You can’t manage what you don’t measure is a term we often hear in agriculture, and it is one that only continues to gain importance in our farming businesses. There are many ways now that we can collect data and information for our farming businesses, and one of those ways is to look at environmental factors. We have touched on Stream Health Assessments and their value on farm, and this week we start to take a look at Visual Soil Assessments which are another great monitoring tool for farmers to make use of. This first part of a two part series will take a look at the key soil indicators of a Visual Soil Assessment, or VSA for short.
So, what is a VSA?
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.
What you Need and Where to Start
To get started with a simple VSA, all you need is a spade. It can also be helpful to have something like an old seed bag to put your soil sample on and make it easier to look at different components such as soil structure.
VSA involves digging a 200mm x 200mm square hole that is approximately 300 mm deep. This soil sample is then used to assess various soil indicators.
It is useful to compare your holes and soil in your paddock to one dug close to the fenceline as this shows the contrast between the soil when it is ‘farmed’ compared to when it is left undisturbed. This is a great way to really compare and contrast the state of soil and how management decisions may be influencing your soil health and condition in your paddocks. It is also useful to undertake a range of VSA samples across different paddocks, land management unit types, and management practices to see how soil differs across the different ways that your farm is managed. It is best to undertake these on the same day to be able to really compare the soil conditions on the same day.
So, what soil indicators do we look at?
Soil Structure
In a VSA, soil structure looks at the size, shape, firmness, porosity, and relative abundance of soil aggregates and clods (Shepherd, 2009). Soils with good structure have easily crumbled, fine and porous aggregates, compared to soils with poor structure that have large, dense, very firm and ‘blocky’ clods (Shepherd, 2009).
Soil structure is very important for pasture performance as it regulates:
- the exchange of water and air through the soil
- soil temperature
- nutrient supply
- root penetration and development
- the resistance of soil to damage from compaction
Soil Porosity
Porosity is important alongside soil structure as it gives insight into how air and water is able to move in the soil. Soils with good structure will be able to have water and air movement through macropores, between and within soil aggregates, however, soils with poor structure may not have food aeration and drainage due to lack of pores that these can move through (Shepherd, 2009).
Soil porosity is also important for the ability for roots to penetrate through the soil, as roots are unable to grow through compacted and tight soils. If roots are not able to grow downwards and access water and nutrients that are available in the soil, this has a detrimental effect on plant health, and overall pasture performance.
Soil Colour
Comparing soil colour in pasture compared to under a fenceline is another important visual indicator.
In general, the darker the colour of the soil, the greater the amount of organic matter and humus in the soil (Shepherd, 2009). This is important as soil organic matter plays an important role in determining soil health as it regulates most physical, chemical, and biological processes in the soil (Shepherd, 2009).
Shepherd (2009) state that soil organic matter:
- promotes infiltration
- helps to develop and stabilise soil structure
- reduces the potential for wind and water erosion
- plays a key role in maintaining cation exchange in the soil
- indicates whether the soil is functioning as a carbon ‘sink’ or as a source of greenhouse gases.
Earthworms
On a previous blog we highlighted the role of earthworms as a indicator of soil biological health. The benefits of earthworms include:
- playing a role in soil formation (microaggregates, soil porosity),
- facilitating nutrient cycling through decomposition and increasing nutrient availability,
- feeding on organic matter and redistributing it throughout the soil profile through worm casts,
- nutrient supply associated with burrowing encourages root growth,
- increasing soil porosity, and
- improving water infiltration rates and water holding capacity.
Soil Smell
Soil smell is also a good indicator of the amount and activity of soil life. This is because soil smell is primarily determined by the type and amount of gases that are given off by the respiration of soil microbes, as well as the type and amount of organic matter and humus that is present in the soil (Shepherd, 2009).
- Good = distinct rich, earthy, sweet, or fresh smell
- Moderate = slight earthy, sweet odour or a ‘mineral’ smell
- Poor = soil has a putrid, sour, chemical or unpleasant smell.
Potential Rooting Depth
Potential rooting depth refers to the depth that roots can potentially exploit before reaching a barrier to root growth (Shepherd, 2009).
The greater the rooting depth, the greater the:
- available water-holding capacity of the soil
- availability of soil nutrients
- resulting dry matter production
- effective utilisation of fertiliser by the plant
- reduction in leaching
- drought resilience
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!