How to interpret a soil analysis ?

It helps in assessing :

• Physical characteristics which give an indication on the capacity of the soil to utilise and provide mineral elements to the plant.
• The chemical composition of the soil, i.e the fertilising mineral element content of the soil.

Physical analysis

This gives an idea of the soil’s constitution. Physical criteria are given in percentages of : clay, silt, sand and organic matter which are the 4 main constituants of an aggregate of soil. The size and proportion of the particles in presence give rise to different soil behaviour in terms of permeability, plasticity, and sensitivity to packing.

Physical analysis is not an absolute necessity on permanent pastures. It is much more relevant when the soil is due to be prepared for sowing a crop for example..

Chemical analysis

A basic chemical analysis (with a CEC) is enough to analyse the fertilsing capacity of a pasture. It represents the capacity of the soil to retain fertilising elements, and the quantity of fertilising mineral elements present in the soil.

• Physico-chemical conditions:
  • Values of Cation Exchange Capacity (CEC)
  • pH
  • Limestone
• Minerals in presence:
  • Exchangeable cations : Ca2+, Mg2+, K+, Na+
  • Absorbable phosphorous
  • Sulphur
  • Trace elements : Iron, manganese, copper, zinc, boron, molybdenum.

Main characteristics of a chemical analysis

La CEC

Cation Exchange Capacity : this is the capacity of the clay and humus in the soil to retain all types of cations (Ca, Mg, K, Na, H, Al, Fe, Mn). The CEC represents the size of the reservoir enabling storage of the fertilising elements.

• A high level means that the soil is rich in clay and humus. This type of soil can retain fertilising elements.
• A low level means a that the soil is poor in clay and humus. Advice : provide organic matter..

S/CEC

Saturation rate S/CEC : this is the ratio of good nutritional element content in the CEC (Ca2+, Mg2+, K+, Na+), which means the quantity of cations. It is linked to the pH

• Eg : Saturation 110 % the soil is rich in limestone and magnesium
• Eg : Saturation 20 % the soil is poor in limestone and magnesium . Advice : provide calcic enrichments when the S/CEC is <80 %

pH

Optimum pH level for good plant life is between 6,2 and 6,6.

• If pH < 5,5 => Advice : Provide calcic enrichment to decrease soil acidity.
• If pH > 7 => Advice : Use the plot more intensively. Providing mineral fertilisers, liquid manure, or apple pomace, acidifies the soil.

OM Organic matter

The ratio of organic matter is quite often high in a pasture. Since the sample of soil is taken from the surface (20cm in depth) it contains quite a lot of organic waste (droppings, dead leaves ….)

• % OM very high : a very high ratio of organic matter is not a problem.
• OM < 3% => Advice : Provide organic matter

 P₂O₅ and K₂O : phosphorus and potassium

These mineral elements are necessary for plant growth and development.

• When there is a lack , Production and nutritional values of grass and hay are reduced (lower protein content)
  • Low level => Advice : Provide organic matter or a P-K enrichment
• In excess, these elements can get washed out. For example, when providing too much liquid manure.
  • High level=> Avice : restrict organic matter enrichment, do not spread any chemical fertilisers.

A soil analysis should be undertaken every 4 to 5 years on a same plot. Samples are best taken in spring or autumn, to avoid extreme temperatures.

The collection procedure is generally well explained by the agronomic laboratories conducting the analysis.

The plot from which the samples will be taken, should be representative of an area, or group of plots on the farm for which there are particularly good reasons to conduct an analysis (high usage, low productivity, deterioration of the plant cover….)
Soil is heterogenous in its constitution, therefore sampling should be representative of the entire plot. Samples are collected excluding the extremities (near the fences, or hedges, near an old manure heap, passageways made by the animals).
On a  field, around 15 samples of the top 10cms of soil are collected, after having eliminated the suface debris. All the samples collected with an auger are then mixed together in a clean bucket, then 500g of the mix are placed in a clean bag supplied by the laboratory.
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The information sheet provided to be sent to the laboratory with the soil sample, should be correctly filled in, so that the laboratory can then select the most appropriate analysis techniques, and criteria calculations. The following information should be given :

• Name of the plot
• Type of soil (do not complete if you are not sure, or if the type of soil is unknown)
• Farming mode
• Previous fertilisations : dates and quantities.

Where can you get a soil analysis carried out?

All agronomical laboratories offer a service for soil analysis. There are laboratories in all regions of France.

The information can be obtained from the local Chamber of Agriculture ( Chambre d'Agriculture Départementale), who may offer the service themselves, or will direct you the nearest laboratory.

How much does a soil analysis cost?

Depending on the type of analysis requested (basic analysis, physical and chemical analysis), the cost will vary. It will cost between 45€ to 60€ exVAT depending on the requested number of criteria  to be analysed.

What are the benefits?

The financial investment in a soil analysis is profitable. The cost is low compared to the price of fertilisers. Conducting appropriate fertilising is economically profitable.

For example : reducing fertiliser or soil enrichments when they are not necessary, or on the contrary increasing forage production thanks to well reasoned fertilising.

The laboratory analysing the soil samples will give advice for fertilisation when returning the results of the soil analysis:

• Calcium and magnesium carbonate enrichments, through liming using either calcinated lime, or less expensive crude lime in the form of marl, dolomite, or soft limestone. An agricultural contractor can then be contacted to spread the lime in the recommended  P,K,Mg  dosages.
• Phosphorpous and potassium (P and K) fertilisation : this is provided with a binary fertiliser (P-K), or animal manure, (compost, manure) which is rich in P and K. The amount to be used is calculated in Units per Hectare (U/ha).

Example : Fertilising requirements of 30 U P2O5 et 40 U K2O
By using a 20/30/30  N,P,K  fertiliser (i.e 30 units of P and 30 units of K in 100kg of fertiliser), you will need 40/30 x 100 = 133kg /ha of this type of fertiliser.

Make sure you request several quotes for the fertiliser to be used according to the enrichment required for a given plot. Fertilisation in phosphorous and potassium will be extended to all the plots on the farm presenting the same soil envirornment.