In our second part of our look at Visual Soil Assessments (VSA’s) we dive into the key plant indicators that a VSA looks at.
A quick recap from the last blog – What is a Visual Soil Assessment and why are they important?
A visual soil assessment is a ‘quick and simple method to assess soil condition and plant performance, and how they are affected and modified by farm-management practices’ (Shepherd, 2009).
While the name VSA implies that the technique is only about soil, the method concurrently gets farmers to look at plant performance indicators. This ensures that farmers are getting insight into the whole picture of how their soils and plants are connected and influencing each other.
Visually assessing soil and plant indicators is important because all physical indicators (excluding soil texture), are dynamic indicators. This means that these indicators are capable of changing under different management and land use pressures (Shepherd, 2009). For this reason, undertaking annual monitoring of VSA provides farmers with data over time that provides insight into how management of soil is impacting their farming operation through soil health and plant performance.
So, what plant indicators do we look at?
Pasture quality
Pasture quality is of importance to all farmers, so it makes sense as to why it is a key plant indicator for a visual soil assessment. Pasture species and pasture composition are also important factors in overall pasture quality.
Pasture quality is taken by observation with key indicators to quality being green leaf herbage and legume content, lack of dead matter, and pasture composition reflecting the original seed mix planted in the paddock, with good, moderate and poor condition being defined in the table below.
You can also undertake a test to measure the Brix level to get further insight into pasture quality. Brix level measures the sugar content in the sap or juice from plants. This gives an indication as to whether plants are effectively photosynthesising and converting sunlight into energy (Quorum Sense, n.d.). Brix is measured using a refractometer and you can learn more about this measurement technique here.
Clover nodules
Assessing clover nodules gives insights into the nitrogen-fixing ability of a pasture (Shepherd, 2009). The density, size, colour and depth of clover nodules is able to tell us this.
If a clover nodule is actively fixing nitrogen, there will be a pink/red colour on the inside of the nodule (Shepherd, 2009). The redder the colour, the more active the nodule is in its fixing of nitrogen.
Clover nodules are assessed by removing three to four clover plants with a space, pushing the spade to a depth of 250 mm. Then, the plant is gently shaken to shake the soil free and expose the root system and clover nodules. Then you will observe the number of clover nodules there are, their size and colour, as well as the depth to which they occur along the roots, and the colour of the leghaemoglobin within the nodule (Shepherd, 2009).
Shepherd (2009) recommends that clover nodules are best assessed in the spring when leaf growth and nitrogen demand is great, but it can also be assessed at other times of the year provided that the plant is not under stress.
Pasture Growth
Just as pasture quality is important, pasture quantity is also of key importance to farmers. It can be measured using a sward stick, or other pasture growth measurement method, and recorded for reference.
Pasture growth rates are important to assess as they are dependent on many things such as:
- good soil structure and aeration
- earthworm and microbial activity
- available water
- good soil fertility
- seasonal weather conditions
- maintenance of good residuals after grazing
Root length and Density
In the soil indicators we looked at potential rooting depth – the depth that roots can potentially exploit before reaching a barrier to root growth (Shepherd, 2009). Roots are also an important plant indicator in a visual soil assessment. This looks at root length and density as they provide good indications about the condition of the plant root system (Shepherd, 2009).
Shepherd (2009:46) state that pastures with deep roots and a high root density are able to explore and utilise a greater proportion of the soil for water and nutrients, compared to pastures that have a shallow and thin root system. This speaks to the importance of having good root length and density, and an evenly distributed root system in order to ensure that the plant is getting all it can from the soil resource.
Deep and dense root systems are really important for raising production while also having significant environmental benefits, such as increased nutrient uptake, reduced reliance on application of fertiliser and nitrogen to generate growth, reduced leaching, as well as increased drought resilience (Shepherd, 2009:46).
Root length and density is assessed by observing this from the soil sample taken for the soil indicators. Shepherd (2009) defines good, moderate, and poor condition as:
- Good = good root length and density with an evenly distributed root system
- Moderate = moderate root length and density with the root system being somewhat patchy
- Poor = poor root length and density with the root system being restricted to limited areas
For more information on Visual Soil Assessments and the indicators, you can access Graham Shepherd’s ‘Visual Soil Assessment Field Guide for Pastures’ here.
And EXTRA FOR EXPERTS: If you want our comprehensive Soil Health Monitoring Template suitable for use with Resolution, the farming app – Download it here!
Visual Soil Assessments are a great place to get started with ecological monitoring. More comprehensive ecological monitoring, such as Ecological Outcome Verification monitoring, uses a much greater set of metrics to look at monitoring soil and ecosystem health. Keep an eye out for a future blog for more info on this, or get in touch with the team at The Whole Story Savory Hub if comprehensive ecological monitoring is something of interest to you.
