Compaction in vineyards is a common problem, frequently caused by the regular passage of heavy machinery. Compacted soils perform poorly, waterlog easily, and do not support healthy microbial populations.
The best way to avoid compacted soils is to develop a healthy soil biology, and use plants with deep root systems, and encourage earthworms. In cases where the tractor wheelings are especially compacted, a subsoiler that does not disturb the topsoil is a lot less destructive than a plough.
Compacted soils have bulk densities greater than about 1.1 to 1.6 g/cm3 (clayey to sandy soils, respectively), few pores (i.e. low total porosity), a high level of discontinuity between pores that restrict aeration, and a small average pore size that obstructs root elongation and drainage. Compacted soil surfaces impair water infiltration and at the same time, accelerate surface runoff and associated topsoil erosion. Soil compaction under the surface diminishes soil water holding capacity, inhibits soil microbe populations and their activities, limits mineral nutrient movement towards roots, heightens the risk of stresses in vines, reduces vine and cover crop growth vigour, and decreases grape yields. Obviously, soil compaction is costly.
A soil pore must be the same size as a root cap or larger for a root to extend through it unencumbered. Compacted soils have a limited number of such pores. To extend into smaller pores, roots must displace soil particles. If the resistance of soil in front of an advancing root tip is stronger than this threshold pressure, the root must change course or cease growing. Actually, in highly compacted soils, roots are largely limited to cracks and cleavage planes.
Plant growth is restricted when soils have an air-filled porosity of less than about 10% of the total porosity at field capacity. Under these conditions, oxygen diffusion towards roots is limited, and harmful carbon dioxide and ethylene can build up. Such an environment increases vine susceptibility to root diseases and may be a contributing factor to the malady currently called sudden vine collapse.
Of the negative effects listed above, those most telling for vineyard managers monitoring their vineyards for soil compaction are the visual cues of low water infiltration, restricted cover crop and vine growth, and collapsed vines. However, visual cues can be unreliable. The degree of soil compaction is best assessed using a penetrometer.
Compaction in vineyards can be caused by a range of factors, such as unhealthy soils, shallow soils, wet or waterlogged soils, hardpans, claypans and vineyard traffic. Tillage implements can further deform and restructure soils through cutting, lifting, displacing, and mixing. For very sandy soils, machine vibration is also an effective compressive force. Soil compaction often reveals itself over months or years and it slowly robs grapevines of growth and production capacity.
Minimising compaction in the first place can be achieved by:
- Growing a suitable cover crop and managing it to maximise root growth and humus build-up in the soil, and correspondingly its resistance to compaction
- Using equipment with even weight distribution, the lightest footprint befitting the task, tyre air pressure adjusted to the lowest feasible level
- Only entering the vineyard with equipment when the soil is sufficiently dry (i.e. well below field capacity in the surface soil)
- Limiting the number of equipment passes through a vineyard to only those that are essential. The latter applies especially to tillage passes