Soil Structure Problems
How Soils Become Structurally Degraded
Soils are naturally a series of layers. The top layer or top soil will naturally have higher levels of organic matter. It is high in nutrients, and it absorbs and holds water when it rains, because the organic matter acts like a sponge.
When soil is plowed though, the top layer is mixed with lower layers; and that changes this natural structure. Tillage isn't the only reason why soil might degrade, but it is significant.
When soil degrades, no matter what the reason; there can be unwanted consequences.
Among other things their may be a reduction in the ability to absorb water; resulting in increased runoff which can cause erosion.
Soil can become compacted, and nutrition levels may decline. The real extent of the problem is difficult to assess as structural decline is often gradual. Mainly medium and fine-textured soils are susceptible.
Surface soils with a clayey sand or coarser texture (<8-10% clay) have minimal secondary structure and are generally not susceptible.
Various hard layers occur in soils, but they do not always indicate structural decline.
Hard Layers in Soil
Several types of hard layers in soils can restrict crop emergence or restrict or prevent root elongation. Hard layers are hard when dry, but vary in hardness on wetting and in the degree to which they impede root growth and/or emergence. For instance, a hard-setting surface limits the timing of cultivations and sowing, can restrict crop emergence and root growth, is hard when dry, but is soft when moist. The four main processes in the development of hard layers in soils:
- transient bonding
This term covers a number of transient cementing agents in soils.
For example, cementation can involve clay bridges (very fine clay coatings) between sand particles or chemical cementation involving the progressive precipitation of slightly soluble compounds (silica, alumino silicates, iron oxides) as very thin coatings, bridges or gels in drying soil.
An important condition is that the soil is dispersive to allow the redistribution of particles. Also, it must undergo wetting and drying cycles to allow these bonds to form. The cementation is transient, being unnoticeable when the soil is wet, but rapidly increases as it dries.
Transient cementation is partly responsible for the dry strength of sand plain soils.
Mechanical: Externally applied mechanical stresses (i.e. vehicles and machinery) will compact soils if the stress exceeds the strength of the soil. Compaction makes the soil dense, the large pores are destroyed and the total air-filled porosity is reduced, leading to slow hydraulic conductivity. Plow pans (also called cultivation pans) and traffic pans form as a result of farming practices, and are not an inherent feature of the soil and are not permanently cemented, although there could be some transient cementation.
Water: Through the action of water potential, effective stresses are generated which compact soil as it dries. The particles are pulled closer together by increasing tension between the retreating water films around them. This process increases the strength and reduces the volume of drying soils.
Compaction also reduces the available space for oxygen in the plant root zones.
For this reason, some of the major consequences of compaction are poor drainage, poor aeration, and hard pan surfaces which cause runoff. Compaction is generally caused by human use of the soil.
Repeated cultivation of some soils leads to a breakdown of soil structure and this also increases the likelihood of compaction.
Mechanical compaction can be prevented by farming practices that minimise cultivation and the passage of machinery.
These include conservation tilling, selection of crops that require reduced cultivation, and use of machinery at times less likely to cause compaction (i.e. when soils aren't too wet or when some protective covering vegetation may be present). For heavily compacted soils deep ripping may be necessary.
Cementation – Pan Formation
Cementation is a process in which soil material hardens irreversibly when calcium carbonate, silica or iron, are precipitated. The porosity decreases gradually until no water can percolate through this layer and it resembles rock. Cemented layers are an inherent feature and may occur within the soil profile or at the surface. Inherent pans: For example, Ferricrete pans and red-brown hard pans are permanently cemented and an inherent feature of the soil.
Slaking in water is an easy way to identify whether pans are cemented.
If a small fragment of the pan is placed in water for an hour and it does not slake, the pan is cemented. Pans that do not slake in water will be a barrier to root growth and prevent root elongation if they are continuous.
A pan restricts the volume of soil available for root growth and indirectly, the plant available water. The effect on plant growth will depend to a large extent on the depth and continuity of the pan. Very shallow pans can affect plant anchorage and stability. In general, if the pan is less than 30 cm below the soil surface, it will restrict crop production, but if it is deeper than 100 cm, the effect is likely to be minimal or even beneficial.
Water repellence or hydrophobicity
Water repellence affects the wetting pattern of soils and results in an uneven wetting pattern. In the paddock, patches of wet soil alternate with patches of dry soil which results in the poor germination of crops and pasture. Water repellent soils require more rain before sowing to wet them than non-repellent soils. This means that seeding may be delayed, reducing the yield potential.
Water repellence can be demonstrated by placing a drop of water on the soil surface. It is advisable to remove a few millimetres from the surface before doing this. If a soil is repellent the droplet will form a bead and not penetrate quickly. Sandy soils are particularly susceptible to water repellence.
Water repellence is generally influenced by past management and land use. Susceptible soils may or may not be water repellent depending on the paddock history and the time of the year. The only reliable method of diagnosing soil condition is to assess each paddock with susceptible soils.
There are two methods:
- Field observations are simple, but the degree of repellence may be strongly influenced by seasonal conditions. If field observations detect water repellence, a laboratory test is recommended.
- Laboratory tests are the most accurate method, and can be used to monitor change in the medium-term.
Time of sampling: Sampling can be done at any time but it is preferable to sample in summer when the soils are at their driest as there will be less variability in results.
Depth of sampling: Highly repellent soil is closely associated with the top 50mm, depending on the amount of soil mixed by cultivation. In many water repellent soils there is a thin surface layer which is “wettable” even though the underlying soil is moderately to severely repellent. It is therefore normal practice to scrape away the top 3 to 5mm before sampling.
In paddocks with a long history of continuous pasture or no-till, the sampling depth should be 5 to 50 mm. In cropped paddocks the same sampling depth can be used, but hydrophobic material is likely to be more evenly mixed through the cultivation layer, so sampling depth will be less critical.
Water repellence can be managed in the following ways:
- Using water harvesting or furrow sowing
- Masking (adding clay to cover the hydrophobic layers)
- Grow species adapted to water repellent soils (e.g. pines, tagasaste, blue lupins)
- Avoiding dry or drying soil (high risk of wind erosion) by furrow sowing onto moisture; or cultivating in the rain
- Lowering surface tension by applying soil wetting agents
- Decomposition (microbial consumption of hydrophobic organic matter)
- Dilution (physically mixing with non-repellent soil) (high risk of wind erosion).
Note: Lime has been tried as an ameliorant on water repellent soils, but is not recommended. It can improve wetting slightly, because of the fine particle size and corresponding surface area effect, but field trials have shown it is relatively ineffective, especially compared with clay amendment