Compaction in Forage Fields

Frequent rains during haying forced many producers to harvest on wet fields. With little inventory to carry over from 2016, there was added pressure to put up forages. Rutted fields are the most visible damage from this year's harvest, but soil compaction can have more severe and lasting consequences.

Compaction is recognized as a concern in row crops, yet the percentage of a row crop field that sees wheel traffic is less than that of a hay field during a given year1. There are a few reasons for this:

  • Row crops are only harvested once, while forages may be cut 2-4 times per year;
  • Forages are more likely to receive multiple manure applications per year; and
  • In Ontario, it is more common to find controlled traffic systems in row crop fields rather than perennial forages

Compaction in forage fields is likely to take the form of ruts, topsoil compaction, or subsoil compaction. Producers can decide their own level of comfort with ruts and surface compaction, so this article will focus mostly on subsoil compaction. See fieldcropnews.com/2017/05/soil-compaction-stay-off-the-field-until-the-soil-is-ready for an overview of the causes of compaction and a discussion of variables such as soil moisture, tires and pressure, and axle loads.

Effects of compaction on forages

The effects of soil compaction often go unnoticed in forage fields, but compaction can have a huge impact on yield. Research has shown that compaction causes between 6 and 74% yield loss in perennial forage stands2. Heavy equipment, high tire pressures, and multiple passes create more severe compaction. One study saw grass yield losses increase between 5 and 15% as the pressure exerted by the tractor on the ground rose from 10 to 30 psi3. The study also showed a 33% reduction in yield when comparing a 3-pass system to a single pass.

Yield impacts from compaction are primarily the result of decreased pore space within and between soil aggregates or clods. This reduces water infiltration and drainage, aeration, root growth, and nutrient availability and uptake. Low aeration can decrease nitrogen availability because of denitrification, and reduced root respiration can restrict potassium uptake. Phosphorous uptake can also be reduced by up to 40% if rooting is significantly impeded4.

Identifying compaction

The first step in addressing compaction is to confirm the existence, location, and severity of the problem. Plant growth and water infiltration problems can be clues, but the most reliable way to identify compaction is by looking at the soil profile in a pit or trench. A trench can be dug across a suspected compaction zone, such as a wheel track. Alternatively, a pit in the field can be compared to a pit in a non-compacted area, like a fencerow. Take undisturbed shovelfuls of soil from the side of the pit at different depths and break them apart by hand to assess root development and the presence of macro-pores. A lack of macro-pores is an indication of compaction.

Alleviating Compaction

Where compaction has been identified, a choice must be made between relying on root activity and shrink-swell processes (freeze/thaw or wet/dry cycles), or attempting tillage to fix it. This decision will depend on the severity of the compaction problem and the crop rotation plan. Weigh tillage decisions against the economics of reduced yield from compaction: if the cost to purchase feed to replace lost yield is less than the cost of tillage, addressing compaction might wait until yields decline further, or the crop rotation provides a better opportunity to address the problem.

Thick tap-rooted plants, such as forage radish, are used in row crop systems to break up soil compaction. If the hay field is ready to be re-seeded or rotated into a different crop, this could be a valid option.

Subsoiling or deep ripping is another option, but results are highly variable and there is significant risk of making the problem worse if it is done improperly. The goal of deep-ripping is to create or reopen cracks through the compacted zone between soil aggregates (not the same as tine leg slots) to restore rooting and drainage, with minimum disturbance to the remainder of the soil profile and maintaining its bearing capacity. It is important to consider working depth, shank geometry, tine geometry, wing lift height, and critical depth before attempting to deep rip. If you are considering subsoiling, have a look at this comprehensive guide to successful subsoiling developed in Quebec by CETAB.

Note that the soil improvements of deep ripping are usually limited to 1-6 years following the intervention, due to recompaction. Ripping is a remedial action, and where compaction is concerned, prevention is better than cure. Compaction can be prevented by not driving on wet soils, reducing axle loads, lowering tire pressure, and minimizing the number of passes over a field.

Summary

  • Soil compaction can cause major yield losses in forages
  • Properly identifying compaction is crucial to deciding what to do about it
  • Weigh tillage decisions against the economics of reduced yield from compaction
  • Biological alleviation requires thick, strong taproots of plants such as forage radish, which can be included as a cover crop in rotation after cereals
  • Subsoiling can be used to address compaction problems, but must be done correctly to avoid worse problems down the line

References

1Soane, B.D. & van Ouwerkirk, C. (eds) 1994. Soil compaction and crop production. Developments in Agricultural Engineering, Vol. 11, Elsevier, Amersdam.

2Jorajuria, D. & Draghi, L. 1997. The distribution of soil compaction with depth and the response of a perennial forage crop. Journal of Agricultural Engineering Research. 66(4):261-265.

3Rasmussen, K.J. & Moller, E. 1981. Regrowth after pre-wilting of grassland crops. II. Soil compaction in connection with harvest and transport. Tidskr. Planteavl. 85: 59-71 (In Danish, with English summary)

4Chamen, W.C.T., Moxey, A.P., Towers, W., Balana, B., Hallet, P.D. 2015. Mitigating arable soil compaction: A review and analysis of available cost and benefit data. Soil & Tillage Research. 146: 10-25.


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