Soil Compaction in Vegetable Fields

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Peas are very unforgiving of delays in harvest and a crop can go from useable to unusable in 24 hours. This means that harvesting must often go on when soils are wet. 2013 has been wet during pea harvest and fields that are harvested during wet periods will likely have ruts and compaction that will affect rotational crops that follow. As other vegetable crops are harvested, from cabbage to snap beans, soil compaction will also be a concern. Soil compaction limits root development and root function and will reduce yield potential in vegetable crops.

There are two processes at play when soils are compacted by equipment. The first is destruction of soil structure. In most Delaware soils, our surface soil structure is granular or crumb in nature and consists of small aggregates. It takes considerable time and good cropping practices to build up soil structure. When ruts are cut in soils, this structure is destroyed, making soils denser. Excessive tillage also destroys soil structure.

A second compaction process is the compression of soil particles, pushing them closer together. This happens with equipment traffic across fields. The heavier the loads carried by equipment passing over soils, the more the compaction. With large equipment and heavy axle loads, significant soil compaction is expected; the heavier the weight on an axle, the more the compaction. A pea/shelled bean harvester weighs about 30 tons with approximately 10 tons per axle. Equipment under 10 tons per axle have much less potential to compact soils than those of 10 tons or greater. It is interesting to note that there is less weight per axle with a pea harvester than with large grain combines and grain carts. Other equipment factors affecting compaction include tire size, tire pressure and operating speeds. Wider tires or dual tires will distribute weight over larger areas, reducing deep compaction but increasing the amount of area with shallow compaction. Higher tire pressures will result in more deep soil compaction and slower speeds will also result in more compaction. It is interesting to note that tracked equipment, while being able to cross wetter fields, can cause as much or more compaction than wheeled equipment.

In wet soil, there is less resistance to soil particle movement and soil is more “plastic”. This means that potential for compaction is greater in wet soils than dry soils. While vegetable growers often do not have a choice and must harvest in wet conditions, other crop operations can sometimes wait until soil conditions are more favorable. A good example would be planting lima beans after peas. Waiting a day or two for soils to dry will improve yield potential by reducing compaction. In addition, lima beans actually have better yield potential as planting progresses later in June.

In managing rutted and compacted fields, ruts should be lightly tilled to refill the ruts with soil and the fields should be chisel plowed. Note major areas with deep compaction to address in the future. Subsoiling in the fall is a short term solution to deep compaction. The use of forage radish cover crops has shown great potential to reduce shallow and deep compaction.