Organic matter is the basis of soil health and fertilisation efficiency

From an agronomic perspective, therefore, a soil is healthy not only because it is able to provide nutrients and water to crops (i.e. it possesses its own fertility), but because it represents a balanced system, where physical, chemical and biological components interact efficiently. When this balance is lost, soil undergoes more or less intense forms of degradation.

• structure, bulk density, water holding capacity and erosion rate among physical parameters;
• pH, cation exchange capacity, electrical conductivity, macro- and micronutrient content among chemical parameters;
• soil organic matter content and organic carbon levels, C/N ratio, respiration rate and enzymatic activities among biological parameters.

It is interesting to note that some of these parameters may be evaluated differently when considering soil health in general versus soil fertility specifically. For example, high levels of nitrogen in water-soluble forms (nitrates) may represent an environmental concern but not necessarily an agronomic issue.

What is certain, however, is that agriculture is closely linked to soil health, both because yields and product quality depend on it and because farming practices directly affect soil condition. Among these practices is fertilisation.

A “holistic” approach to soil health is now central both in agronomic research and in European strategies for soil monitoring and improvement.

• mineral fertilisation provides a rapid effect but carries the risk of nutrient losses if elements are not taken up;
• organic fertilisation positively influences soil physical and biological properties but releases nutrients more slowly from a chemical standpoint; in certain situations, this may not meet the immediate needs of crops;
• organo-mineral fertilisation combines the positive characteristics of both.

How should products be selected?
As previously highlighted, establishing effective long-term fertilisation strategies means considering not only crop nutritional needs but also the characteristics of the soil, with the aim of maintaining or improving its health status, i.e. regeneration.

Such studies have been conducted since 1958 at the Hanninghof Research Center in Dülmen (Germany), owned by Yara International since the 1980s.

Among the long-term trials, one evaluates the effects of different fertilisation strategies on plots managed under a five-year rotation: silage maize – rye – silage maize – rye – potato.

The fertilisation treatments compared are:

• no fertilisation (control);
• fertilisation with mature manure only;
• fertilisation with mineral fertiliser (NPK+Mg) only;
• fertilisation with mature manure and NPK+Mg fertiliser.

In these plots, a series of assessments of soil chemical, physical and microbiological properties have been carried out over the years.

The integration of organic matter and mineral elements proved to promote more favourable conditions for nutrient availability, particularly for phosphorus and magnesium.

The trial also showed that the combined application of organic matter and mineral elements led, compared with other treatments, to an improvement in parameters linked to soil vitality, including organic carbon content and respiration rate (an indicator of active microbiota).

There is not necessarily a conflict between yield and soil health, provided nutrient management is optimised. On the contrary, lack of fertilisation leads to poor crop growth, reduced organic matter inputs and, in the long term, depletion of soil organic matter and soil health.

Soil health cannot be assessed through short-term or occasional observations. Only long-term trials make it possible to capture cumulative effects such as increases in organic carbon, structural stabilisation and the evolution of the biological community. For the agronomist, this means that fertilisation strategies must be evaluated not only in relation to the current season, but also in terms of system trajectory over time.