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Introduction

Many farmers talk too readily of “soil compaction” when crop growth appears inadequate and tile soil seems hard. Hard soil is not necessarily compacted soil. In fact, the term “soil compaction” refers to a quite specific soil phenomenon, where the amount of soil solid in a given area is increased at the expense of the air spaces. This is only one way soil may be altered to become structurally degraded and potentially less productive.

The nature of soil structural degradation and the processes which cause it are commonly misunderstood, It follows that meaningful measures of the phenomenon are seldom collected. If the nature of the degradation has been incorrectly assessed how can the most efficient ameliorating practice (e.g. chiselling, deep ripping, etc) be chosen, and how can its effect oe assessed? Efficient amelioration must be based on sound knowledge of the soil problem being tackled.

This paper has three objectives: to review the more common methods of measuring soil structural degradation and demonstrate how each may shed different light on the nature of the degradation; to present examples of data and so highlight potential drawbacks of each technique to discuss some potential causes of degradation and relate these to the location of degraded areas in the field,

Methods of Quantifying Soil Structural Degradation, With Illustrative Examples

No one measure of soil condition can provide a complete picture of the nature of soil structural degradation. Degradation is a multi-faceted phenomenon which is best assessed by a set of techniques, each of which gives added insight into the nature of the phenomenon. Six measures will be reviewed:

(i) Dry bulk density

This is commonly taken as a direct measure of soil compaction. Defined as the weight of solids per unit of total soil volume, it provides a basis for comparing the compactness of soil particle packing between two samples. Assuming uniform particle size distribution, a sample with more solid material per unit of total soil volume has a higher bulk density; it is a more compact soil.

To measure bulk density, samples collected must be both undisturbed and of known volume. A common technique uses an integral sampler where a sharpened cylindrical metal sleeve of known dimensions is driven into the soil (see Loveday, 1974, Ch.3 for details). The trimmed soil core and sleeve, of known volume (Vb), are dried at 105⁰C to constant weight (to give dry mass of solid; Ms). Dry bulk density is the ratio Ms/Vb.

Most important is that in expansive soils (soils that swell on wetting and shrink on drying) bulk density is dependent on water content. Swell- shrink potential varies greatly between soils. An example will illustrate the degree of dependency in a soil with ‘high" shrink-swell potential: a grey cracking clay. A plot of bulk density sampled over a wide range of soil water contents has, overall, a negative trend (Fig 1). If this line represents the bulk density water content relationship of an experimental site, sampling this site on different occasions (at different water contents) may mistakenly show areas of ‘compacted’ soil. To compare between sites, or between different areas of one site, samples must be collected at one water content, or over a similar range of water contents.

Fig.l: Bulk density of a sample of cracking clay plotted against its water content.

Gravimetric water content %

(ii) Porosity

Soil porosity is a popular way of expressing a soil’s structural condition. The soil pores contain the soil air and water. The air-filled porosity provides a guide to a soil’s aeration status which may be related to air diffusion and the potential for plant root respiration.

Air-filled porosity is the volume of air-filled pore space in a soil at a given moisture content, and is found by subtracting the volumetric water content of a sample from its total porosity (see Loveday, 1974, p.70).

With this index, it is of interest to plot the increase in air-filled porosity on subsequent days after an irrigation or heavy rainfall. Various researchers have presented critical figures of air-filled porosity below which a soil is ‘waterlogged’ (i.e. oxygen diffusion and root respiration stops). A commonly accepted figure is 105, though it is somewhat arbitrary being both soil type and crop dependent. The rapidity with which a soil achieves an air-filled porosity value of l07~ following an irrigation or heavy rainfall may be related to the soil’s structural status, the more compacted soil remaining at low air-filled porosities longer. An example illustrates different recovery rates of air-filled porosity between two similar (texturally and chemically) soils after irrigation (Fig.2). Both sites were sampled on each of 2 and 5 days after an irrigation. By day 5 air-filled porosity was over 10% to 40cm at site A, but only to 20cm at site B. At site B, therefore, plant root respiration beneath 20cm was probably impaired for at least 5 days after every irrigation.

Fig 2. Air-filled porosity profiles for two sites: (A) not degraded, (B) degraded on each of 2 and 5 days after irrigation.

(A) Air-filled porosity	(B) Air-filled porosity %

(iii) Soil water extraction

Investigation of plant water-extraction profiles is a more indirect method of measuring soil degradation than either bulk density or air-filled porosity. In a structurally degraded soil both impaired root activity and changes in soil pore geometry, which leads to smaller pores holding water at higher tensions, may lead to less water extraction. Water profiles from degraded and non-degraded soils growing the same crop may be collected on the same days following an irrigation or heavy rain. Differences in the amount of water extracted (by the plants) from each site over the time period may then be examined. Differences are not solely due to plant extraction, so an indication of losses through evaporation and drainage on the same sites is required.

An example from a heavy clay soil under cotton will be presented. Differences in soil water content between two sampling days (5 to 78 days after an irrigation) were calculated for two sites. Site A showed strong visual signs of structural degradation to 70cm, but site B showed none, Both sites had been planted on the same day, and it was known that through- water movement in each was similar. At each site the differences in water content were summed to each depth sampled (Table 1). To 80cm, 33mm of water was extracted from site B, but only 23mm from A. So, plants growing in the non-degraded site were able to extract 30% more water from the soil than those in the degraded site.

TABLE 1 Water content change between 5 and 78 days after an irrigation for two sites: (A) degraded and (B) not degraded. Expressed as sums of differences of water to any one depth.

Summed to depth (cm)	Summed difference (mm of water)
	Site A	Site B
10	8.72	6.70
20	16.85	16.87
30	21.81	23.82
40	23.96	27.32
50	23.58	30.79
60	22.78	30.08
70	22.78	32.31
80	22.77	33.01

(iv) Penetration resistance

Like soil water extraction, soil resistance to the insertion of a penetrometer is a secondary indicator of soil structural degradation. Common acceptance of the measure as an indicator of soil strength, and the ease and rapidity of data collecting have ensured its popularity. However, uncertainty exists about the meaningfulness of the measure both due to its interdependency with other soil factors, and with interpretation of the measure as regards plant root exploration of a soil.

The inverse relation of penetration resistance and soil water content (Fig.3) is well known.

Fig. 3: Penetration resistance in the 10-20 cm layer at 6 sites on cracking clay, plotted against their water content. The full lines mark the water content range sampled at each site. The dash line gives the overall trend.

Six sites on the same soil type were sampled, each over a different range of soil water content. If associated water content had riot been measured then it could have been concluded (wrongly) that site 5 had higher penetration resistance (was more degraded) than the other sites. When viewed in association with water content it can be seen that all sites quite closely fit one line: a negative exponential. Penetrometer resistance is also affected by soil texture (see Chancellor,1976, pp7-8).

Critical values of penetration resistance above which root growth stops may be found in the literature; a range of 2000 to 2500 kPa is common, However, such values cannot be directly related to root penetration as they cannot account for factors such as the lubrication of the soil by plant root exudates or the ability of a plant to seek out the path of least resistance especially in a structured soil.

(v) Soil micro-morphology

This is the examination of soil features viewed through a microscope. The soil to be examined is taken into the laboratory, impregnated with epoxy resin (similar to Araldite), allowed to set hard, then cut into ‘thin sections’ (each 0.025mm thick), which are set between 2 glass covers lips.

Examination of soil structural degradation with a microscope is still in early days. Two methods have been investigated, each of which are usually evaluated on a relative basis, e.g. uncultivated versus cultivated soil. First, an estimate of bulk density (or inversely, porosity) can be gained by quantifying the solid to void ratio as viewed under a microscope. This may include a statement on the type of voids that constitute the pore space, e,g. their size, shape, degree of interconnectivity, etc. Second, an assessment can be made of the particles of soil solid, particularly the amount of clay material with preferred orientation. In such a study on cracking clay soil, it was known that before cultivation there were no zones of clay with preferred orientation, but that the clay particles had random orientation with predominantly edge to face contacts (Fig.4, type A).

Fig.4. Using the examination of clay orientation as an index of structural degradation.

(A) Random orientation: edge to face alignment predominant	(B) Preferred orientation: face to face alignment predominant.

Where, Where, packets of clay particles

individual clay particles.

Following cultivation under wet soil conditions up to 25 % of the top 20cm of this soil had preferred orientation, i.e. clay predominantly aligned face to face (Fig.4, type B). The source of these zones of higher density is seen to be the smearing of wet soil by metal implements. The majority of the zones were less than 20mm across, so unlikely to be affected by chisel or ripping implements. It would require root exploration and faunal activity (e.g. earthworms) to ameliorate such zones.

(vi) Soil profile description

In many instances the first sign of soil structural deterioration seen by a farmer or agronomist is a change in some visual characteristic of the soil surface in the field. The source of the change may then be investigated in a pit, where certain signs are commonly taken as denoting degradation. The most common is massiveness, which is an area or layer in the soil with less visible voids and less aggregation than the surrounding soil. Disruption of this massive soil leads to rough surface soil, composed of large massive clods. Horizontal, platy structure may also be found, with the soil breaking into thin horizontal layers. Deformities in plant roots may be apparent because of massiveness or platiness.

Though visible signs of degradation may be accurately measured in the soil profile, it is difficult to assess their actual or potential effect on crop growth and economic return. However, full profile examinations of degraded soil will help in locating both problem areas in a field and problem zones in the soil, and this may lead to more accurate placement of ameliorating practices.

Causes of Degradation and Their Field Location

In crop-agriculture there are two sources of potential compaction: vehicle wheels or tracks, and soil-engaging tools. The effect these have on a soil is dependent not only on the size of vehicle, inflation pressure and width of tyres, type and depth of working, etc, but also on the characteristics of the soil. In general terms, the ability of a soil to withstand compression decreases as the soil increases in water content and becomes more plastic, and when the soil becomes more cohesive due to the predominance of clay. Hence, wet clay soil has a much lower strength than dry sandy soil.

The interaction of vehicle traffic, tillage implements and soil degradation is quite complex especially if a range of both soils and machinery is considered. Summaries of such research are available (see Chancellor, 1976, ch. 3 and 4). Only some general points will be made here.

The location of structural degradation in a field is closely related to the type and intensity of the cultivation practices done. For various reasons the distribution of the degree of structural degradation may vary within a field, Wheel-track effects may be more apparent in row crops, or in orchards with permanent pathways. However, during season wheel- track effects may be of little significance in a field which has had intense pre-sowing land preparation where up to 100% of the field may have been repeatedly traversed with heavy equipment. The effect of pre-sowing land preparation may be greater in certain areas of a field, e.g. where soil water content was higher or where more sodic subsoil had been exposed. Head and tail ditch areas in irrigated fields, and gateway areas in dry-land fields also receive disproportionately high traffic and degradation may be more intense in these areas.

Interrelated with variability of structural deterioration across a field is variability in the vertical, i , e. with depth in the soil profile. Many parameters influence the location of degradation in the profile, and the type and degree of the effect. On the soil surface wheel and track effects are usually most obvious, The effect is normally compaction, but with wheelslip, shearing and puddling also occur. Beneath the surface, degradation may be caused either by direct soil contact by an implement, or by energy transfer into the soil from the implement or machinery on the soil surface, Depth of cultivation with implements may vary from 2cm (e.g, a scarifier) to 100cm (e.g. a ripper). Most often the decision to begin cultivation after irrigation or rain is based on surface soil conditions, though the implement may penetrate deep into the subsoil. Under such conditions compaction, through compression, may occur ahead of or beneath the implement, and shearing may occur alongside. Again, energy transfer may cause degradation in soil not actually touched by the implement.

Discussion

Soil structural degradation is often cited as an important element in yield decline, poor crop-utilisation of soil water, increased waterlogging and seed bed roughness. However, understanding of the effect or the nature of degradation is often minimal. It is rare, therefore, that there is a basis for effective, long-lasting ameliorative practice.

Most often, characterisation of structural degradation requires the application of simple techniques of soil science. These, however, must be applied with care and the data they provide assessed with caution if the results are to truly characterise the phenomenon being examined.

Assessment of degradation is usually based on a relative measure; one field to another, one side of a fence to the other, one end of a field to the other. Similarly, assessment of the outcome of any ameliorative practice should be made relative to the soil condition prior to amelioration,

References

1. Chancellor, W. J. (1976). Compaction of Soil by Agricultural Equipment. Division of Agricultural Sciences, Bulletin 1881, University of California, USA.

2. Loveday, J. (1974). Methods for Analysis of Irrigated Soils. Technical Communication 54, Commonwealth Bureau of Soils, Harpenden, UK.