What is organic matter and why is it key?
Organic matter is the set of decomposing plant and animal residues present in the soil. It is the engine of soil fertility and health, as it improves structure, water retention, and nutrient availability. For agricultural input distributors, understanding organic matter is essential to recommend products that increase it, such as compost, biostimulants, and organic fertilizers.
In modern agriculture, organic matter is a key indicator of sustainability. Soils with high organic matter content are more resilient and productive. Therefore, growers seek solutions that restore and maintain it, and here distributors play a crucial role by offering products that promote its accumulation.
Organic matter also influences the biological activity of the soil, feeding the microbiota that decomposes the material and releases nutrients. Without organic matter, the soil degrades and loses productivity. That is why this guide will help you understand its importance and how to communicate it to your customers.
Chemical composition and fractions of organic matter
Soil organic matter consists of a heterogeneous mixture of compounds including carbohydrates (cellulose, hemicellulose, lignin), proteins, lipids, organic acids, and humic substances. The latter are divided into humic acids, fulvic acids, and humins, which represent between 60% and 80% of total organic matter in mineral soils. Humic acids have a high molecular weight (50,000-100,000 Da) and are responsible for cation exchange capacity (CEC), while fulvic acids, with a lower molecular weight (1,000-10,000 Da), are more mobile and act as chelators of micronutrients such as iron, zinc, and manganese.
From a functional perspective, organic matter is classified into three fractions: the active or labile fraction (composed of fresh residues and microorganisms, with a residence time of months to a few years), the intermediate fraction (partially decomposed compounds, with a half-life of 5-20 years), and the passive or stable fraction (humic substances associated with clay minerals, with residence times exceeding 100 years). This latter fraction is what contributes to long-term carbon sequestration, a key aspect for climate change mitigation and for the certification of sustainable agricultural practices.
The optimal organic matter content varies according to soil texture. In sandy soils, a level of 1.5% to 2% may be adequate, while clay soils require levels of 3% to 5% to maintain good structure. The carbon:nitrogen ratio (C:N) of organic matter is a critical indicator: a C:N ratio above 30 indicates slow decomposition and potential nitrogen immobilization, while values below 20 favor rapid mineralization and nutrient release. Distributors must be familiar with these thresholds to recommend amendments with the appropriate C:N ratio depending on the crop stage.
Impact of organic matter on soil physico-chemical properties
Organic matter directly influences soil bulk density. A soil with 3% organic matter can have a bulk density of 1.1 g/cm³, while a degraded soil with less than 1% can reach 1.6 g/cm³. This difference affects root penetration: at densities above 1.4 g/cm³, root growth is significantly reduced. Furthermore, each 1% of organic matter in the topsoil layer (0-20 cm) can retain approximately 20,000 liters of water per hectare, equivalent to 2 mm of additional precipitation available for the crop.
Cation exchange capacity (CEC) is another property that improves with organic matter. While clays have a CEC of 20-50 cmol/kg, humic substances can reach values of 200-300 cmol/kg. A 1% increase in organic matter can raise soil CEC by 2-5 cmol/kg, which translates into greater retention of cations such as calcium (Ca²⁺), magnesium (Mg²⁺), potassium (K⁺), and ammonium (NH₄⁺), reducing leaching losses and improving the efficiency of applied fertilizers.
In terms of pH, organic matter acts as a natural buffer. Humic and fulvic acids contain carboxyl (-COOH) and phenolic (-OH) groups that can donate or accept protons, stabilizing soil pH within a range of 5.5 to 7.5. This property is especially valuable in acidic or alkaline soils, where pH fluctuations can limit nutrient availability. For example, in soils with a pH below 5.5, aluminum (Al³⁺) is released in forms that are toxic to plants, but organic matter can chelate this aluminum and reduce its phytotoxicity.
Soil microbiota: invisible allies
The soil microbiota includes bacteria, fungi, actinomycetes, and other microorganisms that live in the rhizosphere. These organisms are fundamental to plant nutrition, as they participate in the decomposition of organic matter, nitrogen fixation, and phosphorus solubilization. For distributors, understanding the microbiota is key to offering products that enhance it, such as biofertilizers and bioprotectants.
Healthy soil harbors a diverse community of microorganisms that interact with plant roots, improving nutrient uptake and resistance to pathogens. The microbiota also produces phytohormones and bioactive compounds that stimulate plant growth. Therefore, products containing beneficial microorganisms, such as those from Ecoganic, are increasingly in demand.
The relationship between organic matter and microbiota is symbiotic: organic matter provides food for microorganisms, and they, in turn, transform it into available nutrients. Therefore, any agronomic management strategy must consider both elements. Distributors who understand this synergy can offer comprehensive solutions that improve soil health.
Microbial diversity and its specific functions in the rhizosphere
A single gram of fertile soil can contain between 10⁸ and 10⁹ bacteria, 10⁶-10⁷ actinomycetes, 10⁵-10⁶ fungi, and 10⁴-10⁵ protozoa. This diversity is not coincidental: each microbial group performs specific functions that contribute to the health of the agroecosystem. Bacteria of the genera Rhizobium and Bradyrhizobium establish symbiosis with legumes, fixing between 100 and 300 kg of nitrogen per hectare per year. Free-living bacteria such as Azotobacter and Azospirillum fix nitrogen in the rhizosphere, contributing between 10 and 30 kg N/ha/year, and also produce phytohormones such as indoleacetic acid (IAA) that stimulate root development.
Arbuscular mycorrhizal fungi (AMF) form symbiotic associations with more than 80% of terrestrial plants. These fungi extend their hyphae beyond the nutrient depletion zone of the root, increasing phosphorus uptake by up to 90% in soils with low availability of this nutrient. Furthermore, AMF improve soil aggregation through the production of glomalin, a glycoprotein that acts as a natural glue and can represent up to 30% of the stable soil organic carbon. Saprophytic fungi such as Trichoderma spp. decompose lignin and cellulose, releasing nutrients and competing with pathogens through the production of lytic enzymes and antibiotics.
Actinomycetes, although less well-known, are responsible for the characteristic smell of wet earth (geosmin) and produce antimicrobial compounds such as streptomycin and actinomycin. These microorganisms degrade recalcitrant compounds such as chitin and cellulose, participating in the carbon cycle. As for protozoa, although they represent only 1-2% of the microbial biomass, they are key predators that regulate bacterial populations and release nutrients immobilized in the microbial biomass, a process known as the "microbial loop" that can release up to 30 kg N/ha/year.
Factors affecting microbial activity and how to manage them
Soil microbial activity is regulated by multiple environmental and management factors. The optimal temperature for most soil microorganisms ranges between 25°C and 35°C, with reduced activity below 10°C and above 45°C. Soil moisture is equally critical: maximum microbial activity occurs at 60-80% field capacity, while prolonged saturation conditions (>90%) create anaerobic environments that favor pathogens such as Pythium and Fus It depends on the soil type and crop, but in general a level above 2% is recommended in agricultural soils. Values below 1% indicate degraded soils that require organic inputs. A soil analysis can determine the current content and guide recommendations. Organic amendments such as compost, manure, and cover crops are the most common. Additionally, biostimulants containing humic and fulvic acids can contribute to the formation of stable organic matter. Ecoganic offers products that combine these substances with beneficial microorganisms. You can observe it through biological activity: presence of earthworms, rapid decomposition of residues, and vigorous plant growth. Microbial biomass analysis or soil respiration are more precise methods. A soil with good microbiota will have fewer disease problems and better structure. Not necessarily. Biostimulants improve fertilizer efficiency, but they do not completely replace them. They can reduce the required dose, but in very poor soils, additional inputs are needed. The key is integrated management that combines fertilization, organic matter, and microbiology.Frequently Asked Questions
How much organic matter does a soil need?
What products increase organic matter?
How do I know if my soil microbiota is healthy?
Do biostimulants replace chemical fertilizers?
