Soil Structure Breakdown & Compaction

Remedial measures
Extent of the phenomenon
Slumping and hard-setting
Remedial measures
Extent of the problem

Surface capping

When finely aggregated soil with a low organic matter content is exposed to heavy and prolonged rain, the surface tilth disintegrates and a slurry-like layer forms. The finer particles of silt and clay that have detached from aggregates move a short distance into the soil, blocking pores so infiltration of both air and water through the soil surface is reduced. Upon drying, a dense surface crust can form a rigid cap, which may be strong enough to prevent seedlings breaking through (Figure 2).

Figure 2: Heavy rainfall following sowing can result in collapse of surface soil structure. Following drying, a hard cap can form that prevents seedling emergence through the surface. Redrawn from Batey (1988)

Remedial measures

Measures that can be taken to decrease the risk of capping include:

retaining crop residues on the surface as a protective cover,
not ploughing deeper than necessary so that incorporated residues are retained near the surface,
avoiding excessive tillage, particularly with powered cultivators that produce a very fine seedbed,
minimising the length of time that a fine seedbed is exposed to rain,
improving stability of surface aggregates by any action that will increase soil organic matter concentration near the surface.

If a cap forms on the soil surface it is sometimes possible to remedy the situation by:

if a cap forms prior to drilling, lightly harrowing or rolling when the surface is dry to crush the cap and recreate a seedbed, and
if a cap forms after sowing, waiting until the surface is dry before cultivating.
if seedlings have not reached light near the surface, harrowing can be practised, and
if seedlings are already near the surface, shallow inter-row cultivation may displace or fracture the cap and allow emergence.

Extent of the phenomenon

Capping is common on fine sandy and silty soils where organic matter content is low. It is found particularly under continuous arable cultivation and in market garden soils. That a soil is susceptible to capping is not always a constraint to crop production. It can become a problem primarily when heavy rainfall occurs whilst the soil surface is unprotected by a crop canopy. Heavy rainfall following sowing of a crop is most damaging. If conditions remain wet, water will pond on the surface encouraging waterlogging and root diseases. On the other hand, if the surface dries out, a cap will form that will limit seedling emergence. Capping can also limit the infiltration of irrigation water applied during the season. The fact that a soil is susceptible to capping is a good early indicator that soil organic matter levels are declining too much and that steps should be taken to reverse that trend.

Slumping and hard-setting

Some soils with very unstable aggregates, may slump back after ploughing and cultivation to the same density as before. This condition can be followed by hard-setting where the slumped soil that has run together in the wet state dries as one complete dense mass without cracks. Where this phenomenon is serious, it is difficult to prepare a good seedbed, and crop development, especially in the early stages, can be held back by restricted root growth. This problem is one stage worse than surface capping and occurs on soils of similar texture. It is usually associated with a soil management system that has run down the level of soil organic matter too far by intensive cropping and a low input of crop residues.

Remedial measures

to reduce the severity of hard-setting include:
minimising the number of cultivations and avoiding cultivation when the soil is wet,
employing measures to build up soil organic matter content in the cultivated layer, including adding bulky, organic material, retaining crop residues or using green manures, and
altering rotations to give a higher proportion of crops requiring little tillage and, in particular, using a grass crop or pasture in rotation.

Extent of the problem

Hard-setting is a problem only on soils where soil organic matter has degraded to low levels. As with capping, fine sandy and silty soils are most at risk. A dense, hard-to work, plough layer is formed through which it is difficult both for crop roots to ramify and water to infiltrate. It can be seen in some soils that have been subject to regular rotary cultivation (e.g. market garden soils) and in arable soils particularly where row crops (e.g. maize) have been grown continuously. Fortunately, many market garden enterprises are situated on peats, very sandy soils or on soils of volcanic origin and hard-setting problems are therefore avoided.

Soil with Cap

A dense cap can form on the surface of a poorly-structured soil after heavy rainfall or irrigation (see above). This cap can impede further infiltration of water and can also prevent emergence of seedlings. (Photo from British ADAS/MAFF collection.)

Cultivation of a poorly-structured soil can result in the formation of a very fine tilth that is subject to wind erosion (see below). The soil can be seen heaped against the fence-line after a wind erosion event. (Photo by K C Cameron, Lincoln University.)

Eind erosion