Identifying the type of fertilizer best suited to the objective is an integral part of crop nutrition strategies. But what are the main indications to take into account?
In the era of tutorials, we at Agribios Italiana also wanted to produce a mini-guide to help you make your choice.
Basically, the choice of fertilizers to be used should be subordinated to the nutritional needs of the crop to be managed and to the analysis of the soil. The latter allows us to establish what the soil has “in stock” and what is missing, and should not be limited to the simple quantification of the macro-elements – N, P and K – but should also evaluate:
Quantifying the supply of nutrients in the soil is not exhaustive information in itself since what really matters is their bioavailability. Consider the case of Phosphorus, abundantly present in most cultivated soils but unavailable for absorption by the root systems, as it is subject to retrogradation to forms that are insoluble in water; or Iron, which in soils with high pH is present mostly in trivalent form, little or not at all absorbed by plants.
The evaluation of the content of organic matter is important not only for the capacity of the latter to release nutrients (Nitrogen in particular) and to stimulate the proliferation of useful microorganisms, but also for its fundamental role in determining the physical characteristics of the soil, porosity in particular. In the case of excessively low levels, fertilization must necessarily include the distribution of organic products (soil improvers or fertilizers).
The nutritional needs of the specific crop must be calculated on the basis of the desired yields, compared with the soil resources and, if necessary, integrated with fertilization, which in most cases requires an adequate distribution plan that optimizes its effectiveness and minimizes losses of product not absorbed by the plants.
As is known, mineral fertilizers are characterised by rapid nutritional efficacy, but the risk of nutrient loss due to leaching, evaporation or insolubilization is high if distribution and absorption peaks by plants do not coincide. Organic fertilizers are characterised by a slower and more prolonged release over time, largely mediated by the action of soil microorganisms responsible for the mineralization of organic matrices, already present in the soil or added. Finally, organo-mineral fertilizers combine both characteristics: prompt action and prolonged release over time.
In the case of tree crops, it is essential to foresee a planting (or basic) fertilization and annual fertilization, the latter in turn to be divided into post-harvest (or autumn) fertilization and spring fertilization.
Basal dressing fertilization leaves much more freedom of action from an operational point of view compared to annual fertilization, as it is carried out before the plants are planted and any support structures (poles, wires, etc.) are built. The absence of root systems allows working at different depths without running the risk of damaging roots and working organs. Having to serve as a reserve of nutrients for a lifespan of the plant that can be decades, and often also having a soil improver function, basal fertilization normally uses organic products.
Annual fertilization must be set differently depending on the age of the plant (different nutritional needs, different development of the root system).
Autumn fertilization serves to nourish the plant after harvest, aiming to reintegrate what has been removed from the soil related to the production of the year just ended, and to manage any imbalances and discrepancies in the plot (also from a site-specific perspective). It takes advantage of a good absorption activity by the roots that characterises the autumn season and the rainfall of the period, which solubilizes the nutrients and makes them available. Also, in this case, an organic or organo-mineral fertilizer will exert a protective effect on the nutrients, favouring slow, gradual release.
Spring fertilization is intended to give the “finishing touch” to the vegetative awakening, providing the nutrients, Nitrogen in particular, needed in a readily available form for a quick restart. The organo-mineral formulation is recommended in this case.
Herbaceous crops, although contemplating a similar scheme, in consideration of their more or less short production cycle, are more “predatory” than tree crops. The custom is to administer with fertilization what is necessary to complete the crop cycle and harvest the product, without worrying too much about the soil.
Actually, it would be necessary, even in the case of herbaceous crops, to think differently and pay greater attention to maintaining and increasing soil fertility. Otherwise, it will become impoverished and, beyond certain levels, it will be difficult to manage.
If soil analysis represents the most scientific approach to the targeted choice of fertilizers, it is also necessary to consider the cases in which it is not available.
In such situations, the indication is to provide nutrients with organo-mineral fertilizers that, as specified above, combine prompt action and prolonged release over time. In this way, the probability of an adequate response of the crops to fertilization is maximized.
The analysis of the nutritional elements contained in the leaves, blades and petioles is decidedly less widespread than that of the soil but can provide important, and prompt, information on the real nutritional status of the plant, allowing rapid corrections of imbalances with products with both foliar and root distribution. In essence, leaf analysis provides a snapshot of the result of the interactions between the plant and the environment. Or rather, of our specific crop in a specific environment, nourished with specific products. This consideration is even more valid today than in the past, given that the climate changes in progress profoundly alter the interactions mentioned above.
The fertilization technique can influence the quantity and vitality of beneficial microorganisms in the soil in various ways.
First, the application of organic matter of adequate composition to the soil stimulates the proliferation of useful microorganisms already present in the soil.
Second, in the case of soils that are not extremely vital from a microbiological point of view, it is possible to introduce beneficial microorganisms (rhizosphere bacteria and mycorrhizal fungi) to the soil (and possibly to the above-ground part of the crop) thanks to the application of inoculated fertilizers.
In what situations can their use be helpful?
Where there is a high risk of pathologies or parasitosis of the collar or root due to the presence of pathogens or nematodes in the soil, the use of inoculated fertilizers with rhizosphere bacteria is able to create an unfavourable environment for these microorganisms, through antagonism or trophic competition, reducing the risk of the onset of disease and working in synergy with possible pesticide treatments.
This type of fertilizer is also useful for improving the absorption of nutrients and promoting root development, thanks to the action of symbiotic and non-symbiotic microorganisms. Inoculated fertilizers are therefore used in the vegetative awakening of tree plants and in the early stages of the development of herbaceous and tree plants when the root system has not yet explored a sufficiently large volume of soil. The usefulness of these fertilizers is also proven in cases of springs with unfavourable weather conditions, for example, characterised by low temperatures.
