When to do the spade diagnosis: Sample when the soil is moist if the soil is too dry or too wet it can be difficult to distinguish signs of poor structure. Spring or autumn should be the best time of the year and allows management decisions to be made if needed to improve soil structure.

Where to sample: Take samples from areas where you expect good structure (uniform crop growth) and from areas where you expect poor structure (wheeled areas or areas near gates) to allow you to see differences in structure.

The soil structure evaluation is based on the method called VESS (Visual Evaluation of Soil Structure) developed and published by Bruce Ball, Tom Batey and Lars Munkoln (1997). Roots observation and macropores counting has been developed by Joséphine Peigné and Jean-François Vian (ISARA Lyon).

Step one: Soil removal

In soils with a hard surface or under grass, cut out a spade-sized block of soil. Cut down on three sides and then lever the block out leaving one side undisturbed. Alternatively dig out a block and then take a slice from the undisturbed face. Carefully, lay the block on a plastic sheet on the ground or onto a plastic tray. Measure the length of the soil block.

Step two: Soil assessment 

Soil structure evaluation (VESS method, Ball et al., 1997)

1) Block break-up

Gently open the undisturbed side of the block like a book and start to break it up. 

If the block breaks up easily into small fragments then the structure is likely to be good.

If the block is hard to break up then it could either be held together by roots and you will need to pull these apart to expose the soil fragments , or it is compacted and breaks into large lumps.

Break up the block enough to allow you to discover if there are any distinct layers of differing structure. If the block is uniform assess as a whole, if there are two or more such layers, then score separately .

Measure the depth and thickness of any distinct layers.

For each soil layer assess : the degree of firmness (easy to break) and size of soil fragments clods and agreggates. Clods are defined as large, hard, cohesive and rounded aggregates (mores tan 7 cm). See Table 1.

A photograph at this stage provides a useful record and, when put together with others, allows comparisons to be made.

2) Reduced fragments

For each soil layer, break up the soil with your hands into smaller structural units from 1.5 to 2 cm (known as aggregates).

Assess the shape and porosity of soil fragments (see Table 1) and evidence of anaerobism (colour, mottles and smell).

Table 1: Soil structure assessment grid for each soil layer identified at block scale and reduced fragment scale.

3) Soil structure scoring and interpretation

Give a score by matching what you see to the descriptions and the photos in the chart (Fig. 1). A score of Sq1 or Sq2 is good, a score of Sq3 is moderate. Scores of Sq4 and Sq5 are poor and require management action.

Soil scoring: If clods are large, compact lumps that can be broken into non-porous, sub-angular (sharpedged) aggregates this indicates poor structure and a higher score. Small, rounded aggregates or large aggregates that break down easily into smaller rounded aggregates indicate good structure and a lower score. After assigning a score from comparison with the pictures in the chart, adjust it according to the difficulty of breaking apart of the fragments and their appearance. In grassland, roots make it difficult to break up the block but this is not a factor that will increase the score.

Roots

Two observations will be done : (1) Refresh the face of the spade hole, and observe and assess roots according to Table 2 indicators ; (2) complete the observation when you describe soil structure of the block.

Table 2: Observation and assessment of roots in the Spade test

Earthworm macropores

1) Macropores counting

In the same hole of the spade test, we measure the impact of earthworm activity on soil porosity through the presence or absence of anecic burrows in the soil profile (vertical burrows 1 to 3 mm in diameter).

Anecic burrows were counted at the junction between subsoil and topsoil (at a depth of 30 cm generally) in the spade hole on a horizontal plane .

As the counting is done with spade test, the surface is 0.50 m x 0.50 m (0.25 m2) for each spade test in the bottom of hole (you need to increase and refresh the surface). 

For counting burrows:

– Clear a plane surface

– With a knife: remove the floor progressively, and highlight the earthworm burrows. For a better estimation: you can mark each burrows with colored drawing pins.

2) Interpretation

There is no universal threshold for earthworms’ macropores numbers at 30 cm depth. The test is useful to be done: (1) when we want to compare several agricultural practices on the same soil type to see which practices favor earthworms’ macropores and thus deep soil penetration of water and roots, (2) when you follow the long-term effect of agricultural practices (test can be done at several periods to see the evolution) and (3) if a we note crop problems such as yield decrease and that topsoil structure is not concerned, very few earthworms’ macropores at 30 cm depth (less than 50 per m2) can be a sign that soil compaction occurs deeper and that a soil profile observation is needed.

Spade diagnosis and earthworm sampling (see section 3.3.3) can be coupled to see the effect of degree of soil compaction on earthworm population.

• Weeds as bioindicators

The aim of this method is to gain information on soil conditions in an agroecosystem using wild plants (‘weeds’) as bioindicators. Many weeds can grow in different soils and environments, but each species has an optimum range of conditions under which it can be found. According to Grime’s plant strategy classification, weeds are usually characterised by a competitive or ruderal strategy, and only a few have the capacity to adapt to very extreme conditions (stress‐tolerant species). At any abundance rate, some weed species can be typically found under specific soil conditions. Knowing which species can be associated with which soil condition is the basis for using them as bioindicators. Weeds as bioindicators have been known for a long time. In this regard, the authors started by analysing the old, mainly anecdotal literature and integrated more modern, scientifically‐based evidence, which is still rather sparse. Finally, species were clustered into two groups based on the number of records clearly associating them with a given soil characteristic.

Weed species for which the same type of association with a given soil characteristic was reported in three or more different sources were defined as ‘highly reliable’ indicators. Weed species for which an association was reported in two different sources were defined as ‘medium reliable’ indicators. Weed species are listed in the ‘Bioindicator species tables’ shown in the appendix.

The second step was to develop a methodology that allows farmers and agroecosystem managers to extract the best information on weeds as bioindicators of soil conditions from dedicated field sampling. The sampling strategy suggested here cannot be considered exhaustive, but it represents a good compromise between sampling effort and data accuracy. In order to gain more precise information on soil conditions, the use of conventional soil testing techniques is recommended.

• Investigation method

Identifying weed species is not always an easy task, but the species selected here as soil bioindicators are quite different from one another, which should reduce the risk of misclassification. The correct identification of weed species is a prerequisite for using this method. 

When to sample

When the purpose of sampling is to make decisions on which control measure to apply, weeds are usually identified at an early developmental stage. However, sampling for weeds as soil bioindicators can be done at a later growth stage (e.g. flowering), when species are easier to identify. In temperate environments, it is advisable to take a sample more than once per year, for example, in spring before the application of weed control techniques, in summer before crop harvest, and in autumn before soil tillage (if any occurs). By combining the information of these three sampling periods, it is possible to form a clear picture of the most important weed species present in the agroecosystem while minimising the risk of missing some seasonally important, short‐cycled species. 

Where to sample

Weed sampling should be performed in one or more target fields, typically in those frequently showing abnormal soil conditions. Since the evaluation is based on weed community composition and not just on the presence of some individual weeds, the whole target field should be sampled. Considering that the weed community can strongly vary from the field margin to the centre of the field, it is suggested to walk along and across the field before starting the sampling, take note of any areas where the weed composition abruptly changes, and decide whether or not to include outer field areas (e.g. margins) in the sampling.

Materials needed

• Weed identification book
• Clipboard, pad and pencil
• Sampling sheet (see example below, appendix & web‐site)
• Bioindicator species table (see appendix & website)
• Newspaper sheets

Field work

1. Observe the overall field or area that you are going to sample. Walk along and across the whole field to get an idea of whether the sampling area is homogeneous in terms of weed community composition. If it is not, identify the subareas that have a clearly different weed composition. If the field margin vegetation is very different from field vegetation (e.g. due to the presence of ditches, shrubs, fences or other structural elements) exclude it from the sampling.
2. From one side, walk inside the field following a zigzag pattern. Take note of the main weed species present, and visually estimate the percentage of soil cover for each of them. In the sampling sheet, write down the main species encountered in the first sampled sub‐area (e.g. ‘A’). Repeat this procedure for the second (e.g. ‘B’) and any other sampling subareas.
3. SAMPLING NOTES:

3.1 Focus your survey on the overall weed species composition and on the dominant species. Single occurrences and rare species may be of high botanical interest but cannot be considered trustable indicators for soil conditions, especially in highly disturbed situations like agroecosystems.

3.2 In case you cannot identify some of the main species present, take some individual specimens to identify them later. In that case, remove the plant from soil and include part of the root system. The best individuals to be sampled are those with flowers and fruits. If they are too large, you can bend the plant or sample part of it. Then assign it a provisional name based on main plant features (e.g. ‘grass with hairy reddish leaves’ or ‘dicot with purple flowers and long ovary’) and record the species as such on the sampling sheet. Next, conserve the plant between two newspaper sheets after flattening and opening leaves. Afterwards, put some weight on the newspaper sheets. This will keep the features of the specimen as close as possible to those of the live plant.

4. In the sampling sheet, annotate the soil conditions in each sub‐area. Focus on differences between sub‐areas for these features:

• Soil texture (e.g. in which sub‐area is it sandier? In which is it more clayey?)
• Soil compaction (how difficult is it to put a stick into the soil?)
• Soil colour (e.g. which one is darker?)
• Soil moisture

5. Check if the main species found in the sub‐areas are annual or perennial. If you are uncertain, use the following simple test: try to uproot a plant of the target species; if you can easily uproot it including a large amount of the root system it is an annual; if the plant breaks when trying to uproot it is likely a perennial.
6. Now, for each sub‐sample, you have a description of the main weed species present as well as of the main soil characteristics.

Out‐of‐field work

1. Identify the unknown species using the pressed specimens taken in the field and update the sampling sheet accordingly. If you cannot identify these species on your own, seek help from a more experienced colleague.
2. Check which species recorded as dominant in the sampling sheet are present in the ‘Bioindicator species table’ (see annex).
3. Add up the soil cover value of each species belonging to the same bioindicator typology that is present in each sampled sub‐area.
4. In case species that are considered bioindicators of opposite soil characteristics (e.g. dry vs wet soil, acidic vs alkaline soil) appear in the same sub‐area, discard these characteristics: in this case, the bioindicator(s) would be of low reliability.
5. If dominant weed species belonging to different bioindicator typology are not conflicting, the characteristics described in the ‘Bioindicator species table’ can be checked against the actual soil characteristics to verify whether or not the indication given by the table is consistent.
6. Now, for each sampled sub‐area in your field, you have a more detailed description of the main soil characteristics based on the weed species present.

Plant conservation

If you want to conserve the specimens collected in the field keep them in newspaper sheets until the plant is completely dry. Then remove the sheets and attach the specimen to a thick white A3‐sized paper sheet using pins. Add information like the species’ Latin name, the date and place of collection, etc.

• Which conclusions can be drawn?

To have clearer indications of soil characteristics, it is preferable to rely on soil analyses. However, the observation of wild plant (‘weed’) community composition present in a field represents a quick and cheap method to estimate soil characteristics and to draw inferences about the effects of agricultural practices. It should be kept in mind that weed community composition may be affected by several soil factors as well as by past and present management, which may interact at a very small scale. Therefore, the information resulting from using weeds as bioindicators should always be cross‐checked with field records and soil assessments.

Different soil characteristics usually result in different weed species compositions. By focusing on dominant weed species that may be bioindicators, it is possible to extract useful information to tailor farming practices to the actual soil conditions and to improve them where needed. Aspects like soil texture and soil reaction (pH) are less likely to be improved, but others like excess water, soil compaction and reduced soil fertility can be improved by e.g. appropriate drainage, tillage and cover crop practices.