Formulation, application methods, variety response: aspects needed to be better understood for a
Several groups of soil microorganisms (mycorrhizal fungi, rhizosphere bacteria, rhizobia and phosphorus solubilizing microorganisms) can have a significant impact on the growth, yield and quality of horticultural crops, thanks to increased absorption capacity of soil mineral nutrients. Their contribution to plant nutrition can be limited to a single nutrient, as in the case of nitrogen-fixing bacteria, or to a variety of elements, as it happens with mycorrhizal fungi, which are generally useful for the absorption of phosphorus and can also improve the availability and absorption of nitrogen and microelements.
The use of biostimulants based on these microorganisms has gained importance in the last decades as a tool useful to reduce the negative environmental effects generated by the excessive and/or improper application of chemical fertilizers, even in fruit and vegetable crops. However, despite the amount of studies and identification of numerous useful microbial strains, the broad application of microbial-based biostimulants (which is a new category of biostimulants established by the EU Regulation 1009/2019) in agricultural practice is still hampered by several factors and limited to few kinds of products. The main reasons derive from the limited understanding of the complex interrelationships that exist between these microorganisms, the plants and the soil environment.
It is important to underline that the effectiveness of microbial-based biostimulants depends also on their formulation and method of application. It must be clear to farmers and advisors that it is necessary to adapt the cultivation practices when it is intended to introduce the use of microbial-based biostimulants in the management of an orchard or a plantation. This adaptation is required to promote the optimal conditions for the coexistence between the plant and the inoculated microorganism that allow the best exploitation its characteristics.
The variety (i.e. the genotype of the plant) is another factor that can affect the root colonization by the AMF and the plant response to them. For fruit trees, this is particularly important because of the use of rootstocks with different vigour and root system architecture.
All these practical issues are being addressed by the trials carried out in EXCALIBUR. Different kinds of formulations are tested with the three crops concerned by the project (apple, strawberries and tomato). For example, a mixture of different species of mycorrhizal fungi selected by the partner company Inoculum Plus is going to be applied either as a granulated or a wettable powder, depending on the crop and the management system present in the farm where the trials are planned. For example, when planting trees, a granulated formulation will be used, positioning the product just above the roots in the hole. However, for vegetables such as tomato or strawberries, the product will be applied at planting using a wettable powder formulation. This will be used to prepare a suspension on which the root system of the plantlets can be immerged before planting. A wettable powder will be also used for field application through the irrigation system on trees or plants that have been already planted.
The variety response is going to be addressed for apple rootstocks with trials at INHORT in Poland and NIAB-EMR in UK. At INHORT the trial has started by applying the AMF product on apple trees already in the nursery. Two different rootstocks characterized by a limited vigour (M9) or with a medium-high vigour (Antonówka), on which the scion cultivar Alba was grafted, were treated with the mycorrhizal fungi alone or in association with a biostimulant derived from yeast production medium (stillage). The trees will be used to establish an orchard in spring 2021. At the end of the last season, it emerged clearly that the two rootstocks responded differently to the treatments in the nursery, as also assessed during the uprooting of the trees, before their storage under controlled conditions. It will be thus interesting to verify the level of root mycorrhization and the effect of the treatment on the soil biodiversity in the orchard as well as the level of nutrients in the trees during the growing season.
An interesting aspect that will be assessed with this trial is related to the effect of the combined application of AMF and the biostimulant stillage. The stillage is assumed to foster the development of the root system, which should increase the number of roots potentially available for AMF colonization, exerting an additional positive effect to the plant. However, the stillage has been shown to increase also the soil microbial activity, which in turn can also favour both the plant and the mycorrhizal fungi development.
From this brief excursus, it should be clear that to fully exploit the benefits of microbial-based biostimulants it is required to correctly choose the cultivar/rootstock–inoculum combination and the inoculation/application technology. Considering that the EU Commission has included “soil quality” among the priorities for the next research framework program, it is perhaps curious to recall how, in 2009, the famous film “Avatar” by James Cameron anticipated the concept that relationship between the root system of plants and soil microorganisms is the basis of life: by increasing the soil biological fertility we can therefore influence the productivity of plants and improve the quality of the products and our life.
Article and photos provided by INHORT.