five-step sequence is the same regardless of farm size, but the specific machines within each step scale with the annual clearing area. Korean highland operations use three scale configurations:
Small Scale — Up to 15 Hectares Per Year
Step 1: EP-EW-4000 Rock Rake (75 HP, 3.6 m). Step 2 if required: THOR 2.4 (180 HP) — also handles brush mulching and road construction work simultaneously. Step 3: CT-2100 Rock Picker (110 HP, 2.5 m³ bunker). Step 4: PSW-3200 Standard (140 HP). At 15 ha/year on moderate stone density highland fields, the three-machine Steps 1–3–4 sequence (without Step 2 THOR crushing on lighter-stone years) covers the annual clearance requirement within a reasonable spring preparation window. The EP-EW-4000 and CT-2100 both run at 75–110 HP and may share a single medium-sized tractor; the PSW-3200 requires a separate 140 HP machine.
Medium Scale — 15 to 50 Hectares Per Year
Step 1: EP-EW-4000 or THOR 2.4 depending on stone density. Step 2: THOR 2.4 (180 HP) — required on most highland granite fields at this scale. Step 3: CT-2100 Rock Picker. Step 4: PSW-3200 A (140 HP, extended frame for stability at wider width settings). At 50 ha/year, the spring clearing window is tight relative to the planting calendar — the THOR 2.4’s 0.8–1.4 ha/h effective coverage rate in typical highland conditions covers 50 hectares in approximately 35–60 productive machine-hours, allowing the preparation to complete within a reasonable pre-planting period even with weather delays.
Large Scale — Above 50 Hectares Per Year
Step 1: BlackBird Rock Rake (9.5 m, 300 HP) for maximum pre-raking throughput on lighter stone areas. Step 2: THOR 3.0 (230 HP, 3.0 m) for the heaviest stone areas — with possible BlackBird + THOR 3.0 combined coupling on the largest open sections. Step 3: CT-2100. Step 4: PSW-3200 B (140 HP, with integrated 2,000 Kg fertilizer bunker to reduce total field passes). At 100+ hectares per season, parallel tractor operation — one tractor on Steps 1–2, another on Steps 3–4 on sections already crushed — maximizes daily throughput within the narrow spring preparation window before the highland planting calendar opens.
Seasonal Timing — When to Run Each Step in Gangwon-do
In Gangwon-do’s primary highland potato and vegetable zones (Pyeongchang-gun, Hoengseong-gun, Inje-gun at 400–800 m altitude), the spring field preparation calendar is compressed into a short window between the thawing of winter frost and the planting deadline. A typical Gangwon-do highland spring preparation timeline:
March: Soil thaw begins at lower altitudes (below 500 m). Step 1 rock rake operations begin on early-thawing fields. Surface stones from winter frost-heave are visible and ready for clearance. Snow cover on upper sections (above 600 m) may still restrict access until late March.
April (early–mid): All altitude zones accessible. Steps 1–2 (rock rake and crushing) proceed simultaneously across the full field area. The CT-2100 rock picker (Step 3) follows the THOR crushing pass on sections where complete stone removal is required — ginseng plots, potato seedbeds, and high-value vegetable areas. PSW-3200 rotavator primary tillage (Step 4) follows the completed stone clearance sections, working ahead to prepare seedbeds before the planting window opens.
Mid-April to early May: Planting window. All five steps should be complete before this period — the preparation sequence must keep pace with the planting calendar. Operations that fall behind in spring stone clearance compress the planting window and risk late-season yield penalties in Gangwon-do’s short growing season.
The practical implication: a 50-hectare highland potato operation should plan for 3–4 weeks of spring stone clearance and seedbed preparation before the first planting date. Machine availability, particularly tractor time on the critical 140–180 HP machines running Steps 2 and 4, is the operational constraint that determines whether this schedule is achievable. Machine booking or rental confirmation for the clearing season should be arranged in February at the latest for spring-season operations.
Perguntas frequentes
Can I skip Step 2 (rock crushing) and go directly to Step 3 (rock picking) on a heavily stoned field?
Only if all individual stones on the field are below 80 Kg in weight. On Korean highland granite fields with embedded boulders — which commonly exceed 100–300 Kg in South Chungcheong ginseng zones and Gangwon-do highland farms — the rock picker cannot handle these stones safely. Attempting to pick them causes tine deflection and in severe cases, tine damage. The THOR stone crusher must reduce these large stones to below the 80 Kg picking limit before the CT-2100 rock picker can process the field. On fields with uniformly small annual frost-heave stones, Step 2 can be omitted and Step 3 follows Step 1 directly.
How many passes of Step 2 (THOR crushing) are needed for very large boulder fields?
The THOR 2.4 processes stones up to 30 cm in a single pass. Stones above 30 cm may require two passes to reduce to the target fragment size — the first pass impacts the large stone and partially fractures it; the second pass completes the reduction. Very large embedded boulders (above 60–80 cm) should ideally be pre-broken by excavator impact or hydraulic breaker before the THOR pass to avoid excessive stress on the rotor in the first impact. On typical Korean highland granite exposure sites, single-pass processing is effective for the majority of stone population; large boulder pre-treatment adds roughly 5–15% to the total clearance time on the most severely stoned sections.
How long does the complete 5-step sequence take per hectare?
Practical time estimates per hectare for a 3-machine highland operation on medium-density granite fields: Step 1 rock rake (1.0–1.5 ha/h at 4–6 km/h): 0.7–1.0 hours; Step 2 THOR crusher (0.8–1.2 ha/h): 0.8–1.2 hours; Step 3 CT-2100 picker (0.8–1.0 ha/h): 1.0–1.2 hours; Step 4 PSW-3200 rotavator (2–3 ha/h): 0.3–0.5 hours. Total Steps 1–4: approximately 3–4 machine-hours per hectare. On heavy-stone new land, allow 4–6 hours per hectare. Contrast with manual stone picking that takes 15–30 person-hours per hectare on comparable stone density fields in Korean highland conditions.
Do all five machines in this sequence need to be the same brand?
No. Each step uses independently operating machines connected only through the logical workflow — they do not need to be mechanically compatible with each other beyond the standard tractor PTO and hitch connections that all Korean and international tractor brands provide. Steps 1, 2, and 3 are all separate passes on separate days if necessary. The Watanabe machines cover Steps 1 (EP-EW-4000 rock rake), 2 (THOR crusher), 3 (CT-2100 picker), and 4 (PSW-3200 rotavator). If you already own a non-Watanabe rotavator, you can use it for Step 4. The critical factor at each step is confirming the machine specification matches your tractor’s HP and hydraulic valve count — not the brand name.
Tell Us Your Field — We Recommend the Right Step Sequence
Crop type + stone density (light / medium / heavy) + largest stone size + slope conditions + tractor HP → specific machine recommendations for Steps 1–4 with the complete sequence logic explained. All machines in Korea local stock, Ansan-si, Gyeonggi-do.
Editor: Cxm
