The Effect of Growth Activators and Plant Growth-Promoting Rhizobacteria (PGPR) on the Soil Properties, Root Yield, and Technological Quality of Sugar Beet

The strategy “from farm to fork” assumes a reduction in the usage of fertilizers and plant protection products in EU agriculture. The aim of this study, conducted over the years 2017–2019 in several locations in Poland, is to evaluate the application of growth activators with and without plant growth-promoting rhizobacteria to reduce mineral nitrogen fertilization without negative effects on the root yield. We studied the effect of these activators on selected soil properties. The experimental treatments included the application of the growth activators Penergetic (K + P) and Azoter, which contains the bacteria Azotobacter chroococcum, Azospirillum brasilense, and Bacillus megaterium, before sowing or during vegetation. The nitrogen rates were reduced by 30% in comparison to full nitrogen mineral fertilization (control treatment). In selected experiments, the application of Penergetic and Penergetic with Azoter caused a higher content of nitrate nitrogen (N-NO3) and ammonium nitrogen (N-NH4) after the sugar beet harvest as well as higher contents of mineral nitrogen (Nmin), P, K, and Mg in the soil in comparison to the treatment with the full dose of mineral nitrogen fertilization. The obtained results proved that it was possible to reduce the mineral application of nitrogen by 30% without a decrease in the biological and pure sugar yield, and even with an increase in the sugar yield caused by the application of the growth activators Penergetic (K + P) and Azoter.


Introduction
An expected 50% growth in the global population during the 21st century demands advanced technologies and low-input crop management, including precision delivery of nutrients at certain growth stages of crops [1].
In the crop production of sugar beet, new and effective methods of increasing sugar yields have been introduced. One of the innovations is the foliar application of silicon [2][3][4][5][6][7][8][9]. To protect the environment, efforts are typically made to limit the doses of mineral nitrogen. An innovative method is the application of the growth activators Penergetic P and K. A beneficial effect of the application of Penergetic-P for yield was proved in potatoes [10], sugar beet [11], snap-beans [12], and coffee [13]. A positive effect of Penergetic-K on grain yield and its quality was observed in organic wheat production [14] and soybeans [15]. A positive effect of the preparation on the seed germination energy and biometric traits of vegetables was also observed [16] as well for seedlings of winter wheat [17]. Nascente and Cobucci [18] observed positive effects of Penergetic-K and P application on the seed yield of common beans.
Agronomy 2020, 10, x FOR PEER REVIEW 2 of 17 Various groups of bacteria that are able to stimulate plant growth by a mechanism(s) of action are referred to as plant growth-promoting rhizobacteria (PGPR). They affect plant growth and development directly or indirectly either by releasing plant growth regulators (PGRs) or other biologically active substances. This causes an increase in the content of available nutrients, which increase nutrient uptake and reduces the negative effects of pathogenic microorganisms on plants [19]. PGPRs have various types of effects, such as an increase in root growth and nutrient uptake, the stimulation of plant hormones, inhibition of the activity of plant pathogens, improvement of the soil structure, and mineralization of organic pollutants [20].
Biofertilizers increase nutrient availability and crop health without negative environmental effects [21]. Bacterial-based biofertilizers and the plant probiotics market is growing all around the world due to the need for sustainable crop production [22]. One example of the commercial application of Azospirillum sp. inoculants is the use in approximately 3.5 million ha in South America [23]. In the near future, plant growth-promoting bacteria (PGPB) will begin to replace the use of fertilizers and pesticides in agriculture [24]. Rhizospheric bacteria and fungi in the form of biofertilizers and biopesticides allow for reduced application of synthetic chemicals in crop production [25]. Plant growth-promoting rhizobacteria with probiotic potential can improve plant health and yield without negative environmental effects [26].
The aim of the study was to evaluate the effect of growth activators with and without plant growth-promoting rhizobacteria on sugar beet to avoid the negative effects of N fertilization on sugar yield and selected soil properties.
The pH of the soil before the experiments was established as neutral or alkaline, except for Kępina in 2019 (in the layer from 0-30 cm) ( Table 1). The pH of the SOC was in the range of 1.15-3.54 (0-30 cm) and 0.95%-2.97% (30-60 cm). For 0-30 and 30-60 cm, respectively, the N-NO 3 was at 5.7-81.5 and 6.6-23.4 mg kg −1 , the N-NH 4 was at 0.26-5.02 and 0.28-3.60 mg kg −1 , the mineral nitrogen (Nmin) was at 23-337 and 32-105 kg ha −1 , the P was at 46-145 and 27-155 mg kg −1 , the K was at 62-254 and 47-242 mg kg −1 , and the Mg was at 69-153 and 70-137 mg kg −1 . For 0-30 and 30-60 cm, respectively, the content of available micronutrients was (mg kg −1 ): B-2. 10 The amount of rainfall during the growing season (April-October) in the year 2017 was 443-615 mm, in 2018-466 mm, and in 2019, from 364 to 426 mm ( Table 2). During the period of the highest demand of sugar beet for water (June-August), the optimal value of the k-factor was found only in 2017 in July and August 2019 in Pągów and in June 2019 in Kępina. Sugar beet was grown in Sahryń after winter rapeseed, and in other locations after winter wheat. The characteristics of the technology used in the experiments are presented in Table 3.   In the experiments during autumn, we applied (depending on location) Polifoska 6 fertilizer (6% N in ammonium form, 8.7% P as mono and diammonium phosphate, 24.9% K as potassium chloride, and 2.8% S as sulphate), potassium chloride (49.8% K as potassium chloride), and Korn-Kali-potassium chloride with the addition of magnesium salt (33.2% K, 3.6% Mg, 3% Na, and 5% S). In spring, before sowing, we applied Saletrzak Standard 27-ammonium nitrate with the addition of dolomite flour containing calcium and magnesium (13.5% N in the ammonium form and 13.5% N in the nitrate form, 1.4% Ca, and 2.4% Mg) or ammonium sulphate 32 (16% N in the ammonium form and 16% N in the nitrate form).
During the growing season in the experiment, at the six-leaf stage of sugar beet (Biologische Bundesanstalt, Bundessortenamt und Chemical Industry growth scale (BBCH 16)) and 14 days later, foliar nutrition was conducted with micronutrient fertilizers containing boron (2 × 300 g ha −1 B). Protection against weeds, diseases, and pests was performed in accordance with the recommendations of the Institute of Plant Protection-National Research Institute in Poznań.
In the experiment, three treatments were applied: 0-Control (full nitrogen fertilization dose depending on the location-from 112 to 175 kg ha −1 N); 1-Dose of mineral nitrogen lower by 30% in comparison to the full dose before sowing and during vegetation-from 78 to 123 kg ha −1 N; Penergetic-K (400 g ha −1 ) on the straw of the forecrop before it was mixed with the soil; Penergetic-K (400 g ha −1 ) with the first herbicide spray; Penergetic-P (300 g ha −1 ) with a second herbicide spray in spring; and Penergetic-P (300 g ha −1 ) at the six-leaf stage of sugar beet (Biologische Bundesanstalt, Bundessortenamt und Chemical Industry growth scale (BBCH 16)); 2-Dose of mineral nitrogen lower by 30% in comparison to the full dose before sowing and during vegetation-from 78 to 123 kg ha −1 N; Penergetic-K (400 g ha −1 ) + Azoter (10 dm 3 ha −1 ) on the straw of the forecrop before it is mixed with the soil; Penergetic-K (400 g ha −1 ) + Azoter (10 dm 3 ha −1 ) in spring with the first herbicide spraying; Penergetic-P (300 g ha −1 ) with the second herbicide spray; and Penergetic-P (300 g dm 3 ha −1 ) at the six-leaf stage of sugar beet (BBCH 16). Penergetic-K and Penergetic-P are growth activators, the compositions of which are reserved by the manufacturer. They are produced based on bentonite and molasses processed by a special technology. The product contains no other chemical substances. The Penergetic International AG Company produces Penergetic-P and -K, from bentonite clays subjected to the application of electric and magnetic fields. These products are used to increase the photosynthetic efficiency of plants (Penergetic-P) or to improve the performance of organic matter decomposing organisms of the soil (Penergetic-K).
The number of replications was 4, and the total number of plots was 12. Each plot consisted of six rows. The dimensions of a single plot were a length of 16 m and width of 2.7 m (43.2 m 2 ), of which 21.6 m 2 (three middle rows) was used for harvesting. During harvest, the plants were topped by hand on the three middle rows, and the leaves were weighed. The roots were then counted, dug up, and weighed. During the harvest, each plot was collected in accordance with the Polish Standard [39]. The root samples were transported to the Plant Breeding Station of the Kutno Sugar Beet Breeding Company inŚmiłów, where they were processed into pulp. The pulp was transported to Straszków, where the technological quality of the roots was evaluated on the automatic Venema technological line [40]: the sugar content polarimetrically [41], the K and Na by photoelectric flame photometry [41], and the α-amino nitrogen by fluorometric methods [42].
The data were analyzed using analysis of variance and multiple comparison of means using Tukey's procedure. The significance level for all the analyses was set at 0.05. The analyses were performed using Statistica 13 software (TIBCO Software Inc., Palo Alto, CA, USA). Descriptive statistics, such as the minimum, maximum, standard deviation (SD), and coefficient of variation (CV) were calculated.

Results
In certain locations, we observed that combinations No 1 and 2 were characterized by a higher content of nitrate nitrogen (N-NO 3 ) and ammonium nitrogen (N-NH 4 ) and, consequently, higher amounts of mineral nitrogen (Nmin) and higher contents of P, K, and Mg in the soil in comparison to the control treatment, 0 (Table 4). 0-Control; 1-Penergetic (K + P); and 2-Penergetic (K + P) + Azoter.
Variant No 1 (by 5.5%) and variant No 2 (by 5.1%) had significantly greater plant density during harvest in relation to the control object (Table 5). The use of Penergetic activators (variant No 1) resulted in a significant increase in the root yield (by 7.3%), harvest index value (by 3.1%), biological sugar yield (by 6.0%), and pure sugar yield (by 6.3%) and a significant reduction in the potassium content in the roots (by 11.8%), the foliage coefficient (by 10.0%), the standard molasses losses (by 4.0%), the sugar yield losses (by 2.8%), and the value of the alkalinity coefficient (by 14.7%) compared to the control object (Table 5, Figure 2).
Combination No 2 demonstrated a significant increase in the yield of leaves (by 8.1%), the yield of roots (by 11.5%), the total yield of roots and leaves (by 10.2%), the biological yield of sugar (by 12.7%), the pure sugar yield (by 13.4%), and the sugar productivity (by 0.5%) and a significant reduction in the potassium (by 9.4%) and sodium (by 17.9%) content in roots, the standard molasses losses (by 4.0%), the sugar yield losses (by 2.8%), and the value of the alkalinity coefficient (by 12.5%) in relation to the control combination.
The Penergetic + Azoter facility was characterized by a significantly higher yield of leaves (by 13.1%), sugar content in the roots (by 2.0%), total yield of roots and leaves (by 7.1%), fresh mass of the plant leaves (by 11.8%), fresh mass of the plant (by 6.3%), biological yield of sugar (by 6.3%), pure sugar yield (by 6.7%), and content of the refined sugar (by 2.3%) compared to the Penergetic object.
For the majority of the studied traits, a significant effect of the environment (years × location) was observed as well as a significant interaction between the treatments and environments, which indicates that the effect of the combinations varied in different years and locations.
Among the examined traits, the highest variability was found in the sodium content in the roots (CV = 49.3%), and the lowest variability was found in the sugar productivity (CV = 2.3%) ( Table 6). Combination No 2 demonstrated a significant increase in the yield of leaves (by 8.1%), the yield of roots (by 11.5%), the total yield of roots and leaves (by 10.2%), the biological yield of sugar (by 12.7%), the pure sugar yield (by 13.4%), and the sugar productivity (by 0.5%) and a significant reduction in the potassium (by 9.4%) and sodium (by 17.9%) content in roots, the standard molasses losses (by 4.0%), the sugar yield losses (by 2.8%), and the value of the alkalinity coefficient (by 12.5%) in relation to the control combination.
The Penergetic + Azoter facility was characterized by a significantly higher yield of leaves (by 13.1%), sugar content in the roots (by 2.0%), total yield of roots and leaves (by 7.1%), fresh mass of the plant leaves (by 11.8%), fresh mass of the plant (by 6.3%), biological yield of sugar (by 6.3%), pure sugar yield (by 6.7%), and content of the refined sugar (by 2.3%) compared to the Penergetic object.
For the majority of the studied traits, a significant effect of the environment (years × location) was observed as well as a significant interaction between the treatments and environments, which indicates that the effect of the combinations varied in different years and locations.
Among the examined traits, the highest variability was found in the sodium content in the roots (CV = 49.3%), and the lowest variability was found in the sugar productivity (CV = 2.3%) ( Table 6).

Discussion
We obtained, in our study, the increase in the amount of mineral nitrogen in the soil after sugar beet harvesting in combinations with Penergetic activators. This confirmed the results of previous studies in which the use of the Penergetic-K (300 g ha −1 ) activator in pre-sowing spraying in organic winter wheat cultivation resulted in an increase in pH, redox potential, and specific soil electric conductivity [14]. The process of mineralization increased when the oxidation was stronger, and this caused an increase in the amount of mineral nitrogen in the soil after the crop harvest. The combined use of Penergetic with phosphorus and potassium mineral fertilization promoted the activity of fauna and microorganisms present in the 0-8 cm soil layer in soybeans. The activity of fauna and microorganisms in the 9-16 cm soil layer was intensified with the use of Penergetic, alone or with phosphate and potassium fertilization, in soybeans. The use of Penergetic did not significantly increase the soil biological activity in the winter crop [45].
The use of the activators Penergetic-K and Penergetic-P combined with a reduced dose of mineral nitrogen (reduced by 30%), increased the yield of the roots by 7.3%, the fresh root mass by 2.1%, the biological yield of sugar by 6.0%, and the pure sugar yield by 6.3% and reduced the sugar content of the roots by 0.17 p.p. in comparison with the control treatment (without the Penergetic activators and with a full dose of mineral nitrogen). In the literature, there are few results of experiments with the use of Penergetic activators in the cultivation of sugar beet. In one, foliar spray of Penergetic-P (0.1 dm 3 ha −1 ) applied in the growth stage of BBCH 18-19 of sugar beet caused an increase in the before-harvest content of chlorophyll a (by 5.6%), chlorophyll b (by 8.6%), chlorophyll a + b (by 6.4%), and carotenoids (by 3.0%) in comparison to the control [11]. Root yield increased by 14.5%, and the average root mass increased by 9.1%. However, the sucrose content in the roots increased by 0.34 p.p., and the yield of white sugar increased by 17.2%.
The results of experiments with the use of Penergetic activators in the cultivation of crops other than sugar beet are available in the literature. The application of Penergetic-P in potato cultivation increased the share of large tubers from 39% to 55.9% and reduced the proportion of small tubers from 32.0% to 19.3% and, consequently, increased the yield of tubers by 16.6% in comparison to the control [10]. The application of Penergetic before sowing (dose 300 g ha −1 ) in organic winter wheat caused an increase in grain yield by 5.0% in comparison to the control and an improvement of grain quality (increase in the protein content by 2.4% and gluten by 2.9%) [14].
Penergetic in soybeans did not affect the plant growth. However, the grain weight per plant and yield of soybeans were higher (by 20%) in comparison with the application of the fertilizer NPK and micronutrients [15].
Soil and foliar application of Penergetic-P in spring wheat caused an increase in the grain yield in comparison to wheat grown without such application. Penergetic-P applied to the soil and by foliar application resulted in a substantial increase in the number of seeds per spike, and lengthening of the stems compared with wheat in the control treatment [46].
In a greenhouse experiment performed in Brazil in green beans, the dry matter yield of commercial pods was not significantly different between objects with Penergetic-P and Penergetic-K and full mineral fertilization (NPK) versus objects with Penergetic-P and Penergetic-K with a 25% dose of NPK [12]. Penergetic-P positively influenced the germination of radish and tomato seeds as well as the growth and development of cucumber, red beet, tomato, and radish seedlings [16]. The use of Penergetic-P on the sowing day for winter wheat seeds in organic cultivation had a positive effect on the health of seeds and the root system assessed at the beginning of autumn during tilling (BBCH 21-23). Infestation of cocci by Fusarium spp. was reduced from 13.5% in the control to 9.0%, and root diseases were reduced from 71.7% to 55.0% [17].
There are also experimental results proving the beneficial effect of the Penergetic growth activators on the nutrition and yield of coffee, and the effect was dependent on the cover plants on the plantation [13]. Nascente and Cobucci [18] evaluated the effect of the application of Penergetic-K and Penergetic-P on the yield of common beans with diversified fertilization with phosphorus. A greater availability of P to plants when Penergetic was applied was possibly due to the soil colloids and organics or partly due to the increased microbial activity in the soil.
There are no results from studies with the use of Penergetic activators together with plant growth-promoting rhizobacteria. In our own research, we observed that the use of Penergetic activators together with the Azoter activator containing bacteria Azotobacter chroococcum, Azospirillum brasilense, and Bacillus megaterium caused an increase in the yield of roots by 3.9%, the fresh root mass by 4.2%, biological yield of sugar by 6.3%, pure sugar yield by 6.7%, and sugar content in the roots by 0.33 p.p. in comparison to the object with only Penergetic activators.
Inoculation by Azospirillum sp. of dryland crops showed a grain yield increase on winter (by 14.0%) and spring cereals (by 9.5%) and also on legumes (by 6.6%). Inoculation with selected strains of Azospirillum sp. increased crop yields and enhanced the efficacy in plant nutrient availability [23]. The inoculation of maize seeds with Azospirillum brasilense intensified the plant growth, improved biochemical traits, and raised the nitrogen use efficiency under a nitrogen deficit [47]. Little effect of inoculation with Azospirillum brasilense on grain yield was observed for wheat (Triticum aestivum) [48]. A beneficial influence of co-inoculation of Azospirillum lipoferum and Baccilus megaterium was observed for providing balanced N and P nutrition for wheat plants [49].
The research on Azotobacter chroococcum spp. in crop production has manifested its significance in plant nutrition and its contribution to soil fertility. The Azotobacteria genus synthesizes auxins, cytokinins, and GA-like (Gibberellin-like) substances, and these growth materials are the primary substances controlling enhanced growth. These hormonal substances affect the growth of the closely associated higher plants [50].

Conclusions
The results obtained in several locations proved that it was possible to reduce the fertilization of sugar beet with mineral nitrogen by 30% without reducing the biological yield of sugar and pure sugar yield, and even to obtain an increase as a result of the use of the Penergetic-K and Penergetic-P growth activators and the Azoter preparation containing the bacteria Azotobacter chroococcum, Azospirillum brasilense, and Bacillus megaterium. This proved that high sugar yields can be obtained with more environmentally friendly technologies.