Limiting the impacts of machinery crossings
Reducing soil compaction by decreasing soil stress through: reducing the weight of equipment, choosing tires, equipment (radial carcass, wide or high-volume tires, dual wheels), and adjusting inflation pressure.
A soil with good structure offers better rooting, promotes biological activity, and allows for better efficiency of mineral and organic fertilizers.
Impact of compaction
The pressure exerted by machinery traffic in agricultural fields causes compaction, which corresponds to a decrease in soil porosity:
- This compaction impacts the water and air infiltration capacity, the ease of root penetration into the soil, and the proper functioning of biological activity: moisture is uneven within the same field and pressure on seeding elements is sometimes too high.
- Experiments conducted by Arvalis over 5 years showed a significant correlation between the impact of compaction caused by successive machinery passes and the yield of various crops. Yield reductions ranged from 5 to 30% for all modalities, regardless of crop or system (dry or irrigated).
- This demonstrates that preventing soil compaction, especially during harvest, is a major issue for soil fertility.
Measuring compaction
The pressure applied to the soil, expressed in kPa (kilopascal), depends on the weight on each wheel and the contact area of the tire on the soil. It also varies according to the pressure of tire inflation (a less inflated tire has a larger contact area due to tire flattening, thus reducing soil stress):
- A light tractor (<150 HP) with narrow wheels exerts a soil stress of 50 kPa, compared to 40 kPa for a heavier tractor (>150 HP) with wide wheels. This shows how the tire contact area influences soil stress.
- A combine harvester with a full grain tank exerts a soil stress of 120 kPa, three times higher than that of a heavy tractor with wide tires.
The impact on the soil depends on this mechanical aspect (soil stress) and the soil characteristics (texture, moisture, mechanical resistance). Regarding moisture, field capacity defines the threshold at which machinery can pass through fields without causing excessive damage to soil structure. This threshold is approximately 22% moisture, beyond which the impact will be deep (>25 cm) and severe.
Surface compaction (<10 cm), intensified by successive passes, is more pronounced when the tire is narrow and the pressure applied is high:
- These do not increase deep compaction, which occurs from the first pass.
- Deep compaction does not increase with successive passes; it depends more on soil moisture and the total weight of the machine: the heavier the weight, the deeper the compaction.
Legislation, axle weight
To protect roads and structures, French legislation limits loads:
- Single axle: 13 tonnes
- Vehicle with 2 axles: 19 tonnes
- Vehicle with 3 axles: 26 tonnes
- Vehicle with 4 axles: 32 tonnes
- Articulated vehicle / vehicle combination with 4 axles: 38 tonnes
- Articulated vehicle / vehicle combination with 5 axles: 40/44 tonnes
According to Odette Ménard, Canadian soil agronomist, the maximum allowable load without soil damage is 7 tonnes per axle in wet conditions and 10 tonnes per axle in dry conditions (a loaded combine harvester can weigh up to 30 tonnes...).
Legislation does not limit the weight of agricultural vehicles in fields.
Limiting this impact
Control traffic in the field as much as possible:
- Keep trucks out of the field,
- Try to unload while stopped at the edge of the field rather than while moving,
- Do not turn in the middle of the field,
- Follow the same tracks as the combine harvester for the transfer vehicle,
- Establish a transport plan for the refueler/trailers and stick to it (in this context, CTF is interesting: see below).
Do not fill the grain tank to full (nor the refueler):
- Visually, one tends to want to pass only once to make fewer surface ruts. But in terms of impact on soil structure, it is wiser to make several passes with a lighter load to avoid deep compaction, which is much harder to recover from than surface compaction.
Use necessarily radial tires (which better distribute weight), wide and at low pressure, especially if conditions are wet:
- The use of diagonal tires is more problematic because they absorb soil stress less effectively.
- A radial tire has a wider and flatter footprint that absorbs more.
From 6 meters wide, CTF ("controlled traffic farming" = satellite guidance system) can be considered.
- Used on a large scale since the late 1990s in Australia, this technique allows limiting compaction caused by machinery traffic.
- The idea is to define permanent traffic lanes on fields using large working widths and narrow tires to minimize the area of machine traffic.
- CTF allows reducing soil compaction to only 15% of the surface, thus potentially little overall yield loss related to this issue.
Central tire inflation allows reducing soil pressure:
- This technology allows quickly adapting tire pressure according to the cultural operation to be performed, on road or in fields.
- However, this option remains expensive and should be used judiciously.
Use cover crops to maintain and improve structure:
- Surface compaction (<20 cm) can be corrected by superficial soil tillage, unlike deep compaction (>20-30 cm) which cannot be mechanically corrected.
- The presence of roots and the action of earthworms can recreate porosity, provided they are present long enough in the field. Moreover, porosity created by soil biology is the most resistant to pressure. Indeed, macropores created by tillage are poorly connected and resist poorly to stresses.
- Conversely, microporosity favored by conservation agriculture practices has better resilience and facilitates root exploration.
Application of the technique
Crop types
This technique is easily generalizable to all crops. Crops more affected by soil compaction are those with:
- Short vegetation cycle (e.g., spring crops rather than autumn crops)
- Taproot system rather than fibrous
- Large root size
Soil types
- Calcareous soils are less sensitive to compaction.
- Clay soils are less sensitive to compaction phenomena than loam or sandy soils due to structural activity linked to freeze/thaw and wetting/drying cycles.
Climatic conditions
Water status plays a key role: compaction phenomena are worsened by working in wet conditions.
Effects on cropping system sustainability
"Environmental" criteria
Effect on air quality: Reduction of GHG emissions.
Effect on water quality: Reduction of pesticides and turbidity.
Effect on fossil resource consumption: Reduction of fossil energy consumption.
"Agronomic" criteria
Limiting compacted zones and ruts has a positive impact on:
Productivity: better root colonization of the profile and optimal resource use (water and minerals) by the crop. Limiting irreversible deep compaction with major effects on rooting.
Soil fertility: maintaining a favorable structural state for mineralization.
Water stress: maintaining porosity favorable to water infiltration into the soil (limiting runoff losses) and better root colonization of the profile.
Functional Biodiversity: maintaining a structure favorable to exploration by earthworms which generally promote "soil life" (soil aeration…).
Physical soil properties: maintaining a favorable structure (structural porosity) for root development and water and gas exchanges, thus for soil warming.
"Economic" criteria
Mechanization costs: Investment in efficient but costly equipment (bins, etc.). Requires good pressure gauges and an air compressor.
Fuel consumption: Limiting compacted zones and ruts, thus reducing mechanical soil work (fewer passes, fewer energy-intensive operations, e.g., plowing, subsoiling).
"Social" criteria
Working time: Additional handling: tire pressure adjustment and/or mounting dual wheels in the field (which must be removed after work to comply with maximum authorized road width).
For further information
- Subsoiling - Understanding the physical behavior of soil, J. Labreuche (Arvalis), H. Boizard (INRA), Perspectives agricoles n°282, p28-31, Press article, 2002
- Study of the medium and long-term effect of cropping systems on the structure of a loam-clay soil in the northern Paris Basin, H. Boizard (INRA), General Soil Study n°11, p11-20, Peer-reviewed article, 2004
- Modeling compaction under the effect of agricultural machinery traffic, P. Défossez, General Soil Study n°11, p21-32, Peer-reviewed article, 2004
Sources
Limiting the impacts of machinery traffic, GECO
Limiting damage to soil structure, AgroLeague
