Soils: Visual Soil Assessment (VSA)

Introduction

The Procedure

1. When Should Soil Condition Assessment be Carried Out?

The following recommendations are given as a general guide:

  • For arable-cropped soils – Test once a year after harvest and before cultivation. You could make a second test after the final cultivation to check the condition of the seedbed.

VSA can be carried out effectively and reliably over a range of soil moisture levels, a characteristic that enhances the robustness of VSA as a tool. However, it is suggested that the VSA is carried out when it is judged that the soil is at the correct moisture content for cultivation, or is sufficiently dry to prevent compaction by wheeled traffic.

If you are not sure, apply the ‘worm test’. Roll a ‘worm’ of soil on the palm of one hand with the fingers of the other until it is 50mm long and 4mm thick for cropped soils. If the soil cracks before the worm is made, or you cannot form a worm (e.g. if the soil is sandy), the soil is suitable for testing. If you can make the worm, the soil is too wet for testing.

As long as the soil moisture content is right, test at a similar time each year. This will make your results more comparable from year to year.

2. Setting Up

It is important to be properly prepared to carry out soil quality assessments.

  • Time – Allow about one hour per field. The assessment process takes about 15 minutes for each sample, and you should sample three or four sites in each field.
  • Reference sample – Take a small soil sample from an un-cultivated area. The field to be sampled will have had a history of grazing or cropping. Taking a spade-depth sample from an area of the field boundary where there has been little if any cultivation or treading, allows you to see the relatively unaltered soil. This helps with giving the correct visual score to the soil colour matrix indicator.
  • Sites – When carrying out field assessments, avoid areas such as headlands or loading areas, which may have had heavier traffic than the rest of the site. VSA can also be used, however, to assess the effects of high traffic loading on soil quality; tramlines, for example, can be selected and the results compared with low traffic areas. Select sites that are representative of the field.
    It is important to record the position of the assessment sites in your field accurately so that you can come back to them for future monitoring.
  • Set up the equipment - At the chosen site, put the square of wood in the bottom of the plastic basin, and spread out and anchor down the plastic bag beside it.
3. Site Information

Complete the site information section at the top of the scorecard. Then record any special aspects you think relevant in the notes section at the bottom of the reverse side of the scorecard (for example, wet weather at harvest last season; soil heavily poached by stock grazing stubble; topsoil blew off two years ago, etc.).

4. Carrying Out The Test
  • Take the test sample – Dig out a 20cm cube of topsoil with the spade. If the topsoil is less than 20cm deep, trim off the subsoil before moving on to the next step. The sample provides the soil from which most of the soil state indicators are assessed.
  • The drop shatter test – Drop the same test sample a maximum of three times from a height of 1m (waist height) onto the wooden square in the plastic basin. Then transfer the soil onto the large plastic bag and grade so that the coarsest clods are at one end and the finest aggregates are at the other end.

Systematically work through the scorecard, assigning a visual score (VS) to each indicator by comparing the soil laid out on the plastic bag with the photographs and description in the relevant section of the field guide.

5. The Plant Indicators

You can normally complete the plant indicator scorecard at the time you carry out the soil indicator assessment, by comparing your recollection of crop development or observations of the pasture, with the photographs in the field guide manual. But some plant indicators, such as the degree and nature of root development and grain development, cannot be assessed at the same time as the soil indicators. Ideally, these should be assessed at plant maturity.

The drop shatter test. The sample is dropped a maximum of three times from a height of 1m (waist height) onto the wooden square in the plastic basin. The soil is then transferred onto the large plastic bag and graded so that the coarsest clods are at one end and the finest aggregates are at the other end.

The plant indicators are scored and ranked in the same way as soil indicators: a weighting factor is used to indicate the relative importance of each indicator, and the contribution of each to the final determination of soil condition.

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