Measuring Soil Health

Introduction

Soil underpins the entire farm system. Healthy, well-managed soils support productive and healthy crops and pasture, which in turn supports a profitable and resilient farming system. A soil that accumulates organic matter will be sequestering carbon, improved fertility and water holding capacity and increased productivity.

Soil analysis can be a useful tool for understanding overall soil health and identifying areas that may require management or action. Soil analysis doesn’t have to be limited to sending samples to the lab for analysis, it can be as simple as getting out your spade and digging deeper into soil structure.

This page provides an overview of the types of tests that you can do to understand overall soil health.

1. Soil Texture

Why is this important?

Soil texture refers to the relative properties of clay, silt and sand in a soil. Soil texture cannot be altered but is important to understand as it impacts on soil structure, aggregate stability, the amount of carbon present and the soil’s ability to sequester more carbon.  Soil texture will help to identify the risk factors that impact on your soil texture, and allow you to develop mitigation options to avoid adverse effects (like compaction, water logging and erosion).

How is it assessed?

To understand soil texture, rub some moist soil between finger and thumb. Sand is a larger particle size so tends to feel gritty, and doesn’t hold its shape when moulded into a ball. Silt feels smooth, silky or floury. Clay feels sticky when wet, looks shiny when smeared and holds together in a ball. This diagram in the RB209 explains how to hand texture your soil.

2. Soil Structure

Why is this important?

Good soil structure is vital for crop productivity and soil health. It supports and regulates biological activity, water movement and storage, soil temperature, gas exchanges and nutrient cycling. The structure of soil should allow for an even distribution of air, water, mineral particles and soil organic matter. 

How is it assessed?

A typical method of assessing soil structure is VESS (Visual Evaluation of Soil Structure). This is a scoring system which rates the soil in terms of its structural condition from 1 (friable and good structure) to 5 (very compact and impacting on plant root growth and function). The VESS test can be completed at the top of the soil profile pit (between 0-10cm) and then lower down (between 10-30cm) to assess condition throughout the soil profile. More detail on the VESS method can be found here.

3. Bulk Density

Why is this important?

Bulk density is the mass of soil in a given volume. Bulk density can be used as an indicator of pore space, soil compaction and will normally increase with soil depth.  Bulk density is also a critical part of being able to calculate the carbon stock within a field.

Bulk density is usually reported as g /cm3 of soil. It can range from between 0.8g/cm3 soil to 1.8g/cm3, and will vary depending on soil type. Lighter, sandier soils will have a higher bulk density than clay soils. If the soil’s bulk density is over 1.6g/cm3 it can  impact on root growth.

How is it measured?

We measure this at three different depths (O-10cm, 10-30cm and 30-50cm) that correspond to the depths that we measure organic matter and organic carbon at. It can be measured in various ways, however at FCT we use the undisturbed core method. This requires using an open ended steel cylinder to extract the known volume of soil from each of the depths down the soil profile. The soil is then removed, processed (stones and roots removed, stones weighed and assessed for volume), dried and weighed. Bulk density is measured in g/cm3.

By taking measurements at three depths, we can obtain a picture of the carbon yield across the soil profile. Carbon yield (reported as t/ha) provides a much more nuanced metric than a simple percentage of organic matter, and allows for a better understanding of where the carbon is held within the soil profile. 

4. Soil Organic Matter (SOM)

Why is this important?

SOM is the organic component of soil, made up of materials such as plant residues, living organisms and decomposing organic matter. Soil organic matter contributes to healthy soil function and crop productivity in many ways including enhancing soil aggregation and the soil’s water holding capacity, allowing optimal nutrient cycling and providing food for the living organisms which inhabit the soil.

Soil organic matter can be broken down into three distinct groups, this includes plant roots and the living microbial biomass; active soil organic matter and stable soil organic matter, often referred to as humus. The average amount of organic matter in UK agricultural soils can vary between 1 – 7%. The soil organic matter fraction also includes the soil organic carbon. Often the soil organic carbon is calculated as 58% of the soil organic matter, although this can vary depending on the soil type.

Analysing SOM at three different depths within the soil provides an understanding of how the carbon is dispersed throughout the soil profile. Generally carbon near the surface will fluctuate more than carbon held at depth due to carbon cycling.

How is it measured?

There are two main methods that are used to test for soil carbon / soil organic matter. It is important to be consistent in your testing approach:

  • Loss on Ignition: Most common test for SOM. Tends to be a cheaper test and the best for helping inform on-farm management decisions. This test is not standardised so can vary between labs, so is important to remain consistent with lab choice. The analysis measures soil organic matter content, which can then be converted using a calculation method to determine the relative carbon content. LOI provides a more rounded approach for assessing soil health.
  • Dumas: A more accurate and standardized test for analysing soil organic carbon, however it does not assess overall soil health. In alkaline soils, it’s important to ensure that the lab method accounts for inorganic carbon as well as providing the organic carbon content which is reported as a percentage. Both are important parts of the farm carbon cycle but react differently to management practices. The amount of soil which is analysed is very small (often 2g) as such, it is important to take good samples that are representative of the area being tested.

5. Soil Organic Carbon Yield

What is it?

The amount of carbon held within your fields (to the depths measured). It is reported in tonnes per ha, and can provide a more detailed result than just a soil organic carbon percentage.

How is it calculated?

We multiply 1 ha by the depth of soil (0-10cm, 10-30cm or 30-50cm), the bulk density and the soil organic carbon percentage. This gives the amount of carbon in tonnes/ha in your soil at each depth. Soil organic carbon yield can only be calculated when the bulk density is assessed.

6. Nutrient Analysis & pH

Why is nutrient testing important?

Testing soils for their nutrient status provides an indication of the nutrients available to the crop from the soil. Typically these are phosphorus (P), potassium (K) and magnesium (Mg) but more detailed nutrient analysis can be carried out by the lab on request which may include soil mineral nitrogen testing, or the availability of trace elements.

How is it measured?

We send soil samples to labs for analysis. Nutrients typically are measured in mg/l. The indices reported come from the Defra Index scale and depend on the concentration of nutrients within the soil sample. 

Why is understanding pH important?

Soil pH is a measure of the acidity and alkalinity of the soil. The natural soil pH is determined by the chemical composition but this can be altered through natural and agricultural processes. Soil pH affects the availability of nutrients within the soil and therefore crop productivity, and is therefore a key parameter to understand. 

pH can range from strongly acid (less than pH 5.5) to strongly alkaline (more than 8.5). The target pH for grassland is around 6 and for arable soils is 6.5. If the pH results are low, lime can be added. If the pH is low, then any applied nutrients will not be utilised effectively, as such, addressing pH issues will help with fertiliser use efficiency.

7. Aggregate Stability

Why is this important?

Soil aggregates are the building blocks that make up soil. How stable these aggregates are is an important factor in long term soil health and the development of a resilient soil ecosystem that will deliver on-farm benefits. Soil aggregation is also considered a good indicator of soil organic matter levels. 

How is it measured?

A handful of soil from each profile pit is taken away and air dried for 4 days. Once dry, three lumps of soil are submerged in water and assessed for how well they hold together after 5 minutes and then again after two hours. The lumps of soil are scored using a scale of 0-4 with 0 being good and the lump remaining intact and 4 the score when the lump breaks down.

8. Earthworm Counts

Why are earthworms important?

Earthworms are one of the indicators for soil biology and soil health. They are important soil engineers, redistributing and mobilising nutrients, cycling organic matter and carbon throughout the soil profile, and improving water infiltration.

Earthworms in agricultural soils can be grouped into three ecological types:

  • Epigeic – litter dwelling earthworms
  • Endogeic – topsoil earthworms
  • Anecic – deep burrowing earthworms

How are they counted?

To measure earthworm numbers, we dig a soil pit that is 20cm x 20cm x 30cm deep and hand sort the soil to count the number of earthworms present. This can then be broken down into types and numbers of adults and juveniles. The higher the value the more worms were present. More details can be found at GreatSoils.

9. Infiltration 

Why is this important?

Soil water infiltration is a good indicator of soil structure which can highlight areas of compaction. A short infiltration time can indicate that the soil is healthy due to the high number of pore spaces allowing the water to infiltrate. Pore spaces are important for root development, soil aeration and water retention. Where compaction is present, the soil pores are effectively squashed together leading to reduced infiltration and risk of runoff and erosion. 

How is it measured?

To measure soil infiltration a cylinder and a known volume of water is required. The cylinder is inserted into the soil a few inches and the water poured in. A stopwatch is required to measure the time it takes for the soil to infiltrate. A detailed guide on carrying out the infiltration test can be found here.