## 4 Conclusions  
The highest grain yield was noted following the application of mineral fertilisation and the three microbial preparations combined (_Paenibacillus azotofixanus_, _ Bacillus megaterium_ and _ Bacillus subtilis subtilis_), as well as NPK with _Paenibacillus azotofixanus_. The preparation containing _Paenibacillus azotofixanus_ performed well under optimal conditions for wheat growth and development, while all the preparations applied in combination (_P. azotofixanus_, _B. megaterium_, _B. subtilis_) performed well under extreme weather conditions by supplementing each other. The microbial preparations had a significant effect on the leaf greenness index (SPAD) at both test dates (BBCH\({}_{31}\) and BBCH\({}_{51}\)). No interaction was noted between the years of study and the preparations applied on the SPAD values. The highest leaf photosynthetic index at both observation dates was noted following the application of NPK + _P. azotofixanus_ and NPK and all the preparations combined (_P. azotofixanus_, _B. megaterium_ and _B. subtilis_). At the development phase of BBCH\({}_{31}\), when fertilising wheat with NPK alone and applying NPK and the microbial preparations individually, an increase in the photosynthetic rate value was obtained at the maximum amount of rainfall and at the maximum temperature. At the BBCH\({}_{51}\) stage, the maximum increase in the photosynthetic rate was obtained with the minimum amount of rainfall and at the maximum temperature in the fertilisation variants of NPK, NPK + _B. subtilis_, NPK + _B. megaterium_ and NPK + all the preparations in combination. The highest N-NO\({}_{3}\), N-NH\({}_{4}\) and P contents in the soil were noted following the application of NPK and all microbial preparations combined. Strong correlations were noted between the leaf greenness index (SPAD) and the photosynthetic index value and  
Figure 8: Diagram illustrating the correlation between the photosynthetic index (A), SPAD, protein content in grains, N-NO\({}_{3}\), N-NH\({}_{4}\) and P contents in the soil and the winter wheat grain yield.  
the protein content in wheat grains and between the N-NO\({}_{3}\), N-NH\({}_{4}\) and P contents in the soil and the wheat grain yield.  
Conceptualisation, A.S., K.W. and E.K.; methodology, A.S., K.W. and E.K.; validation, A.S., K.W. and E.K.; formal analysis, A.S., K.W. and E.K.; investigation, A.S.; resources, A.S., K.W. and E.K.; data curation, A.S., K.W. and E.K.; writing--original draft preparation, A.S., K.W. and E.K.; writing--review and editing, A.S., K.W. and E.K.; visualisation, A.S. and E.K.; supervision, A.S., K.W. and E.K.; project administration, A.S.; funding acquisition, A.S., K.W. and E.K. All authors have read and agreed to the published version of the manuscript.  
Project financially supported by the Minister of Education and Science under the program entitled \"Regional Initiative of Excellence\" for the years 2019-2023, Project No. 010/RID/2018/19, amount of funding 12.000.000 PLN. The results presented in this paper were obtained as part of a comprehensive study financed by the University of Warmia and Mazury in Olsztyn, Faculty of Agriculture and Forestry, Department of Agroecosystems and Horticulture (grant No. 30.610.015-110).  
Not applicable.  
The authors declare no conflict of interest.