Module 01 >

Monitoring Soils

Module 02 >

Interpreting Soil Results

Module 03 >

Soil Fertility and Nutrient Management

Module 04 >

Common Soil Constraints

Module 05 >

Soil Carbon Capture

Module 06 >

Digital Agriculture for Soils

Module 07 >

Using Biologicals to Build Soil Organic Matter and Resilient Soils

Module 08 >

Managing Irrigated Soils in the Riverina Region of NSW


Interpreting Soil Results

Module 01 >

Monitoring Soils

Module 02 >

Interpreting Soil Results

Module 03 >

Soil Fertility and Nutrient Management

Module 04 >

Common Soil Constraints

Module 05 >

Soil Carbon Capture

Module 06 >

Digital Agriculture for Soils

Module 07 >

Using Biologicals to Build Soil Organic Matter and Resilient Soils

Module 08 >

Managing Irrigated Soils in the Riverina Region of NSW


Laboratories usually send results as a table, like the ones in case study 1 and case study 2. Always check the units and test method when interpreting results.

Many results need interpreting with context, so it is not possible to give an ideal range for each nutrient. For example, a ‘low’
potassium result is different between canola and wheat and will change depending on the soil type and season. Your local agronomist is the ideal person to interpret soil nutrient results and calculate nutrient application rates for your situation.

Some laboratories will give guidelines or ideal ranges. Interpret these with care as they might not be relevant to your situation.


Always check the units and test method when interpreting results.

Quick Guide

Units and conversions

Units of measurement

mg/L = milligrams per litre
meq/100g = milli-equivalent per 100 gram of soil
meq % = milli-equivalent percent cmol/kg =
centimole per kilogram; also written as cmol+ kg-1
ppm = parts per million
μS/cm = microSiemens per centimetre
mS/cm = milliSiemens per centimetre
dS/m = deciSiemens per meter
ha = hectares
t = tonnes


1 mg/kg = 1 ppm = 1 mg/g
1 meq/100g = 1 meq % = 1 cmol/kg = 1 ppm
1 ha = 2.5 acres
1 t = 1000 kilograms
1 deciSiemens/metre (dS/m) = 1
milliSiemens/centimetre (mS/cm) = 1000
microSiemens/cm (μS/cm)


Why test? To work out liming rates, and to check for nutrient toxicities or deficiencies caused by acidity or alkalinity.

Related tests

Exchangeable cations such as aluminium (Al3+). Strongly acidic soils have elevated exchangeable aluminium.

How is it measured?

Soil pH is a measure of acidity or alkalinity. pH is measured in either water or calcium chloride (pHCaCl2). Check whether pH is listed as pHw or pHCaCl2. Some laboratories will include both. The two methods give slightly different readings.
pHw = pHCaCl2+ (0.5)

Interpretation guidelines
Ideal pH range for most crops is 5.5 – 7.0. However, some crops can tolerate a wide soil pH range, though yield starts to suffer as soil pH becomes too acidic or alkaline. Check local cultivar guides or speak to your agronomist about the ideal pH for your crop. Acidity loving crops like blueberries prefer a pH < 5.

Example soil test result and interpretation

The pH of this soil sample is ideal for most crops, but too high for acid-loving crops. There should not be any nutrient toxicities or deficiencies induced by pH.

Problems caused by acidic or alkaline soils

Figure 1. The effect of soil pH on nutrient availability, toxicities, deficiencies, and imbalances.

Source: Msimbira & Smith, 2020.

Issues with...

Acidic soils

  • Nutrient deficiencies (phosphorus, calcium, magnesium, potassium, molybdenum) Toxicities (iron, aluminium, manganese) when pH < 4.5
  • Reduced microbial activity
  • Poor legume nodulation

Alkaline soils

  • Potential salinity and soil dispersion.
  • Copper, iron, manganese, zinc, phosphorus deficiency

Salinity – Electrical Conductivity (EC)

Why test? To check if soil salinity is affecting crop growth. Plants are more susceptible to salinity in their germination and seedling stage than in later stages of growth.

Electrical conductivity (EC) is a measure of soil salinity. The more salts in the soil solution, the higher the EC reading.

Related tests

Electrical conductivity (EC) is a measure of soil salinity. The more salts in the soil solution, the higher the EC reading.

Watch out for

Gypsum can elevate salinity results.

How is it measured?

EC1:5 is measured by mixing 1 part soil to 5 parts water, shaking the sample, filtering out the solids, then measuring the salinity of the water. This EC value does not directly represent the salinity of the soil. Soil texture influences how salt levels affect plant growth. EC1:5 values are multiplied by a texture factor to give a better idea of soil salinity and presented as ECe.

ECe uses the 1:5 measurement multiplied by a soil texture factor (see Table 1).

ECse (EC saturated extract) directly measures salinity the saturated extract of a soil sample. This method is less common because it is time consuming and expensive.

Table 1. Conversation factors from EC1:5 to ECe.


Check the units (dS/m, mS/m, or uS/m) and method when interpreting results. Most soil salinity measurements are presented in deciSiemens per metre (dS/m) but sometimes other units are used. Check both parts of the units – whether dS, mS or μS, and whether m or cm after the slash. The following table converts between the different units.

Interpretation guidelines

Ideal range: Generally, < 0.15 dS/m (EC1:5) or < 2 dS/m (ECe) will not affect crop growth.
Crops vary in their tolerance to salinity. While there are some guidelines on interpreting EC1:5 measurements, most research on plant tolerance to salinity use ECe. If you are reading EC1:5 interpretation guides, they should specify a range for different soil textures i.e., a range for sandy soils, clays, loams, etc.

Table 2. Interpreting ECe

Example soil test result and interpretation


Why test? High chloride levels can disrupt plant growth and make it harder for roots to take up water.

Related tests

Salinity as denoted by EC. High chloride can be a sign of soil salinity. Interpret EC and chloride results together.

How is it measured?

Chloride results are presented as either mg/kg or ppm. Sometimes chloride is estimated by multiplying the EC value by 640.

Interpretation guidelines

Values above 120 mg/kg in sand to sandy loam, >180 mg/kg in loam to clay loam, and >300 mg/kg in clays can indicate a salinity issue.

Example soil test result and interpretation


EC and chloride levels are very low in this sample. Soil salinity is not a problem for crop growth.

What is salinity vs sodicity?

Salinity is sometimes confused with sodicity. Sodic soils have too much exchangeable sodium and are often dispersive, whereas salinity can be caused by a variety of salts, not just sodium. A soil can be both saline and sodic. See the sections on soil dispersion and exchangeable cations for more information.

Organic Matter

Why test? Higher organic matter levels are associated with better soil structure, fertility, and overall soil health.

Related tests

Effective Cation Exchange Capacity (ECEC), Nitrogen. Organic matter is a key driver of soil ECEC (a measure of soil fertility). Nitrogen is released (mineralised) from organic matter throughout the season, providing a source of nitrogen to crops.

Watch out for

Plant material such as roots and leaves in the sample can inflate the result. Check the test method.

Interpretation guidelines

Ideal range: 1 – 2% (broadacre cropping), 2 – 5% (irrigated cropping, pastures in high rainfall zones). The desired range will vary based on soil texture, ECEC and rainfall zone, but generally <1% is very low. Lower rainfall areas, sandier soils (and therefore lower ECEC) will have less OM. Pastures in high rainfall zones could be expected to have 2 – 5% organic matter.

How is it measured?

Organic matter (%) is estimated from organic carbon (%) tests. There are three main ways to measure soil organic carbon:

  • Walkley-Black: Only measures readily oxidisable/decomposable carbon, not total organic carbon (TOC). This method measures about 80% of the TOC.
  • Dumas (LECO): Measures total organic carbon but is not suitable for calcareous soils as lime will be combusted along with organic carbon. If using LECO on a calcareous soil, the lab will do a fizz test and adjust the value.
  • Loss on ignition: Better for soils with high organic matter content. It measures the weight of a dry soil sample before and after burning away the organic matter. It is not ideal for calcareous soils as hot enough temperatures can burn off the carbonates and inflate the results.

Labs measure organic carbon and convert the number to organic matter by multiplying by 1.72 or 1.75. E.g., if organic carbon = 1.45%, the organic matter content is 1.45 x 1.72 = 2.5%.

OM% = TOC% X (1.72 OR 1.75*)

*Labs use slightly different conversion factors.
The test result page should list the conversion factor.

Example soil test result and interpretation

What is Soil Carbon Tests For Carbon Farming?

Soil carbon baseline testing for carbon farming carbon credits is to be done at accredited labs which follows the guidelines of Carbon Farming Initiative - Estimation of Soil Organic Carbon Sequestration using Measurement and Models - Methodology Determination 2021.

In general, gravimetric soil organic carbon content (grams organic carbon / 100g of oven dried soil) is calculated based on TOC (acid-washed/oven-dried), whole soil and fine fraction mass (oven-dried), gravel mass, soil moisture content of air-dried sample, and gravimetric water content.

Cation Exchange Capacity (CEC) and Exchangeable Cations

Why test? Cation exchange capacity (CEC) is a gauge of soil fertility. It describes the amount of exchangeable cations that a soil can hold.

CEC varies depending on clay %, the type of clay, organic matter, and soil pH. Generally, the more clay and/or organic matter, the higher the CEC. Soils with a low CEC have a low resistance to changes in soil chemistry caused by land use.

CEC influences

  • soil fertility (ability to retain nutrients). Higher CEC = greater ability.
  • soil structural stability. Higher CEC soils tend to be more stable.
  • nutrient availability. Soils with a low CEC are more likely to develop nutrient deficiencies.
  • soil pH changes more slowly in soils with a higher CEC.
  • nutrient applications. Low CEC soils have a greater risk of nutrient leaching.

Related tests

pH, Soil dispersion. Acidic soils can have elevated exchangeable aluminium. Dispersive soils may have high (>5%) exchangeable sodium.

How is it measured?

Results are presented as either cation exchange capacity or effective cation exchange capacity. The result is a sum of the percentages of exchangeable cations.

Figure 2. How the colloids hold cations to determine cation exchange capacity (ECE)

Exchangeable Cations

The most common cations in the soil are calcium (Ca2+), magnesium (Mg2+), potassium (K+), sodium (Na+) and aluminium (Al3+). They are often presented as a %, but some labs will also present kg/ha, ppm (mg/kg) and cmol+/kg.

If the soil test report does not provide exchangeable cation percent, you can calculate it:
(cmol+kg-1 / CEC) x 100.

For example, if potassium (K+) = 1.1 cmol+/kg and the ECEC is 7.5:
(1.1/7.5) x 100 = 14.6%.

Some laboratory results will give effective cation exchange capacity (ECEC). This calculation includes the alkalinity inducing cations Ca2+, Mg2+, K+, Na+ and acidity inducing cations H+ and Al3+. It is more accurate than CEC.

Modern tests use cmol+/kg as the unit of measurement. Older tests used meq/100 g or meq%, which is the same as cmol+/kg.

cmol+/kg = meq/100g = meq%

Interpretation guidelines

Ideal range: Varies depending on the soil. Low fertility soils have a CEC <5 cmol+/kg.

Interpretation guidelines

Ideal range: Varies depending on the soil. Low fertility soils have a CEC <5 cmol+/kg.

Example soil test result and interpretation:

Did you know?

Cation exchange capacity can be improved by adding organic matter. Soil organic matter acts as colloids, attracting positively charged elements and compounds including cations and other nutrients.

Figure 3: ’The soil cation exchange capacity' in "Soils for Nutrition: State of the Art" by the Food and Agriculture Organization of the United Nations (FAO), 2022.

Exchangeable Cation

The ideal balance of exchangeable cations is debated in soil science circles. Sometimes an ideal range is listed, which is about 15-25% Mg2+, 5 – 15% K+, >50% Ca2+and <6% Na+. It’s best not to focus too much on cation ratios, except when looking at exchangeable sodium, exchangeable aluminium, and the calcium to magnesium ratio.

Exchangeable Sodium Percent (ESP)

A soil with an exchangeable sodium percent (ESP) >6 is considered sodic. This means the soil could be dispersive. Dispersive soils collapse in water, and dry into a hard, structureless mass. However, ESP alone is not enough to gauge dispersion, as it is affected by soil salinity and other soil factors. For example, soil salinity suppresses dispersion (See Figure 4).

If ESP is > 5, it is worth checking if your soil is dispersive. This is best done by an ASWAT or EAT test (See section on Soil Dispersion). ESP is useful to see sodium itself is hindering crop root growth, and to calculate liming rates if the soil is dispersive.

Exchangeable Sodium
Figure 4: Relationship between Exchangeable Sodium Percentage (ESP) and Electrical Conductivity (EC)
Exchangeable Aluminium

Very acidic soils can have high exchangeable aluminium (Al3+) which is toxic to plant roots. Exchangeable aluminium only becomes an issue when pH in water drops below 5.5. Exchangeable aluminium levels >1% could indicate a problem for sensitive species. Aluminium is an issue for sensitive crops when levels >2 mg/kg.

Example soil test result and interpretation
Ca: Mg ratio

Many soil test results report the Ca2+: Mg2+ ratio as an indicator of soil stability. This ratio is best used as a general guideline when troubleshooting as soils can have a wide range of cation ratios with little impact on soil structure:

  • if the ratio is < 2:1, the soil may have reduced stability. Do a dispersion test to check.
  • a ratio of > 10:1 could indicate Mg deficiency in plants and animals.

The Ca:Mg ratio of this soil is >2, This value, coupled with the low ESP (1.7%) suggest soil stability or dispersion are not an issue.


Nutrient results need to be interpreted for each situation. Determining whether a result is low or high depends on the critical value for the specific nutrient.
The critical value is the number required to achieve 90% of the yield potential (See Figure 5) and will vary depending on the soil, climate, crop, and growth stage. If the test result is above the critical range, the crop probably won’t respond to additional nutrients. Test results below the critical value indicate crop growth is limited by nutrient deficiency and will respond to applied nutrients (See Figure 5).

See Case study 1 and 2 to for an example on how to interpret test results according to critical values.

Figure 5: Crop yield responses to critical values for soil nutrients.


The three main forms of crop available nitrogen in the soil are nitrate-N (NO3-), ammonium-N (NH4+) and organic N. Nitrate and ammonia are the easiest for plants to use. Generally, nitrogen needs to be mineralised from organic matter or applied in a readily available form for crops to use it.

Organic N and ammonium-N are not very mobile in the soil. 0 – 10cm soil tests tend to gauge these numbers well. Nitrate-N is mobile and better assessed with deeper soil testing.

Figure 6. Soil nitrogen (N) cycle. The cycling of N through different forms in the soil determines the quantity of N that is made available for plant uptake.

Source: Agriculture WA.

How is it measured?

The common laboratory tests are total nitrogen, ammonium, and nitrate-N. Laboratories often have their own test methods.

Total nitrogen is the total amount of N in the soil. Most of it will be bound up in organic matter (the organic N) and not readily available for crops to use. Total N is not a guide to the amount of available N. The result is used to calculate the carbon:nitrogen ratio (C/N ratio).

Nitrate-N and ammonium-N are presented as mg/kg or ppm.

Interpretation guidelines

Nitrate-N and ammonium-N can vary considerably in the soil. Need local experience to interpret the results, as they depend on soil moisture, temperature, time of sampling. Generally, if nitrate-N is < 5 mg/kg, the value is considered low.

Total-N is often tested with the LECO (as with testing organic carbon) and results are given as a %.

Example soil test result and interpretation
Interpretation 1

Carbon: Nitrogen ratio

Soil test results sometimes present a C:N ratio. This ratio can significantly impact crop residue decomposition affecting residue cover on soil and crop nutrient cycling, predominantly N. It is important to understand C:N ratio of different crop residues when planning cash crops rotations and cover crops in rotations.

For organic materials, when the C:N ratio is above about 20:1 (20 times more C than N), decomposition slows. In broadacre cropping, this can mean growers need to apply N fertiliser to help stubble breakdown, otherwise soil microbes will use soil N stocks to do their work, temporarily creating an N deficit.

Soils with C:N ratio of 24:1 is the best for soil microbes to make nutrients like N, P and Zn available to plants.


Phosphorus in soil is in several forms such as Available P (PO43-), quantity P (bound) and diffusional P. Quantity P is the component of P off soil solid phases that may be potentially available during the course of the life of the plant when the intensity pool is depleted by plant uptake. Phosphorus availability is highly limited factors such as pH.

How is it measured?

There are four main phosphorus tests. It is important to choose the right test for your situation. Phosphorus moves in and out of different pools in the soil and the different tests can measure different pools of P.

The different test methods extract varying amounts of P from the different pools. Most tests measure readily available P plus some organic/inorganic P which becomes available over the season.

Figure 7: The P Cycle in Soils’ in "Soils for Nutrition: State of the Art" by the Food and Agriculture Organization of the United Nations (FAO), 2022.

Interpretation guidelines

Colwell P test results should be interpreted with the phosphorus buffering index (PBI), which describes the ability of a soil to ‘lock up’ P (see PBI below).

The critical value will depend on the crop, location, and P test used for Colwell, Olsen and Bray methods. As the DGT-P test is still relatively new, there are fewer interpretation guidelines.

Example soil test result and interpretation
Phosphorus Buffering Index

The phosphorus buffering index (PBI) describes the ability of soil to ‘lock up’ or immobilise phosphorus. The higher the PBI, the more P it will lock up which affects P fertiliser application rates.

P quickly binds on exchange sites on higher PBI soils, making it unavailable for plants to use. Soils with a low PBI do not lock up much P, meaning a greater risk of P leaching.

Higher PBI soils need more P fertiliser. Over time the exchange sites are ‘filled’ and more P is available for crops to use.

Table below shows PBI and Colwell P critical values for pastures, wheat, and potatoes. The critical value is the Colwell P soil test value needed to achieve 90% yield with the respective PBI.


Reference: Wheat- Moody, 2007; Potato - APAL


A pasture on a soil with a PBI of 80, needs a Colwell P value of at least 34 mg/kg. Potatoes on the same soil need 65 mgP/kg.


Available potassium is measured as either extractable K (Colwell-K) or exchangeable K. Colwell-K usually has higher results because the test measures exchangeable K+ + water soluble K+ + a small amount of fixed K.  

On sandy soils, there is a rough relationship where exchangeable K+ x 391 = Colwell-K.

If CEC has been tested, the results should list exchangeable K.

To convert between cmol+/kg and mg/kg:

mg/kg of extractable or exchangeable K+ = 391 x cmol+/kg

Interpretation guidelines

Use the following as general guidelines only to see if test results are significantly higher or lower than these values. For proper crop nutrition, use critical values suited to your region, soil type and specific crop. For example Lucerne has higher K demands. Also, K demand increases with clay content.

Example soil test result and interpretation

Only exchangeable potassium was measured on this soil sample. The result, 430 mg/kg is well above the general guidelines given above, indicating potassium deficiency is not an issue on this soil at the moment.


Legumes need sulphur to fix nitrogen. Sulphur leaches easily and deep soil testing gives as more accurate picture of soil sulphur sticks.

How is it measured?

Most sulphur tests measure the sulphate form of S, the form plants take up. The two common tests are KCl-40 and MCP.

The KCl-40 test is commonly used as it takes into account the available S and some of the S that will become available when organic matter mineralizes over the season. It is considered more accurate because it tests these two pools. For soils high in organic matter, request the KCl-40 test. This test tends to be more reliable on sandier soils but can give low readings after long, dry periods or leaching rains.

The MCP test does not sample the organic pool of S and can underestimate S supply.

Example soil test result and interpretation
This test used the KCl method, estimating available S and some S from the organic pool that will become available over the season. With 7.8 mg S/kg, sulphur is low to marginal, and crops will benefit from S fertiliser.

Trace elements

Trace elements (micronutrients) are only needed in small amounts but are just as important as macro nutrients (N, P, K, etc.). The commonly tested trace elements are manganese (Mn), iron (Fe), boron (B), zinc (Zn), and copper (Cu).

How is it measured?

Trace elements are usually tested using the DTPA method, but this method is best used in neutral to alkaline soils. The EDTA method is more accurate in acid soils.

Interpretation guidelines

Soil testing is not very reliable to gauge plant available nutrients but is better used to track overall nutrient stocks. Plant tissue testing is a better gauge of trace element deficiencies and toxicities than soil testing. When it comes to trace elements it’s better to ask the plant directly with tissue testing than infer from soil test results.

As with the macro nutrients, critical levels will vary between soil types and plants.
Things to look out for:

  • High pH soils and carbonates make Mn, Zn, Fe and Cu less available. Fe deficiency often observed on high pH soils
  • Acidic soils have lower availability of Mo.
  • Fe deficiency can be amplified by poor drainage or wet conditions
  • Sandy soils are often inherently low in trace elements
  • Can have Mn and Cu deficiencies in drier conditions where levels are marginal

Levels outside those in the table might warrant a closer look.



Soil Dispersion

Why test? To check if the soil needs gypsum; to gauge the erosion potential of the soil.

Related tests: exchangeable sodium percent and salinity. Dispersive soils tend to have elevated (>6%) levels of exchangeable sodium. Salinity can suppress dispersion.

How is it measured?

Dispersion testing can be done at home, or samples can be sent to the lab. Air-dried peds are placed into a dish of deionised water, and dispersion and slaking are measured after 10 minutes, 2 hours, and 24 hours. Methods vary slightly depending on whether the lab is using the Emerson Aggregate Test or the ASWAT Test.

Figure 8. A simple soil dispersion test - DPIRD WA

Interpretation guidelines
Lab results show a number as the result, and may include the interpretation e.g., ‘moderately dispersive’. The more dispersive the soil is, the more gypsum is needed to improve soil structure. The ESP is used to calculate the gypsum rate. Soil dispersion is outlined in much greater detail in Managing dispersive soils: practicalities and economics.


Soil texture describes whether a soil is a sand, loam, sandy loam etc. It can be tested by sieving the soil and measuring the proportion of sand, silt and clay, but is usually measured by ‘feel’ - working the soil by hand and measuring the length of ribbons of soil. In the lab, soil texture is determined using the soil texture triangle (Figure 9).

Sandier soils tend to have lower organic matter, hold less water and nutrients, and are prone to water repellence. Sandier soils usually benefit from split nutrient applications as they are more prone to leaching, particularly of N, K and S. Clay soils are more fertile but can have structural problems (high soil strength, dispersion) and are more prone to waterlogging.

Figure 9. Soil texture triangle
Source:  OER Commons


Soil colour is a guide to minerals and drainage. Iron plays a decent role in soil colour. In well-drained soils, iron Is red. As oxygen levels decrease, iron becomes more yellow.

Brighter colours indicate better drainage.
Regularly waterlogged soils tend to have grey, green or even blueycolours.

Soil mottles, patches of different colour, indicate the soil is periodically waterlogged.


Troubleshooting odd results

Sometimes the laboratory results don’t make sense. The most common cause of strange results are user error while sampling and storing samples, or choosing the wrong test method for the soil.

Other causes of error:

  • Mislabelling samples
  • Leaving soil samples sealed in plastic containers for too long without being dried which can impact speciation of nutrients
  • Leaving soil samples in the sun in high temperatures
  • Low pesticide results. sampling for pesticides often need to be stored in a foil or glass container as some may adhere to plastic sample bags

Natural fluctuations

If monitoring soil properties, the results will fluctuate over time with the season and soil moisture conditions. Many soil properties are affected by seasonal changes, including pH, soil carbon, bulk density and nutrients. Collecting samples at the same time each year helps with consistency.

For an example, pHw tends to be lower in drier conditions, with values fluctuating by up to 0.6 pH units in a year. Soil pHCaCl2 is less influenced by seasons.