The Effect of Organic Biostimulants on Beneficial Soil Microorganism Activity ()
1. Introduction
Biostimulants and organic fertilizers have been used by farmers for thousands of years [1] . Soil is a living entity with many components containing a balance of microbes, nutrients, animals, water, and air [2] . Commonly, farmers use additives to soil to improve crop yield and growth [3] . Four rates of biostimulant (0, 15, 30, 45 mL/3.8 L of water) were evaluated for increased microorganism activity. These additives can be made of many substances with some being organic and some being synthetic. The soil carbon-nitrogen ratio is essential for maintaining soil health. By applying sources of carbon and nitrogen, a farmer can improve the stability, health, and workability of soils [4] . Biostimulants, such as blackstrap molasses, can increase the microbial activity of the soil, and therefore the health of the plants being grown [5] . Biostimulants have also been found to improve plant growth and productivity [6] . Additionally, the application of organic fertilizers provides essential elements for plants and microorganisms. Additionally, the application of organic, animal-based fertilizer as opposed to synthetic, mineral-based fertilizer has been shown to increase biomass and uptake of nutrients [7] . The scope, though, of the many benefits of biostimulants and organic fertilizer is not fully known. However, CO2 evolution has been established as a method to measure microbial activity [8] . The Solvita burst-test, a method of measuring CO2 evolution from microorganisms, is useful in determining the changes in populations of organisms in a sealed vessel. The Solvita quick burst method can be accomplished in 24 hours using a CO2-sensitive paddle that can be read using a digital Solvita reader. This greatly simplifies and expedites measurement of microbial activity [8] . Research to determine the benefits of biostimulants and organic fertilizer is essential.
Fertilizer sources can influence both soil and plant uptake. Slow-release fertilizers have become the preferred method of farms today to provide plant crops necessary fertility. Since ancient Roman times natural organic fertilizers (animal manure, green waste, compost), have provided slow-release forms of essential plant nutrients [9] . Soil microbial breakdown of organic products produces a natural release of N through the N cycle, P through the P cycle and likewise for the other elements [10] [11] . Previous research shows a steady release pattern of essential fertilizer elements compared with rapid release of synthetically manufactured quick release fertilizers [12] . Fertilizer off-site runoff and aquifer contaminations can occur when excessive soluble fertilizer sources exceed the capacity of the plant and soil uptake. Quick release fertilizers are prone to runoff when infrequent fertilizer applications occur, particularly with low cation exchange capacity soils. Today, organic farming is being looked at again to reduce pollution and encourage beneficial soil organisms and increase organic buildup in the soil. Guidroz [13] reported increased organic matter accumulation in an organic vegetable farm in Baton Rouge, Louisiana. This in turn improves nutrient availability, an increase in soil health and reduced fertilizer pollution. Leaf tissue analysis is a good method of determining nutrient uptake [14] [15] .
The objectives of these experiments were to determine if biostimulants beneficially increase soil microorganism activity in soilless medium, and additionally to measure the impact of synthetic and organic fertilizers with blackstrap molasses on plant nutrient uptake nutrient runoff.
2. Materials and Methods
Study 1: On April 6, 2020 a study was initiated to determine the efficacy of biostimulants on soil microbial activity. Organic blackstrap molasses solutions were applied at 4 rates (0, 15, 30, and 45 mL/3.8 L of water) with 100 mL of treatment solutions applied to 3 L nursery containers filled with a pine bark substrate. Each treatment had 3 replications. The solutions of blackstrap molasses were drenched over the soil surface, simulating drip irrigation, and CO2 evolution was measured on a weekly basis for 4 weeks. Containers were irrigated every other day as needed with 100 mL of water. A sample (30 mL) of the substrate was harvested and was incubated 24 hours before measuring the CO2 evolution using a Solvita CO2 burst test, an established method of measuring soil health [8] [16] . Data was recorded and analyzed using SAS.
Study 2: On May 20, 2020 a second study was initiated to determine the efficacy of biostimulants and different types of fertilizer sources (6% N-2% P2O5-4% K2O) on soil microbial activity and plant health. Fertilizer rate was 1lb N/yd3. Treatments consisted of 1) synthetic fertilizer applied without molasses solution, 2) synthetic fertilizer with molasses solution at the rate of 30 mL/3.8 L of water, 3) organic fertilizer without molasses solution, and 4) organic fertilizer with molasses solution at the rate of 30 mL/3.8 L of water. Each treatment had 5 replications. The solutions of blackstrap molasses were drenched, and CO2 evolution was measured on a weekly basis for 5 weeks. Containers were irrigated every other day as needed with 100 mL of water. A sample (30 mL) of the substrate was harvested and was incubated 24 hours before measuring the CO2 evolution using a Solvita CO2 burst test. Plant tissue was collected and analyzed using the LSU soil testing lab to determine nutrient content [14] . Data was recorded and analyzed using SAS.
Study 3: On March 27, 2021 a third study was initiated to determine the change in soil pH, EC and loss of fertilizer nutrients. Fertilizers (6% N-2% P2O5-4% K2O) regardless of source (synthetic or organic) were applied at the rate of 1 lb N/yd3. Four-inch nursery containers having a soil capacity of 225 ml were filled with treatment amendments at the established rate. Each fertilizer source had 3 replications. Containers were filled and positioned onto 1 L mason jars to collect excess water which will be referred to as effluent over a 7-day period. Containers were drenched daily with 125 ml of water. Effluent was measured after the 7-day period and used to calculate pH, EC and total nutrient load (nitrates, P, K, Ca, Mg, and S). Data was analyzed using SAS at the 0.05 level.
3. Results and Discussion
Soil health: Soil microbial activity measured over a 6-week period resulted in significant differences. After 1 week, all treatments were significantly different from each other. The highest CO2 evolution increased with rate of molasses (Figure 1). After 2 weeks, the highest rate had the greatest CO2 evolution compared to all treatments. The two middle rates were statistically similar, and both greater than the control. All molasses rates were statistically different and greater than the control. At week 4, all treatments were statistically similar except the highest treatment, which was slightly higher than the other treatments (Figure 1). The organic fertilizer treatment with no molasses was significantly higher than all other treatments after 1 week (Figure 2). After 2 weeks, organic fertilizer with molasses was significantly higher than all other treatments. After 3 weeks, organic fertilizer with molasses was significantly greater than the other treatments.
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Figure 1. CO2 evolution of different rates of organic molasses in pine bark substrate after 4 weeks.
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Figure 2. CO2 evolution of different rates of organic molasses and different fertilizers after 5 weeks.
Organic fertilizer without molasses was greater than remaining treatments, which were statistically similar (Figure 2). After week 5, organic fertilizer with molasses was greater than all other treatments.
Plant Health: St. Augustinegrass N (Figure 3) and K (Figure 4) leaf levels were statistically similar between all treatments, however, P (Figure 5), Ca (Figure 6), Mg (Figure 7), and S (Figure 8) levels for organic fertilizer and organic
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Figure 3. St. Augustinegrass N leaf tissue nutrient content comparing fertilizer rate and source. ZMeans with the same letter are not significantly different at the 0.05 level.
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Figure 4. St. Augustinegrass K leaf tissue nutrient content comparing fertilizer rate and source. ZMeans with the same letter are not significantly different at the 0.05 level.
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Figure 5. St. Augustinegrass P leaf tissue nutrient content comparing fertilizer rate and source. ZMeans with the same letter are not significantly different at the 0.05 level.
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Figure 6. St. Augustinegrass Ca leaf tissue nutrient content comparing fertilizer rate and source. ZMeans with the same letter are not significantly different at the 0.05 level.
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Figure 7. St. Augustinegrass Mg leaf tissue nutrient content comparing fertilizer rate and source. ZMeans with the same letter are not significantly different at the 0.05 level.
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Figure 8. St. Augustinegrass S leaf tissue nutrient content comparing fertilizer rate and source. ZMeans with the same letter are not significantly different at the 0.05 level.
fertilizer + 30 mL of molasses were significantly higher than the organic fertilizer treatments. All tissue levels met the minimum acceptable range except for synthetic fertilizer treatments for Ca leaf tissue levels, which were less than 0.3% [14] .
Tomato N leaf tissue levels (Figure 9) were statistically similar between all treatments, however P (Figure 10), Ca (Figure 11), Mg (Figure 12), and S (Figure 13) levels for organic fertilizer and organic fertilizer + 30 mL of molasses were significantly higher in the organic fertilizer treatments. Potassium (Figure 14) tissue levels for synthetic fertilizer + molasses (30 mL/3.8 L of water)
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Figure 9. Tomato N leaf tissue nutrient content comparing fertilizer rate and source. ZMeans with the same letter are not significantly different at the 0.05 level.
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Figure 10. Tomato P leaf tissue nutrient content comparing fertilizer rate and source. ZMeans with the same letter are not significantly different at the 0.05 level.
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Figure 11. Tomato Ca leaf tissue nutrient content comparing fertilizer rate and source. ZMeans with the same letter are not significantly different at the 0.05 level.
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Figure 12. Tomato Mg leaf tissue nutrient content comparing fertilizer rate and source. ZMeans with the same letter are not significantly different at the 0.05 level.
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Figure 13. Tomato S leaf tissue nutrient content comparing fertilizer rate and source. ZMeans with the same letter are not significantly different at the 0.05 level.
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Figure 14. Tomato K leaf tissue nutrient content comparing fertilizer rate and source. ZMeans with the same letter are not significantly different at the 0.05 level.
maintained higher levels than all treatments, while synthetic fertilizer was greater than organic fertilizer levels compared to both organic fertilizer treatments. All tissue levels met the minimum acceptable range [15] .
Growth Parameter: Although there were significant soil and plant health differences, there were no significant differences measured for grass and tomato plant height and biomass accumulation (graphs not shown).
Nutrient Effluent: Potassium concentrations were below 1 ppm which was below detection for both fertilizers (data not shown). Analysis revealed significant differences between nutrient levels, pH, and EC levels between fertilizer sources. Soil pH was greater for the organic fertilizer source effluent as expected (Figure 15). Electro-conductivity (EC) and nutrient levels were significantly greater in the synthetic granular fertilizer compared to the organic fertilizer (Figure 16 and Figure 17). These results relate to the nutrient results in plant tissue where there were increases in organically fertilized plants (Figures 18-22).
Increased soil microbial activity was observed with the use of biostimulants. This has been reported by several researchers previously [6] . Additionally, organic fertilizer alone increased soil health. This has also been shown in prior research [2] [13] . Essential plant elements were shown to increase significantly when using biostimulants. Maintaining the acceptable ranges of essential elements
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Figure 15. Soil water effluent comparing synthetic and organic fertilizers after 7 days of watering. Z* = Significant at the 0.05 level.
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Figure 16. Soil water effluent electroconductivity comparing synthetic and organic fertilizers after 7 days of watering. Z* = Significant at the 0.05 level.
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Figure 17. Total nutrient load concentration of soil water effluent. Z* = Significant at the 0.05 level.
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Figure 18. Soil water Ca comparing synthetic and organic fertilizers after 7 days of watering. Z* = Significant at the 0.05 level.
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Figure 19. Soil water Mg comparing synthetic and organic fertilizers after 7 days of watering. Z* = Significant at the 0.05 level.
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Figure 20. Soil water P comparing synthetic and organic fertilizers after 7 days of watering. Z* = Significant at the 0.05 level.
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Figure 21. Soil water S comparing synthetic and organic fertilizers after 7 days of watering. Z* = Significant at the 0.05 level.
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Figure 22. Soil water NO3- comparing synthetic and organic fertilizers after 7 days of watering. Z* = Significant at the 0.05 level.
is necessary for proper plant health. Although in our study there were no growth differences measured, proper nutrition is vital for plant growth and development. Reducing nutrient runoff is an important best management practice. Increased nutrient losses can result in increased off-site runoff and aquifer contamination. Increasing plant usage can also improve plant growth and yield resulting in improved nutrient efficiency [7] . Best management practices strive to improve crop production, lower cost, and reduce environmental impacts. Organic fertilizer seemed to approach best management practices more closely compared to the synthetic fertilizer source.
4. Conclusions
Soil biostimulants did significantly increase the microorganism activity. In study 1, the higher rates of molasses solution had significantly higher rates of CO2 evolution. The higher rate had increased activity after 1 week by more than 100%. This increase persisted for 4 weeks. Overall, blackstrap molasses increased microbial activity. In study 2, there were significant differences between fertilizer sources and molasses treatments. The benefit of molasses was dependent upon fertilizer source and dependent variables. Organic fertilizer with molasses maintained the highest activity after 5 weeks for all treatments. Although there was increased microbial activity and fertility interaction, there was no measured increase or decrease in plant growth. Further research should be considered to elucidate the full impact of molasses and the use of organic fertilizers. Blackstrap molasses does have a significant, beneficial effect on plant health and microbial growth. Blackstrap molasses combined with fertilizers may have horticultural benefits for crop production. However, more research needs to be done to determine if the use of blackstrap molasses is economically feasible for commercial production.
In conclusion,
- Biostimulants increased microorganism activity;
- Biostimulants with organic fertilizer had the highest rate of microorganism activity;
- Biostimulants had a positive effect on plant health.
Application to Society
Blackstrap molasses does have a significant, beneficial effect on plant health and microbial growth. Blackstrap molasses combined with fertilizers may have horticultural benefits for crop production. Organic fertilizers combined with molasses did increase essential element uptake, which can improve crop growth. However, more research needs to be done to determine if the use of blackstrap molasses is economical feasible for commercial production. Fertilizer management can reduce pollution and increase plant growth.