Guidance on the interpretation of physical and chemical soil test results.

Guidance on the interpretation of physical and chemical soil test results.

As we have seen in previous publications, soil is a vital natural resource that supports life on our planet. Not only does it provide the basis for the growth of plants, crops and trees, but it also plays a critical role in regulating the water cycle and storing carbon. To ensure its long-term sustainability and productivity, it is essential to assess and maintain soil health. Methods for assessing soil health play a crucial role in this process.

What is soil health?

We would like to take this opportunity to point out that this little guide will be continually updated to add and improve the information to help you improve the health of your soil.

Soil health refers to the ability of the soil to perform its natural functions and provide a favourable environment for plant growth. A healthy soil is one that can provide essential nutrients, adequate structure, good water retention and adequate microbial activity, among other things. Assessment of soil health is essential to ensure the sustainability of agriculture and the conservation of the environment.

Soil health assessment methods

Often underestimated and passively observed, soil is the silent foundation that supports all life on our planet. Beneath our feet, this precious natural resource holds a hidden complexity that influences food production, ecosystem health and climate change mitigation.

Soil health assessment methods have become an essential tool for unlocking the secrets beneath the earth’s surface and ensuring its long-term vitality. From chemical analysis to biological assessment and erosion resistance, this section explores the various techniques that allow us to understand and conserve this invaluable resource.

To do this, scientists carry out the following investigations:

Soil chemical analysis: This method involves taking soil samples and analysing them in the laboratory to determine the concentration of essential nutrients such as nitrogen, phosphorus, potassium and other micronutrients. The results of this analysis provide valuable information about the fertility of the soil and allow fertilisation practices to be adjusted.

Soil pH

Soil pH is a measure of the acidity or alkalinity of the soil. The ideal pH for most crops is between 6.0 and 7.0. Soils with a pH below 6.0 are acidic and soils with a pH above 7.0 are alkaline.
-Ideal: 6.0-7.0
-Acidic: <6.0
-Alkaline: >7.0

Nutrient content

Soil nutrient content refers to the amount of nutrients available to plants. Essential plant nutrients include nitrogen, phosphorus, potassium, calcium, magnesium and sulphur.
– Nitrogen (N): 0.1-0.2%.
– Phosphorus (P): 10-20 ppm
– Potassium (K): 100-200 ppm
– Calcium (Ca): 2-4 meq/100 g
– Magnesium (Mg): 1-2 meq/100 g
– Sulphur (S): 10-20 ppm

Organic matter

Soil organic matter is the living and dead parts of soil organisms. Organic matter helps to improve the structure, water holding capacity and fertility of the soil.

Soil physical analysis: Soil texture, structure and water holding capacity are critical aspects of soil physical health. Soil physical assessment methods include the determination of bulk density, porosity and permeability. These data are fundamental to understanding the soil’s ability to hold water and allow plant root growth.


Soil texture refers to the proportion of sand, silt and clay particles it contains. Coarse textured soils, such as sandy soils, have good drainage capacity but hold little water. Fine textured soils, such as loamy soils, hold a lot of water but have poor drainage. Medium textured soils, such as loams, have a good balance between drainage and water retention.
– Loam: 25-50% sand, 25-50% silt, 10-25% clay
– Sandy: >50% sand
– Clay loam: >50% clay
– Sandy loam: 25-50% sand, >50% silt and clay
– Loamy loam: 25-50% silt and clay, >50% sand


Soil structure refers to the way in which soil particles are arranged. Soils with good structure have pores and spaces that allow water and air to circulate freely. Soils with poor structure have clogged pores and spaces, making it difficult for water and air to circulate.
Good: Granular, porous structure with good drainage and water retention.
Poor: Compact structure, with poor drainage and water retention.


Soil density refers to the amount of mass contained in a given volume of soil. Soil density varies with soil texture and structure. Sandy soils have a low density, while clay soils have a high density.
– Loamy: 1.2-1.5 g/cm³.
– Sandy: <1.2 g/cm³.
– Clayey: >1.5 g/cm³.

Water holding capacity

The water holding capacity of a soil is the amount of water it can hold. The water holding capacity of soils varies according to their texture and structure. Clay soils have a high water-holding capacity, while sandy soils have a low water-holding capacity.
Clay: 15-30% Sandy: 5-15% Clay: 15-30% Sand: 5-15% Sandy: 5-15
Sandy: 5-15% Clayey: 30-50% Clayey: 30-50% Clayey: 30-50% Clayey: 30-50
Clayey: 30-50% Clayey: 30-50% Clayey: 30-50% Clayey: 30-50% Clayey: 30-50

Assessing soil biology: Soil micro-organisms such as bacteria, fungi and earthworms play a key role in soil health. Biological assessment techniques include measuring microbial biomass, enzyme activity and microbial community diversity. A soil with a diverse and active microbial community is healthier and more resistant to disease.

Number of bacteria

The number of bacteria in the soil is an indicator of microbial activity. A healthy soil will have a high number of bacteria.
– 100 million to 1 billion per gram of soil.

Number of fungi

Fungi are also important for soil health. A healthy soil has a high number of fungi.
– 10 to 100 million per gram of soil.

Microbial diversity

-Microbial diversity is an indicator of soil health. A healthy soil will have high microbial diversity

Enzyme activity

-Enzymes are proteins that catalyse chemical reactions in the soil. Healthy soil has high levels of enzyme activity.


It should be understood that soil health assessment is fundamental to decision making in agriculture and sustainable natural resource management. It allows farmers and scientists to better understand soil needs and to adopt management practices that improve soil quality. It also contributes to biodiversity conservation and climate change mitigation by improving soil carbon sequestration and water conservation.

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