Fertilization strategy and soil knowledge must go hand in hand

The serious state of impoverishment and the tiredness in which most of the Italian and European farmed land finds itself is a topic that has been extensively covered, given its urgency.

At the time of writing this study, the Fieragricola Tech event in Verona (29-30 January 2025) just ended, where the scheduled conferences and workshops have repeatedly touched on this topic, however, underlining the opportunity for a regenerative approach to soil management and more generally to agricultural practices.

Conservative agriculture, already proposed for years as an alternative to intensive agriculture, no longer seems sufficient to guarantee an acceptable quality of cultivated land. We need to regenerate and it must be done while farming: in view of the exponential growth of the world population, it is not plausible to put the land to rest except in minimum percentages.

If on the one hand, as reported in the report “Land consumption, territorial dynamics and ecosystem services – 2024 Edition”, presented in November last year by Ispra, the surface occupied by cement, asphalt and other artificial roofs is growing inexorably in Italy, on the other hand, as academics and researchers have reported on several occasions, most European soils are not in good health and the organic matter content of land used for agricultural and livestock activities is often less than 1%. The Po Valley, which in academic textbooks that generations of students studied was described as a fertile area par excellence, does not escape this trend.

It is therefore necessary to rethink agriculture and abandon the highly disruptive practices of the balances that regulate the flows of organic carbon in the soil, from the soil to the atmosphere and from the atmosphere to the soil.

All agricultural practices are potentially responsible for the decrease of soil organic matter: think, for example, of tillage, especially those that mix horizons and compact, or of phytoiatric treatments that, reaching part of the soil, influence the cycles of the microbiota.

Trying to return organic matter to the soil by reasoning in quantitative terms only risks generating results opposite to those desired, if the characteristics of the organic matter supplied are not taken into account.

There are at least two characteristics to take into account: the humification rate (i.e. the ratio between Carbon from humic and fulvic acids and total Carbon) and the ratio between Carbon and Nitrogen.

A high rate of humification of the organic matrix of which an organic or organo-mineral fertilizer is composed is an indication of the presence of a high content of stable organic substance. Humic substances positively influence the chemical, physical and biological properties of the soil, also promoting biodiversity, thanks to their direct and indirect stimulating action on the microflora.

The “fresh” organic matter, slightly or not at all humidified, is instead rapidly degraded by microorganisms and, in addition to having a lower capacity to positively influence the physical properties of the soil, can lead to an increase in CO2 emissions and a peak of nitrogen release, potentially subject to leaching.

The different organic matrices provided to the soil can also be characterized by different relationships between Carbon and Nitrogen, which also profoundly modify its intended use and effect on the physical, chemical and biological properties of the soil.

Most of the microorganisms responsible for the mineralization of organic matter need a C/N ratio of around 24/1, optimal for their metabolism. When the ratio in the organic matrix is higher (as in cereal straw, for example), in order to degrade it microorganisms seek nitrogen elsewhere, typically in the organic matter of the soil, removing it at least momentarily from the plants. If, on the other hand, it is lower, the microorganisms consume the carbon contained in the matrix and release excess nitrogen, which can be absorbed by plants but also lost through leaching.

A correct C/N ratio in an organic or organo-mineral fertilizer, ultimately, favours a prolonged release of nitrogen over time.

To the two parameters described above, which concern the composition of the fertilizer, a third is added of equal importance for determining the success of a fertilization strategy: the organic carbon content of the receiving soil.

A very recent publication by a German research group emphasized the importance of the initial supply of organic carbon in soil to establish the actual result of fertilization with fresh plant residues of oats and peas, in particular the increase in carbon stored in the soil.

Three agricultural soils with different organic carbon content were incubated, adding the two plant residues, characterized by a different C/N ratio, marked with ¹³C.  The tracking of flows has made it possible to demonstrate that the ‘return on investment in Carbon’, or the amount of ‘new’ Carbon retained in relation to the amount of organic matter added, is closely related to the amount of native Carbon in the soil. More specifically, the addition of fresh organic matter to soils with a lack of carbon leads to a higher release of CO2 than soils with high endowment, a fact most likely attributable to the different responses on the part of the microbiota.

The addition of both plant residues has led to changes in the microbial community, with an increase in the presence of fungi compared to bacteria, more pronounced with oat residues and less with pea residues, testifying to the fact that not all organic matrices have the same effect on soils.

Therefore, the researchers conclude:

• that it is important to consider the fate of organic fertilizers with different nitrogen content when you want to replenish the carbon stocks of degraded soils;
• fertilization management should focus on the replenishment of Carbon taking into account the existing content, to avoid the vicious cycle of loss due to mineralization, with a consequent increase in greenhouse gas emissions.