Soil Structure Breakdown & Compaction

Cereals
Maize and sweetcorn
Oil seed rape
Peas and beans
Forage crops (turnips, rape, kale)
Vegetables
Fallow

Arable crops

Cereals

Cereal crops such as wheat, barley and oats are botanically members of the grass family and, therefore, have some characteristics similar to grass crops. There are two important differences. Firstly, they are not perennial so for up to half of the year there is no significant input of organic matter from roots (i.e. between ploughing and plant emergence when the soil is bare, and at the end of the growing season when land bears a ripening crop or stubble). When the soil is cultivated, prior to sowing a following annual crop, breakdown of soil organic matter is encouraged. Secondly, the number of plants (and thus root density) per hectare is normally lower than that for grasses, thus further contributing to lower organic matter inputs.

Even so, the root system of cereals is extensive near the soil surface and can extend to considerable depths (1 m to 2 m). The input of organic matter to the soil is substantial in the form of roots even if straw is removed or burnt. Incorporation of straw into the soil further increases organic matter inputs. For these reasons, production of cereals is less damaging to soil structure than the culture of most other arable crops. Indeed, on degraded land that has been cropped extensively with row or root crops, cereals have a good reputation as structure-improvers. Winter cereals will be more beneficial than spring crops since they have a longer period of growth and organic matter inputs are therefore greater.

Maize and sweetcorn

Maize and sweetcorn are planted at low densities (relatively large distances apart in relatively widely spaced rows). As a result, much of the soil is effectively fallow and receives little or no organic matter inputs. The root system is not fibrous and extensive like that of cereals so that organic matter inputs from this source are rather low. In addition, in many cases, virtually all of the above-ground dry matter is removed (for dairy fodder and/or in the harvested crop). As a result, repeated cropping of the soil with maize can be very damaging to soil structure. Where stubble is grazed and ploughed-in, more organic matter is returned to the soil and there is a less negative effect on soil structure.

Oil seed rape

The number of plants per hectare is usually considerably less than that for cereals, and rape does not have a fine fibrous root system like cereals. As a result, soil structural breakdown is likely to be slightly greater than under cereals. In addition, a very fine seedbed is needed for the small rape seeds. Production of such a seedbed can promote organic matter breakdown and also capping and/or wind erosion problems. Because rape has coarse storage roots, its growth can be particularly susceptible to compacted soil layers.

Peas and beans

In common with other legumes, these crops are well-known for their ability to fix atmospheric nitrogen. Organic matter inputs from these crops are likely to be small since they do not have an extensive root system and crop residues are often removed or burnt. There can be a risk with contract-crops since there is only a short period for harvest where quality is high. Heavy viners may have to operate when soils are wet and this can lead to severe compaction. Interestingly, because legumes have a taproot, their growth can be more affected by compaction than cereals. In addition, they are also sensitive to lack of aeration (wet feet), which occurs in wet, compacted soils.

Forage crops (turnips, rape, kale)

These crops are often drilled in late summer when soil conditions are favourable for seedbed preparation. However, they are normally grazed by sheep or cattle in winter when the soil is wet and feed is short. The risk to soil conditions depends very much on its state when grazing occurs. Treading damage will be greatest when the soil is wet and will be much greater when cattle rather than sheep are grazed (see page 22 on treading damage). With sheep, treading-induced compaction is normally shallow and will usually be remedied by subsequent ploughing. With cattle, compaction may extend to deeper layers and be more difficult to rectify. It is important to avoid compaction problems where possible. For summer crops used for stock finishing, the risk of treading damage is much less.

Vegetables

Soil structural breakdown is particularly common under continuous vegetable production. Seedbed preparation is usually achieved with rotary cultivation, which favours rapid breakdown of soil organic matter. In addition, crops frequently have a sparse root system and are often planted at low densities (widely spaced with wide row spacings). Much of the above-ground plant is removed at harvest and, as a result, very little organic matter can be returned to the soil. From a soil structural standpoint, it is important to incorporate as much crop residue as possible back into the soil. For crops that are harvested in the winter, using heavy machinery when the soil is wet, soil compaction may also be a problem.

Fallow

In the past, a fallow period when no crop was grown and the land was kept clean of weeds was a well-established practice. The reason for fallow periods was usually either to reduce the weed population or, in drier regions, to allow a period for recharge of soil water. In relation to maintaining soil organic matter content and soil structure, a fallow period represents a considerable hazard and is not recommended. During the fallow period there is no or very little input of organic matter. The decomposition of organic residues and native soil organic matter continues so that there is a net loss of soil organic matter. An additional problem with bare fallow is that the soil surface is unprotected and exposed to raindrop impact, which can lead to soil capping and possible erosion.

Continued cultivation causes breakdown of soil organic matter, a reduction of soil aggregation and the formation of a dense, compact plough layer.

The soil above has been under grass/clover pasture for 20 years. It is well-structured with a friable nature and has many pores and voids caused by the ramified growth of grass roots and the activity of a large earthworm population. The soil below has been under intense vegetable production with annual rotary cultivation for about 10 years. The plough layer has become dense and massive and it provides a poor environment for crop root growth. (Photographs by R J Haynes.)