Tomatoes and soil compaction?

Anne Verhallen, Soil Management Specialist – Horticulture, OMAFRA, Ridgetown

Yes, sands get compacted…   Tips for managing soil compaction…   Rules for subsoiling…

Tomatoes and soil compaction – yep it is really out there. Not a big surprise after the wet weather and soggy soil conditions of 2011. A dry year like 2012 shows all the weak spots in fields. However, we have been seeing a lot of soil compaction in tomato fields and getting reports of restricted root systems.

Tomato soils are perfectly situated for compaction. Generally tomatoes are grown on the lighter soils – sandy loams, loams and possibly the lighter end of the clay loams in some cases. All soils in Ontario are a mixture of soil particle sizes – but loams and sandy loams probably have the largest range of particle size. This is important because it helps to explain why they compact so beautifully. Consider how we make roads – a mixture of sand and gravel for the base, add some water to allow the sand and gravel to slide and pack tighter then finish with some vibrating heavy equipment. Sounds a lot like most mechanized harvest operations.

There is a cost that comes with compaction – it leaves a field much more open to any other stress, whether it is lack of rainfall or disease. Commonly research on the impact of compaction in field crops has suggested yield reductions of 0 to 75%. Weather, soil type, rotation and other management factors of course influence the long-term impact of compaction. A survey of the research on tomato response to soil compaction suggests something quite similar – yield reductions of 0 to 80%. A study from Australia reports that tomatoes are fairly tolerant of mild compaction. Tomatoes do generally have good root systems if weather conditions and disease allow. However the same study reports that under severe compaction even accurate management of drip irrigation can not completely compensate for the stress caused by compaction. This suggests that a restricted root system cannot always be overcome by managing the delivery of water and nutrients to the plant.

Managing Compacted Soils

Compaction generally does not happen overnight, it builds up over time. In turn it takes time to fix the problem.

• Take a hard look at the crop rotation. Wider crop rotations that include a variety of plant types, particularly deep rooted forages and longer term grasses like wheat can help build better soil structure.
• Tillage and tires – reduce the number of passes over the field, reduce the weight of the equipment per tire and reduce tire pressures where possible. Tomato fields see a lot of traffic from transplanting to harvest.
• Avoid parking trailers in the field – or set a “landing area” for them that does not encroach into the field. It is always amazing the compaction that we see where trailers are typically parked.
• Cover crop aggressively. Cover crops can help to improve the crop rotation and improve soil structure over time. Use cover crops to create a shock absorber on the head lands.
• Use deep tillage as a prescription tool for the worst areas of compaction – see the rules below.

If you must subsoil, ALWAYS follow these rules:

1. Check the depth of the compacted layers.
2. Make sure the implement is operating just below the bottom of the compacted layer. Running too deep will just push more soil up against the bottom of the compacted layer rather than shattering the compaction.
3. Make sure the soil is dry enough at depth that it will actually shatter. Smearing a trench in the soil will not improve the situation.
4. Plant a cover crop or rotational crop that will stabilize the fractures created and stay off the field while this happens.
5. Change your practices to reduce the potential to re-establish the compacted layer

Keep in mind that Ontario research has not shown long-term benefits from deep tillage. The improvements in drainage are usually only present for 1 year.