VSA is based on the visual assessment of key soil ‘state’ and plant ‘performance’ indicators of soil quality, presented on a score card.
Soil quality is ranked by assessment of the soil indicators alone. It does not require knowledge of paddock history. Plant indicators, however, require knowledge of immediate crop and paddock history. Because of this, only those who have this information will be able to complete the plant indicator score card satisfactorily.
Visual Scoring (VS)
Each indicator is given a visual score (VS) of 0 (poor), 1 (moderate), or 2 (good), based on the soil condition observed when comparing the paddock sample with three photographs in the field guide manual. The scoring is flexible, so if the sample you are assessing does not clearly align with any one of the photographs but sits between two, a score in between can be given, for example 0.5 or 1.5. An explanation of the scoring criteria accompanies each set of photographs.
Because some soil factors or indicators are relatively more important for soil condition than others, VSA provides a weighting factor of 1, 2 or 3. For example, soil structure is a more important indicator (a factor of 3) than clod development (a factor of 1). The score you give each indicator is multiplied by the weighting factor to give a VS ranking. The total of the VS rankings gives the overall ranking score for the sample you are assessing.
The VSA Toolkit
The equipment needed for the VSA 'toolkit' is simple and inexpensive. It comprises:
- 1 spade - to dig out a 20 cm cube of topsoil.
- 1 plastic basin (approx. 35x35x19 cm) - to contain the soil when carrying out the drop shatter test.
- 1 hard square board (approx. 26x26x1.8 cm) - to fit the bottom of the plastic basin on to which a soil cube is dropped for the shatter test.
- 1 heavy duty plastic bag (approx. 74x49 cm) - on which to spread the soil, after the shatter test has been carried out.
- 1 VSA field guide (weather proof) - to make the photographic comparisons.
- 1 pad of score cards - to record the visual score (VS) for each indicator. Separate pads are needed for cropping and pastoral grazing on flat to rolling land.
1. When Should Soil Condition Assessment be Carried Out?
The following recommendations are given as a general guide:
- For cropping 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.
- For pastoral grazing soils - Test once a year in late winter or early spring.
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.
2. Setting Up
It is important to be properly prepared to carry out soil condition assessments.
- Time - Allow about an hour per paddock. The assessment process takes about 15 minutes for each sample, and you should sample three or four sites in each paddock.
- Reference sample - Take a small soil sample from under the fence. The paddock to be sampled will have had a history of grazing or cropping. Taking a spade-depth sample from under a fenceline where there has been little if any cultivation or treading, allows you to see the relatively unaltered soil.
- Sites - Select sites that are representative of the paddock. It is important to record the position of the assessment sites in your paddock accurately so you can come back to them for future monitoring.
- Set up the gear - 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 score card. Then record any special aspects you think relevant in the notes section at the bottom of the reverse side of the score card (for example, wet weather at harvest last season; soil heavily pugged by stock grazing stubble; topsoil blew off two years ago, etc.).
4. Carrying Out The Test
- Take the test sample - Dig out a 20 cm cube of topsoil with the spade. If the topsoil is less than 20 cm 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 1 m (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 score card, 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 score card 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 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.
Using the VSA Results
VSA allows you to assess soil condition in a paddock but does not solve any identified soil condition issues. Once soil is degraded, it can take a long time (sometimes decades) to recover. To help land managers preserve or improve soil condition, guidelines are included in Volume 2 of the field guide for the sustainable management of cropping and pastoral grazing on flat to rolling country.