Measuring soil nutrients, pH and organic matter

A soil sample is simple and straightforward to collect, and analysis will tell you the levels of nutrients, pH and organic matter. This information can be used to plan the use of nutrients and lime, saving you money while also protecting the environment.

Laboratory analysis of a representative soil sample will provide valuable information on the level of:

• Phosphate
• Potash
• Magnesium
• pH
• Soil organic matter*
• Micronutrients*

*Additional charges may apply, please ask the laboratory.

This information informs your nutrient management plan. Where sampling is robust, results can determine the need for variable rate nutrient application. It is usually safe to use soil analysis results for phosphorus, potassium and magnesium as a basis for fertiliser recommendations for up to four years from the date of sampling.

Soil sampling must be done accurately to avoid misleading results and expensive mistakes.

The soil in each field should be sampled:

• Every three to five years
• Ideally, at the same point in the rotation
• Well before growing a sensitive crop (e.g. sugar beet)
• Immediately after the harvest of the previous crop

Do not sample:

• Within six months of a lime or fertiliser application (except nitrogen)
• When the soil is very dry
• In headland or in the immediate vicinity of hedges, trees or other unusual features

The soil sample must be representative of the area sampled. Areas of land known to differ in some important respects (e.g. soil type, previous cropping, applications of manure, fertiliser or lime) should be sampled separately.

Small areas known to differ from the majority of a field should be excluded from the sample.

Ideally, the sampled area should be no larger than four hectares.

Clean tools before starting and before sampling a new area.

Walk a ‘W’ pattern across the sampling area, stopping at least 25 times.

• Arable and horticultural rotations: At each point, collect a subsample (core) to 15 cm depth using a gouge corer or screw auger.

However, if the land is min-tilled, phosphate and potash will tend to accumulate near the soil surface, and a 15 cm sample will overestimate nutrient concentrations to normal plough depth. In this case, samples are better taken to about 23 cm. 

• Grassland: At each point, collect a subsample (core) using a gouge corer or screw auger.
  • Sample to 7.5 cm depth in long-term grassland fields
  • Sample to 15 cm depth in short-term (5 years) leys or grassland about to be ploughed and re-seeded

To minimise the impact of local variation (e.g. the presence of old fertiliser granules), mix samples collected in the field in a clean bag or bucket to form a bulked sample. Send a well-mixed representative subsample to the laboratory for analysis. Use appropriate packaging (normally available from the laboratory) and label samples clearly, providing as much information about the field and crop as possible.

Although laboratory analysis of pH is useful, soils where acidity is known to occur may need more frequent testing than the 3-5 year cycle used for phosphate, potash and magnesium.

Since acidity can occur in patches, spot testing with a soil indicator test across the field is often useful.

Soil indicator tests can be useful on soils which contain fragments of free lime, since these can give a misleadingly high pH when analysed following grinding in the laboratory.

Soil organic matter can be measured in different ways. The most common in the UK are the loss on ignition (LOI) or dry combustion (Dumas) methods. Where you are trying to detect changes over time, it is important to use the same laboratory and method year on year. Variations in the results from the same sample can be due to the use of different temperatures, duration of heating and pre-treatments in different laboratories.

The presence of roots or added organic materials (residues, manures, composts) in the sample can give odd results, so obtaining a well-mixed representative sample for analysis from a number of separate soil samples collected in the field and mixed together is very important.

Loss on ignition method

Loss on ignition (LOI) is the most common method used to estimate the amount of organic matter present in a soil sample. The soil sample is oven dried and then heated to 400°C; at this temperature organic matter is burned off. By measuring the weight lost by the sample the amount of soil organic matter can be estimated.

Dry combustion (Dumas) method

This is a measure of the total carbon content of the soil (after removing any mineral carbonate) by dry combustion and elemental analysis. The amount of carbon measured can be converted into an estimate of organic matter (and vice versa).

Analysis for micronutrient deficiencies may be done on the same soil samples taken every 3 to 5 years for routine analysis of phosphate, potash, magnesium and pH.

If a deficiency is suspected, ensure that an analysis package is chosen that includes the micronutrients in question.

The soil health scorecard provides a framework to monitor soil health on a rotational basis.

The Excel-based tool uses soil analysis results for core soil health indicators and compares them to typical ranges for UK soil types and climate regions (benchmarks).

A completed scorecard automatically produces a 'traffic-light' snapshot of soil health to guide your management decisions.

A list of soil testing companies

Information on nutrient management

Find out about the concept of base cation saturation ratio – the Albrecht system

Find out more about the role of soil organic matter