## 2 Materials and Methods  
### The Field Experiment  
The field experiment was conducted on an organic farm located in the village of Wlazy, near the town of Siedlce, Poland (52.1379376 N, 16.0029782 E). The experiment was conducted on Stagnic Luvisol (according to the WRB). The soil pH was neutral (pH in KCl 6.1). The 0-20 cm soil layer contained 1.05% of organic matter; pH 6.2 (measured in 1 M KCl). At the beginning of the experiment, the content of available P, K, and Mg was as follows: P--8.3 mg:100 g\({}^{-1}\) soil, K--12.1 mg:100 g\({}^{-1}\) soil, and Mg--4.2 mg:100 g\({}^{-1}\) soil.  
The experiment was conducted in the years 2019-2021. Eight variants were randomly used each year, each in three repetitions:  
1--control variant (without undersown crops, without PGPR);  
2--red clover catch crop;  
3--red clover and Italian ryegrass catch crop;  
4--Italian ryegrass catch crop;  
5--control variant (without undersown catch crops) + PGPR;  
6--red clover catch crop + PGPR;  
7--red clover catch crop + Italian ryegrass + PGPR;  
8--Italian ryegrass catch crop + PGPR.  
The area of the plot was 20 m\({}^{2}\) (4 \(\times\) 5 m).  
Spring barley with undersown cover crops was grown at the site where winter rye had been cultivated. After harvesting the forecrop, post-harvest tillage was applied. In late October a field for the experiment was prepared and goat manure was applied with a spreader over the entire area at a dose of 15 t ha\({}^{-1}\). The plot was then ploughed before winter and the field was left until spring. Over the three years of the experiment, the average content of nitrogen, phosphorus, potassium, and magnesium in the goat manure was as follows: 0.54%, 0.28%, 0.87%, and 0.15%.  
In spring, in early April, a pre-sowing cultivator was used. Afterwards, the plots were delimited, and spring barley seeds were sown in all of them with a plot seeder to the amount of 160 kg ha\({}^{-1}\). On the same day, undersown cover crops were sown in the plots: red clover--18 kg ha\({}^{-1}\), a mixture of red clover and Italian ryegrass 9 + 15 kg ha\({}^{-1}\), and Italian ryegrass 30 kg ha\({}^{-1}\).  
Spring barley was sown with a grain drill in rows spaced at 12.5 cm and at a depth of 5-6 cm. The companion crops were then sown in the barley rows, at a depth of 1-2 cm, spaced at 12.5 cm. Spring barley seeds were purchased from the 'STARY FOLWARK' organic farm. The red clover and Italian ryegrass seeds also came from organic cultivation and were purchased from the seed company DSV Polska Sp. z o.o. (DSV Poland Ltd., Wagrowiec, Poland). The seeds were certified by AGROBIOTEST Sp. z o.o (AG-ROBIOTEST Ltd., Warsaw, Poland). The PGPR consortium (_Bacillus subtilis, Bacillus amyloliguefaciens, Pseudomonas fluorescens_) protecting crops from molds was applied twice during the growing season (first term '0' on the day of sowing, second term--BBCH 29-30). The inoculant was applied at a dose of 1 L 200 L\({}^{-1}\) water ha\({}^{-1}\). The bacteria in the consortium were concentrated at 10\({}^{12}\) CFU (colony form units) mL\({}^{-1}\).  
In the control plots, where only spring barley was sown without undersown cover crops, mechanical care (a weeder narrow after the emergence of plants and a medium narrow at the development of 5-6 leaves) was applied for weed control. Mechanical weed control was not applied in the plots with spring barley and undersown cover crops.  
Undersown cover crops initially grow slowly and tolerate shading well. Therefore, they can be sown in plantations where cereals are grown as the main crops. After the application of PGPR the increase in the biomass of undersown cover crops can be expected to be greater than in the plots where the bacteria have not been applied because PGPR act as biostimulants. Undersown cover crops are harvested in August and September, after the harvest of the main crop. Therefore, in October, their biomass can be used as green manure.