Tillage: primary, secondary, conservation and zero tillage
Why soil is tilled, the implements for primary and secondary tillage, conservation and zero tillage, effects on soil, and field capacity and draft sums.
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Tillage is the mechanical manipulation of soil to make it suitable for growing crops: loosening it, burying residues and weeds, and preparing a seedbed. For thousands of years it was the defining act of farming. Today, as the costs of erosion, fuel and lost organic matter have become clear, the question is not only how to till but how little tillage a crop really needs.
Tillage systems and implements are standard topics in agronomy and agricultural engineering, with calculations of field capacity and draft.
Why till?
- Seedbed preparation: a firm, fine, moist layer that gives seed good contact with soil, with looser soil around it for roots.
- Weed control: burying or cutting weeds before sowing and between rows.
- Residue management: burying or mixing crop residues and manure.
- Loosening compacted soil and improving infiltration in the short term.
- Incorporating fertilisers, lime and amendments.
- Special needs: puddling for transplanted rice, making ridges and beds, levelling.
The ideal soil condition for planting is good tilth: crumbly, well-aggregated soil with a mix of fine and coarse pores. Tillage is best done at the right moisture: soil should be friable, crumbling when pressed. Too wet, and tillage smears and compacts it; too dry, and it breaks into hard clods and needs much more power.
Primary and secondary tillage
Primary tillage is the first, deeper, rougher operation after harvest or on new land:
- Mouldboard plough: cuts and inverts a furrow slice, burying residues and weeds completely. Effective, but leaves the soil bare and exposed to erosion.
- Disc plough: concave discs cut and partly invert; suits hard, sticky, stony or root-filled soils.
- Chisel plough: rigid or spring tines loosen the soil without inverting it, leaving much residue on the surface.
- Subsoiler (ripper): a heavy tine pulled deep, below normal tillage depth, to break compacted layers and plough pans.
- Rotary tiller (rotavator): powered blades chop and mix soil and residue; can make a seedbed in one pass but pulverises structure if overused.
- Traditional animal-drawn ploughs and hand hoes remain the main tools on many smallholdings.
Secondary tillage follows, working shallower to break clods, level the surface, kill weed flushes and firm the seedbed:
- Disc harrows, tine (spring or rigid) harrows and cultivators.
- Rollers and packers to break clods and firm the soil.
- Levellers and planks, including laser land levelling, for even water distribution in irrigated fields.
- Inter-row cultivators and hoes for weeding standing crops.
Tillage systems
| System | What it involves | Surface residue after planting |
|---|---|---|
| Conventional (intensive) tillage | full-width inversion ploughing plus several secondary passes | usually less than 15 per cent |
| Reduced (minimum) tillage | fewer or shallower passes, often non-inverting | 15–30 per cent |
| Conservation tillage | any system leaving at least 30 per cent of the surface covered by residue | 30 per cent or more |
| Mulch tillage | full-width, non-inverting tillage leaving residue on the surface | 30 per cent or more |
| Strip tillage | only a narrow strip tilled where the row will be | high between strips |
| Ridge tillage | crops planted on permanent ridges, rebuilt each season | moderate to high |
| Zero (no) tillage | seed placed directly into undisturbed soil through residue | high |
Puddling, tilling flooded soil for transplanted rice, is a special case: it deliberately destroys structure to reduce percolation and control weeds.
Effects of tillage on soil
Short term, tillage can loosen soil, improve infiltration, warm and dry the seedbed, and control weeds. Repeated over years, intensive tillage tends to:
- Speed up organic matter decomposition, by breaking aggregates and aerating soil.
- Increase erosion, by leaving the soil bare and breaking surface aggregates; tillage itself also moves soil downslope (tillage erosion).
- Form plough pans, compacted layers just below working depth.
- Disrupt soil life, such as fungal networks and earthworm burrows.
- Use more fuel, time and labour.
Reduced and zero tillage reverse many of these, but bring their own challenges: weed control without burial, residue handling, cooler and wetter seedbeds in some climates, and the need for suitable seeders.
Calculations
Field capacity of a plough
A tractor pulls a plough with a working width of 1.2 m at 5 km/h. Field efficiency (allowing for turns, overlaps and stops) is 80 per cent.
- Theoretical field capacity = speed × width ÷ 10 = 5 × 1.2 ÷ 10 = 0.60 ha/h.
- Effective field capacity = 0.60 × 0.80 = 0.48 ha/h.
- Time to plough 10 ha = 10 ÷ 0.48 ≈ 20.8 hours.
(The factor 10 converts km/h × m to hectares per hour: 1 km/h × 1 m = 1,000 m2 per hour = 0.1 ha/h.)
Draft and drawbar power
The same plough works 20 cm deep. Assume a specific soil resistance (draft per unit area of furrow cross-section) of 50 kN/m2, a typical figure for a medium loam; heavy clays can be twice as high or more.
- Furrow cross-section = 1.2 m × 0.20 m = 0.24 m2.
- Draft = 50 kN/m2 × 0.24 m2 = 12 kN.
- Speed = 5 km/h = 5,000 ÷ 3,600 = 1.39 m/s.
- Drawbar power = draft × speed = 12 kN × 1.39 m/s ≈ 16.7 kW.
Because tractors lose power through wheel slip and rolling resistance, the engine power needed is considerably higher, often about one and a half to two times the drawbar power in tillage. Ploughing wet clay, ploughing deeper or driving faster all raise the power needed.
Choosing a tillage system
- Soil and climate: well-drained, erodible soils in dry or windy areas gain most from conservation and zero tillage; poorly drained, cold soils may need some tillage or raised beds.
- Crop: small seeds need a fine seedbed; robust seeds can be direct-seeded.
- Weeds and residue: heavy residue needs seeders that cut through it; weed pressure shapes the choice.
- Machinery and cost: fuel, labour and timeliness often decide in practice.
Key points
- Tillage prepares the seedbed, controls weeds, manages residues and loosens soil; it is best done when soil is friable.
- Primary tillage (ploughs, chisels, subsoilers, rotary tillers) is deep and rough; secondary tillage (harrows, cultivators, rollers) refines the surface.
- Conservation tillage leaves at least 30 per cent residue cover; zero tillage sows directly into undisturbed soil.
- Intensive tillage speeds organic matter loss, erosion and plough-pan formation.
- Field capacity (ha/h) = speed (km/h) × width (m) ÷ 10 × field efficiency.
- Draft = specific resistance × furrow area; drawbar power = draft × speed.
What the exams ask
- School-leaving and entrance papers: objectives of tillage, primary versus secondary tillage, and examples of implements.
- Degree and research-fellowship entrances: tillage systems and residue thresholds, effects on soil properties, puddling, zero tillage and its constraints.
- Agricultural engineering examinations: field capacity, efficiency, draft and power calculations, implement selection and matching implements to tractors.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Is ploughing needed every year?
Rarely. Many crops grow well with reduced or no tillage once the system is managed for weeds and residues. Occasional deep loosening may still be needed to correct compaction.
Why does a rotary tiller leave the soil so fine, and is that good?
Its powered blades shatter clods into fine particles. A fine seedbed helps small seeds, but repeated rotary tillage destroys aggregates, encourages crusting and speeds organic matter loss.
What is a plough pan?
A compacted layer just below the depth of repeated tillage, caused by the pressure of implements and wheels. It restricts roots and drainage and can be broken with a subsoiler, followed by changes that prevent it re-forming.
Sources
- Lal, R. "Tillage effects on soil degradation, soil resilience, soil quality, and sustainability." Soil and Tillage Research 27, 1–8 (1993).
- Hunt, D. and Wilson, D. Farm Power and Machinery Management, 11th edition. Waveland Press, 2016.
- ASABE. Agricultural Machinery Management Data (ASAE D497) and Agricultural Machinery Management (EP496). American Society of Agricultural and Biological Engineers.
- Brady, N. C. and Weil, R. R. The Nature and Properties of Soils, 15th edition. Pearson, 2017. Chapter 4.
- FAO. Conservation Agriculture. Food and Agriculture Organization of the United Nations, online resources.
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