The organic matter in “soil-oriented” fertilization

The prevalence of one particle size class over another results in different soil characteristics, both physically (affecting workability, thermal properties, or water retention capacity) and chemically (determining the ability to retain or release nutrients). Specifically, sand improves aeration and drainage, silt retains water, and clay, thanks to its cation exchange capacity, retains nutrients.

However, soils dominated by one mineral component show imbalances. Sandy soils are prone to severe leaching and nutrient loss; silty soils tend to form surface crusts and compaction, especially when heavy machinery or intensive cultivation methods are used; and clay soils are subject to compaction and waterlogging.

While theoretically the “ideal” soil for agriculture is one where the three components are present in equal proportions, it is well known that some crops prefer soils dominated by one component. For example, sandy soils are particularly advantageous for crops requiring good drainage, such as root vegetables and horticultural plants, as these soils warm up faster during the growing season, promoting early crop establishment and efficient water movement.

In cases of pronounced imbalance, the use of organic amendments and/or organic and organo-mineral fertilizers is always beneficial: the addition of humified organic matter promotes the formation of stable aggregates between organic and mineral fractions, positively influencing soil structure, reducing compaction, and facilitating air and water circulation. Moreover, these aggregates counteract nutrient loss through leaching, thanks to the chelating action of humic and fulvic acids.

Finally, remember that soil texture also influences fertilizer application methods (particularly split applications).

Acidic soils, with pH below 6.0, often show deficiencies in nutrients such as Calcium, Magnesium, and Phosphorus, and may contain toxic concentrations of Aluminum and Manganese. For alkaline soils, with pH above 7.5, the main issue is the low availability of micronutrients such as Iron, Manganese, and Zinc, which tend to precipitate and become unavailable to plants.

Soil pH can be corrected once, before planting. But often subsequent interventions are required, more or less frequent depending on severity. Traditional products used to manage abnormal pH include amendments, such as lime (calcitic or dolomitic) or wood ash for acidic soils (these compounds also supply Calcium and Magnesium), and iron sulfate or ammonium sulfate for alkaline soils.

In cases of unbalanced pH, soil organic matter works synergistically with amendments, enhancing nutrient bioavailability introduced or made available by amendments, prolonging their effectiveness over time, and reducing the need for repeated corrective interventions.

Organic or organo-mineral fertilization, or the use of organic amendments, is essential to manage this issue. The structural improvement provided by organic matter positively affects the soil water balance, a key factor in salinity management, reducing dryness on one hand and promoting proper drainage on the other.

Without going into detail for individual crops, we will simply distinguish between annual and perennial crops.

In a perennial crop, which remains on the same soil for decades, base fertilization always includes an adequate supply of organic matter, useful for creating a favorable environment for progressive root development and maintaining an optimal balance of soil water availability, while also ensuring a slow and prolonged release of nutrients, especially nitrogen. Naturally, the topsoil will gradually lose organic matter because it is more exposed to natural events and tillage, which accelerate mineralization. Therefore, especially in soils with one or more of the above-mentioned issues, in perennial crops (orchards, olive groves, vineyards), surface organic matter must be periodically replenished (taking care not to damage roots during incorporation), also to restore nutrient levels, which are removed similarly year after year, albeit with variations linked to plant age.

For annual crops, soil type guides fertilization choices, with greater operational flexibility (base fertilization can be done annually and in the absence of crops, before sowing or transplanting) and significant differences in cases of monoculture or crop rotation. Monoculture causes repeated nutrient removal over time and may exacerbate soil issues (e.g., depletion of specific nutrients in sandy soils), while rotations promote soil “regeneration” in terms of health and nutrient supply, though repeated tillage may still damage structure.

In all cases, safeguarding and increasing soil organic matter content plays a key role in maintaining fertility.

The microbiota of the rhizosphere may have a composition more or less favorable to crop health and optimal nutrition. Limiting or problematic soils—along with climate change and intensive farming practices—can cause imbalances between beneficial and harmful microorganisms (pathogens).

Forward-looking fertilization strategies, those aimed not only at solving immediate problems but at long-term regeneration goals, must consider this aspect. Once again, the role of organic matter is central, as it creates conditions favorable to soil microorganisms by improving structure and providing nutrients to them. The prevalence of harmful microorganisms in the rhizosphere may be linked to crop conditions (perennial crops and monoculture, for example, can “select” certain pathogenic microorganisms), soil imbalances (e.g., waterlogging in clay soils can favor fungi causing root rot), or climate changes (which select species more resistant to extreme conditions, reducing biodiversity).

In such cases, an adequate supply of organic matter may not be enough, and it may be necessary to add selected pools of beneficial microorganisms with inoculated organic fertilizers.