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Introduction
Soil plays an important role in various operational and management contexts, with implications and interests that vary depending on its different uses.
Depending on the area of application, it is necessary to ensure proper management and identify the appropriate quality indicators to be used during the evaluation and study phase.
Areas of Use:
❖ Agriculture
❖ Fruit-growing and viticulture
❖ Woody crops
❖ Protected horticulture
❖ Nursery production
❖ Ornamental greenery
❖ Sports turf
Soil Quality Indicators
An indicator is a parameter, or a value derived from it, through which information about the state of a phenomenon can be obtained.
There are different classes of parameters and indicators used to evaluate soil characteristics according to its intended use:
- Physical characteristics
- Chemical characteristics
- Biological characteristics
Among the chemical characteristics, there are certain parameters, measurable through elemental analysis, that provide significant information.
Organic Matter
Soil organic matter is a potential environmental indicator, as it is related to numerous aspects of productivity and sustainability in agroecosystems, as well as environmental conservation.
It represents a reserve of nutrients and energy for microorganisms and plants and affects their availability through exchange mechanisms. The term soil organic matter refers to all the organic material present in the soil, including both living and non-living components.
The living components of soil organic matter include active roots and living organisms, while the non-living components include root exudates, decomposing plant and animal material, humus, and charcoal. Soil organic matter is largely composed of carbon, oxygen, and hydrogen, but it also contains numerous essential nutrients for plant growth, such as nitrogen, phosphorus, sulfur, and other nutrients.
Assessment of Soil Organic Matter
Soil organic matter (SOM) is commonly, though inaccurately, used as a synonym for soil organic carbon (SOC).
In fact, soil organic matter differs because it also includes other elements such as hydrogen, oxygen, phosphorus, sulfur, and nitrogen, whereas soil organic carbon represents a measure of the organic carbon content in soils.
The Importance of Elements
Soil organic matter is composed of approximately 58% carbon.
Soil carbon analysis is important because soil carbon plays a fundamental role in soil health and fertility and is also a significant contributor to global carbon cycles. Understanding the quantity and distribution of carbon in the soil allows for a better assessment of the effects of land use and management practices on soil health and can also support climate change mitigation strategies by increasing soil carbon sequestration.
Nitrogen is necessary for healthy plant growth and proper physiological development; amounts below the optimal requirement limit potential yield and protein quality. Excess nitrogen can also negatively affect plant growth and production.
Farmers should aim to supply nitrogen in a way that meets crop needs to reduce environmental losses and maximize fertilizer efficiency. Plants use nitrogen to increase vegetative growth and establish productive potential; this expands the photosynthetic area, promoting greater carbohydrate production and grain filling.
The C/N ratio in soil is the ratio between organic carbon and nitrogen and represents a key indicator of soil health, microbial activity, and nutrient availability. A balanced and healthy ratio is ideal for microbial life and efficient decomposition, with optimal ranges often indicated between approximately 12:1 and 24:1.
A low ratio (less carbon and more nitrogen) can lead to faster decomposition and quicker nitrogen release, whereas a high ratio (more carbon and less nitrogen) can cause microorganisms to temporarily “immobilize” available nitrogen in the soil to degrade carbon-rich material, risking nitrogen deficiency for plants.
Soil mineralization, crop residue recycling, and leaching are the main soil processes that determine sulfur levels available to plants during the growing season.
Most of the sulfur present in soil is found in organic matter. Therefore, sulfur supply depends on organic matter concentration and the rate of its decomposition, which is necessary to maintain nutrient availability. Organic matter can mineralize organic sulfur into sulfate (SO₄²⁻) when the carbon-to-sulfur ratio is sufficiently low to provide a net supply.
Sulfate not absorbed by plants is subject to leaching; thus, in coarse-textured soils and under high rainfall conditions, the availability of assimilable sulfur is generally limited.
We can therefore see how the analysis of the elements carbon, nitrogen, and sulfur is of great importance in soil studies.
International Standards
There are various analytical techniques for determining carbon, nitrogen, and sulfur in soil samples.
In particular, some international standards define specific methods for these analyses:
❖ ISO 15178 – Soil quality — Determination of total sulfur by dry combustion
❖ ISO 13878 – Soil quality — Determination of total nitrogen content by dry combustion
❖ ISO 10694 – Soil quality — Determination of organic and total carbon after dry combustion
Elemental Analyzers
“Dry combustion” is generally defined as a method in which the sample is burned in oxygen, or in a mixture of oxygen and a carrier gas, under conditions that convert it into ash and gaseous combustion products. These products consist mainly of carbon dioxide, water vapor, elemental nitrogen and/or nitrogen oxides, sulfur oxides and oxyacids, and hydrogen halides.
The combustion products are treated to ensure that any hydrogen associated with sulfur or halides present in the combustion products is released as water vapor. Nitrogen oxides are reduced to nitrogen, and combustion products that could interfere with subsequent gas analysis procedures are removed. The mass fractions of carbon dioxide, water vapor, and nitrogen in the gas stream are then quantitatively determined using appropriate instrumental gas analysis procedures.
The sulfur content in a soil sample is determined by heating the sample to a minimum temperature in a gas stream containing oxygen. Both organically and inorganically bound sulfur are converted into SO2. This reaction may, in some cases, require higher temperatures or the addition of catalysts, modifiers, or accelerators. At temperatures below 1350°C, SO3 may form in the presence of excess oxygen. This SO3 must be reduced to SO2 using a suitable reagent, such as copper. The SO2 produced by combustion is measured using infrared spectrometry, thermal conductivity, or another suitable detection technique.
This procedure is known as high-temperature catalytic combustion.
All elemental analyzers of the 80 series by NC Technologies comply with the above-mentioned standards.
Depending on the chemical element of interest, one can choose from the following analyzers:
• ECS 8020
• ECS 8024 NC SOIL SPECIAL
• ECS 8040
Analytical Tips
Since the chemical characteristics of soils are not always the same, the concentration of elements can vary significantly from one sample type to another.
Moreover, the amount of sample needed for analysis to achieve good repeatability depends on its homogeneity:
- If the sample is homogeneous, the amount to be analyzed can be limited (micro weighing).
- If the sample is heterogeneous, the amount to be analyzed must be increased to ensure good repeatability (semi-macro weighing).
The Golden Rule of Elemental Analysis
Even if an instrument can analyze up to 200 mg or more, the most important aspect is to weigh the minimum amount of sample necessary to obtain repeatable analyses.
This approach ensures reliable results while saving helium, oxygen, and chemical reagents.
