The lack of germination and plant vigour in the location of the wheel tracks is a clear indication of surface soil compaction. Where pugging of the surface by animals is the cause of surface soil compaction the impact on the crop is more wide spread but less severe due to the shallow soil disturbance at seeding.
Compaction of surface soil may be relatively easily observed from machinery traffic patterns and livestock trampling patterns. Some topsoils will show a very smooth surface and a dense collapsed topsoil when compaction has been caused by soil structural instability and waterlogging.
Subsoil constraints are often more difficult to identify than topsoil influences on crop performance and poor topsoil growth conditions may also mask the influence of poor subsoil growth conditions; thus a poorly germinated and fertilised crop may not reveal additional limitations to growth from subsoil conditions. Many subsoil growth issues are only clearly expressed when the crop is well managed in relation to topsoil conditions.
Moist soil within 30-40cm of the surface in cropped areas in drying profile (that is, no recent significant rain) indicate that there has been insufficient root growth at these depths to extract water. This restricted root growth may be due to compaction or other causes such as subsoil acidity or transient salinity. Tillage (deeper than 20cm) when the soil is dry or slightly wet will often bring up large dense soil clods if there is a compaction layer. Compacted soils may be physically difficult for the tines to penetrate and increase the draft force of the tractor.
Subsoil compaction effects on crop or pasture can be suspected from evidence such as:
- Dry topsoil despite subsoil moisture after a relatively poor yielding crop and a dry finish to the season suggests there have been insufficient roots at depth to extract water.
- Deep working points at seeding that bring large dense clods of soil to the surface is often evidence of a dense compacted layer in the subsoil.
- Difficulty for tines to penetrate and increased draft force of the tractor, especially when working across previous directions of cropping, indicate compact subsurface zones in the paddock.
- Linear patterns of delayed flowering (green strips), particularly in broadleaf crops such as lupin can be a useful indication of subsurface compaction effects on crops and pasture. Growth patterns from windrow burning should not be confused with patterns from compaction.
- Having 1-in-20 to 1-in-50 individual plants performing a lot better than the rest of the crop is a very common symptom of subsurface compaction. Those better plants have found old root channels and so enjoy better nitrogen and sulphur nutrition than the rest of the crop because they have deeper roots. This effect is obvious early in the growth before stem elongation and even after then it might still be visible.
- Deformed and bent roots of individual plants in poorer growth areas (dig up to examine), especially in canola, can be strong evidence for subsurface compaction. Note that aluminium toxicity in acidic subsoil can also affect the root growth of some crop species.
- Poor growth and yield in wheel tracks from a wet harvest or a change of direction in seeding may be visible in the earlier parts of the growing season.
- Poor nitrogen use efficiency evidenced by poor/low tillering despite adequate early N applications/status.
Compacted layers in the subsoil can be observed by digging a soil pit; often having a distinct upper and lower boundary and a blocky appearance or structure which may be fractured. The compacted layer will feel more dense and stronger than the soil above or below it. When pressure is applied to clods from a compacted layer they tend to crack where pressure is applied rather than on natural break lines (as happens with well structured soils).
The effects of slow and restricted root growth are difficult to identify by observing plant growth in the paddock because there is rarely an opportunity to observe a direct comparison between compacted and uncompacted soil. Observation of plant response to soil disturbance which would break up a compacted layer, such as water lines or power cable installation gives a good indication of subsurface compaction in the paddock. It should be realised however, that those effects will not be repeated with ripping due to the shallower working depth of the latter.
Crop and pasture indicators of soil compaction
Poor germination and lack of plant vigour in a linear pattern coinciding with wheel marks indicate surface soil compaction. Where pugging of the surface soil by sheep or cattle is the cause of surface soil compaction, the effects on the crops will be more uniform or sometimes confined to areas of heavy animal traffic.
Subsoil compaction is indicated by:
- Platy soil structures with horizontal orientation, blocky structures with sharp linear edges and roots flattened between the faces of such dense soil shapes or massive layers of low porosity, sometimes with plant roots growing horizontally above them.
- Very few visible soil pores (channels or cracks).
- Root growth confined to cracks between large soil blocks and pre-existing pores.
- Root tips wider than other roots and growing in a twisted path. (Take care not to confuse this with the effects of toxic aluminium due to soil acidity.)
- Poor (shallow) root growth. Swollen root tips happen as roots try to force their way through the hard soil.
- Horizontal root growth over the hard soil layers (especially canola).
- Root density and root branching are reduced in the compacted layer.
- Crops and pastures growing on compacted soil may become water stressed more quickly during dry periods and hay-off more rapidly at the end of the season. Often these areas produce pinched grain.
Hand probes are metal rods that are pushed into the soil by hand. Compacted layers can be felt because it will be more difficult to push the probe through the compacted layer but it will become easier again once the probe has passed through the compacted layer.
Comparisons of resistance to the probe with similar soil in uncompacted zones such as along fence lines or in remnant vegetation areas can highlight the effect and help with the diagnosis. Testing needs to be done when the soil is wet (preferably the drained upper limit to depth) as dry layers in the soil will feel hard regardless of compaction status.
Hand probes can be made from steel rod (about 8–10mm diameter) sharpened to a point on one end or even heavy gauge, 3mm, fencing wire about 40cm long with one end looped to make a handle. In both cases depth increments can be added to the shank of the probe so that depth of the hard layer can be determined. Hand probes can also be purchased commercially from field survey and environmental suppliers.
A cone penetrometer measures and records the force required to insert a standard-sized cone into a soil profile. The cone penetrometer is inserted into the soil at a steady speed (commonly 30mm/s) by hand and the instrument uses a force gauge to measure the penetration force (cone index) in megapascals (MPa) or kilopascals (kPa) required to penetrate moist soil.
The penetrometer records the force at selected intervals (often every 20mm) and records both the penetration resistance and depth at which it was measured at each interval. The data is stored in a data logger and can then be downloaded. Penetration resistance has been related to crop root growth in wet soils close to the drained upper limit. In general, crop root growth restriction starts when penetration resistance exceeds 1.5MPa and severe restriction at 2.5MPa or more in soils at field capacity.
Cone penetrometers are useful for:
- Assessing the effectiveness of deep ripping for reducing soil strength by comparing before and after ripping or between ripped and unripped soils.
- Comparing the soil strength of uncleared soil with no traffic to traffic affected soil in the paddock.
Precautions with probes and penetrometers
A few precautions need to be observed when using cone penetrometers and soil probes:
- Soil penetration resistance should be assessed when the soil is at field capacity (drained upper limit).
- Dry subsurface layers will also resist the penetrometer and can appear to be compacted layers.
- Soils can be wet up by applying water to a distinct area of soil before using a probe or penetrometer.
- Soil strength should only be compared between soils of the same type that have a similar water content. Soils with different texture and structure or water content will give varying results.
- Higher force is required to push a penetrometer through stony and gravelly in the soil. Care should be taken not to misinterpret this as a compacted layer.
Using test strips of possible treatments
Visual observations and measurements explained above provide strategies more likely to evaluate the most cost effective management solutions for the current farm business. Test strips of possible solutions such as deep ripping are often the most effective general strategy. If mechanical relief of compacted soil provides no measureable improvement in root exploration of the soil and plant performance then another constraint may be present and need addressing.