Sargassum sp. as a Bio-Fertilizer: Is It Really a Key Towards Sustainable Agriculture for the Bahamas?

doi:10.21203/rs.3.rs-3101286/v1

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Research Article

Sargassum sp. as a Bio-Fertilizer: Is It Really a Key Towards Sustainable Agriculture for the Bahamas?

https://doi.org/10.21203/rs.3.rs-3101286/v1

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Background Macroalgae blooms (Sargassum sp.) occur annually in the Bahamas due to the integration of various events related to human intercession with the roles of algae in biogeochemical cycles. These blooms are of great concern, as they are associated with many negative effects; thus, the primary aims of this study were to assess the quality of soils collected from South, Central, and North Long Island, and to determine if Sargassumsp. can be used as a biofertilizer for soils on Long Island. A 60-day pot trial method was established to determine the efficacy of different concentrations (1%, 5%, and 10%) of Sargassum sp. as a biofertilizer on cherry tomato cultivation. Additionally, the soil quality prior and after fertilizer amendment was evaluated.

Results The results show that Sargassum sp. increased nutrient content of the soil, specifically nitrate nitrogen and phosphorus; however, plant growth performance parameters (plant height, leaf number, bud number, flower number, and root and shoot weights) were negatively impacted.

Conclusions Due to the obtained results, it is recommended that serious consideration be taken when utilizing Sargassum sp. as a biofertilizer because the pH and type of soil in Long Island, Bahamas affects the bioavailability of the nutrients in algae.

Agronomy

Alkaline soil

microalgal biofertilizer

Sargassum species

tomato cultivation

soil nutrients

Long Island Bahamas

Sargassum sp. was used as a soil amendment for Lycopersicon sp.
Sargassum sp. increased soil nutrients, soil organic matter content, and salinity levels.
Sargassum sp. treatments negatively affected the growth of tomatoes compared to the control group.

The increasing food prices and limited agricultural sectors have resulted in a significant economic problem for Bahamians. This generated an increase in sustainable organic farming, also referred to as ‘backyard farming’ (Simpson Miller, 2022). Additionally, The Bahamas has poorly developed alkaline soils referred to as protosols which have a pH range of 7.5 to 8.5 (Thomas, 2017; Taylor & Ngatia, 2021). The Bahamas also experiences hurricanes and floods which result in salt intrusion. Salinity negatively impacts the soil by increasing soil pH and decreasing organic matter content and the soil microbial biomass (Zhang et al., 2019; Wang et al., 2022). Despite the subtropical climate, the soil present on the limestone archipelago of The Bahamas is undesirable; however, it can be amended using fertilizers.

Fertilizers seem to be the best solution, but inorganic or synthetic fertilizers can be an economic challenge and extremely problematic for the environment (Mishra et al., 2013). The use of synthetic fertilizers, such as nitrogen, phosphorus, and potassium (NPK) fertilizer is expensive; sustainable farmers are unable to afford it. Sustainable farmers are those that adopt conservative methods of cultivation that meet their needs without compromising environmental health nor the ability for upcoming generations to acquire their needs (Campos, 2022). Moreover, the use of inorganic fertilizers has polluted water sources, contaminated the soil, killed microbes and organisms, reduced soil fertility, and increased the vulnerability of plants to disease. These consequential results have led to the driven necessity for biofertilizers (Mishra et al., 2013). Biofertilizers are substances that can be plant derived (lignocellulosic biomass) or animal derived, consisting of microorganisms that support plant growth by enhancing the nutrient supply when applied to the seeds, plants, or soil (Daniel et al., 2022).

Numerous studies have found that organic fertilizers, specifically algae biofertilizers, can maximize plant growth rate and crop yield while improving soil quality (Bind et al., 2023; Das et al.,2019; Kholssi et al., 2022). Furthermore, most importantly, the use of algal biofertilizer will combat the major issue of annual macroalgal blooms in the country. Macroalgae blooms are a natural phenomenon; however, their magnitude, duration, and frequency has proliferated due to both direct and indirect anthropogenic influences (Saedi et al., 2023). Godínez-Ortega et al. (2021) specifically articulates that these blooms are due to a combination of several incidences primarily related to human intercession with the roles of algae in biogeochemical cycles. Moreover, global warming and climatic change also contribute to the increase in macroalgae blooms (Zerrifi et al., 2018). Each year the beaches and coastlines of The Bahamas are engulfed with an abundant amount of Sargassum seaweed, specifically Sargassum fluitans and Sargassum natans (Mendez-Tejeda & Rosado, 2019). There are several negative impacts associated with the mass influx of Sargassum sp. This influx impacts marine fauna, the livelihood of coastal communities, and the tourism sector (Mendez-Tejeda & Rosado, 2019).

Furthermore, the use of organic fertilizers, especially algae biofertilizers, instead of synthetic fertilizers is the way forward. Seaweeds have been used historically as a fertilizer for many years, but the applications are advancing. Thompson et al. (2020) conducted a study in which the economic feasibility of utilizing seaweeds, specifically Sargassum sp., for biogas and biofertilizer production was evaluated. Moreover, in a study conducted by Coppens et al. (2016) to determine how microalgae affected the quality of tomatoes, it was found that the biofertilizer increased the sugar and carotenoid concentrations of the tomatoes. This resulted in tomatoes with better quality and higher economic value. As it pertains to the growth and yield rates, a study done in 2018 found that utilizing marine microalgae in the cultivation of rice increased yields by 7–20.9% (Dineshkumar et al., 2018). Additionally, in an experiment to determine the effectiveness of algae biofertilizer on onion cultivation, it was confirmed that microalgal biofertilizers have the potential to supercede inorganic fertilizers due to its successful increase in seed germination, seed quality, plant growth parameters, physical chemical parameters, and yield production (Dineshkumar et al., 2020). Prior research has agreed with the idea that biofertilizers, specifically algae, are the most desirable when compared to inorganic fertilizers. The implementation of algal biofertilizer use will conduce a healthy environment, socio-economic equity, and economic profitability globally (Win et al., 2018). Utilizing algal biofertilizers as an alternative to inorganic fertilizers correlates with long term, organic, agricultural sustainability.

Although many studies and reviews have been conducted on the effects of algae biofertilizers on the cultivation of various plants, The Bahamas’ soil type and pH are different from many of the soils used in previous research. This study will assist in expanding the knowledge of macroalgae biofertilizer usage, specifically for the Bahamian soil type. The Bahamas has the potential to give the burden of macroalgal blooms both agricultural and economic value as a biofertilizer. Furthermore, most of the previous research only focuses on liquid extracts of algae as a biofertilizer instead of using shredded macroalgae or the macroalgae in its natural form. Thus, conducting an experimental study to determine the potential of Sargassum species in its granular form as a biofertilizer in alkaline soils will be beneficial.

The research questions for this study include: In using soil from South, Central, and North Long Island, what soil to algae biofertilizer ratio is needed for maximum growth performance of tomato plants during a 60-day period? What is the physicochemical status of the soil prior to the addition of the algae biofertilizer? How does different application rates of the algae biofertilizer affect the physicochemical properties of the soils using health indicators? And is there a difference in plant health and growth with the use of the control fertilizer (NPK fertilizer) and the experimental fertilizer (algae biofertilizer)?

The aim of this research is to determine if Sargassum sp. is really a key towards sustainable agriculture for The Bahamas. The objectives are to assess the soil quality of soils collected in South, Central, and North Long Island using health indicators (pH, macronutrients (N, P, K), organic matter content, and soil water holding capacity), to determine if Sargassum sp. can be used as a biofertilizer for soils on Long Island, to investigate how Sargassum sp. affects soil health and fertility, and to establish which application rate of Sargassum sp. is most ideal for cherry tomato cultivation within a 60-day period. It is hypothesized that: soil from South Long Island will have the best quality as compared to soils from Central and North Long Island; algae biofertilizer will improve soil health and fertility; and algae biofertilizer will result in better plant growth performance as compared to the NPK fertilizer.

Algae collection and preparation

On 24th June and 26th June 2022, a total of 14.75 kg of damp Sargassum sp. was collected from Clarence Town, Long Island, Bahamas (23.10058° N, 74.96306° W). The Sargassum sp. was desalinated by placing it in a black fifty-five-gallon drum filled with water from a groundwater well located in Morrisville, Long Island, Bahamas (23.03597° N, 74.90030° W). From the 26th of June to the 29th of June, 2022, the algae was soaked in the drum and the water was changed daily. From 30th June to 6th July 2022, the algae were laid on a black tarp to thoroughly dry in the sun. The algae were shredded into an almost powder-like form and stored in ziploc bags until needed (Kaladharan et al., 2021). This algae biofertilizer was added to the pots based on soil weight ratio. The treatments were mixed in batches and divided by 5 for each treatment (1%, 5%, and 10% application rates).

Experimental design

A 60-day pot trial method was utilized to test the effect of NPK fertilizer (Black Kow 0.5–0.5–0.5) and various application rates of brown algae, specifically Sargassum sp., on the growth of Yates Small Fry cherry tomatoes from 1st September 2022 to 30th October 2022 (Kumar & Nikhil, 2016). Black Kow was utilized according to the manufacturers’ protocol. In addition, the macroalgae was classified to the genus level using the phenotypic characteristics described by Mattio & Payri (2011). Moreover, this pot trial method was completed outdoors in Morrisville, Long Island, Bahamas (23.03597° N, 74.90030° W) to ensure that the plants and soil experienced the environmental conditions of the island.

However, due to the high temperatures, the plants were placed in an area shaded by black, mesh greenhouse shade netting. To further optimize experimental conditions, the plants were undisturbed and watered twice a day (in the morning and afternoon, except when it rained). Seeds were planted in Jolly Gardener Potting Soil and transplanted into 1-gallon pots 6 weeks after sprouting, which was the 1st of September 2022. Five replicates were used for each treatment with 1 plant per pot. There were four treatments and a control group for each soil collected, giving a total of 75 pots and 12 treatments - Control group, 1% Algae, 5% Algae, 10% Algae, and NPK fertilizer. Soil amendment was calculated based on the soils’ dry weight.

Soil collection and analysis

Soils were collected from three locations in Long Island, Bahamas to complete the 60-day pot trial method and determine which soil was best for the cultivation of tomato plants. Soil was collected from South, Central, and North Long Island, specifically from Roses (22.96511° N, 74.88258° W), Grays (23.21997° N, 75.10003° W), and Stella Maris (23.59067° N, 75.26932° W). The areas of soil collection in Roses (South Long Island) and Grays (Central Long Island) had been recently cleared by a tractor. Soil analysis was done prior and after the experiment to determine soil pH levels, nitrate nitrogen, nitrite nitrogen, potassium, and phosphorus concentration levels using the LaMotte Agricultural Combination Soil Kit (Model STH − 14 - Soil Testing Outfit) (Jacobo et al., 2021). Prior to analysis, the soil samples were air dried and sieved with a 5 mm sieve.

Soil type was determined using the Ribbon Method established by the Food and Agriculture Organization of the United Nations (FAO, 2020). A 5 in 1 TDS/ EC/ pH/ Salinity/ Temperature meter (Brand: RRMY-Digital Multimeter) was used to measure soil salinity level percentage, salinity concentrations, total dissolved solids (TDS) value, and electrical conductivity (EC) value. The accuracy of the readings for salinity percentage was ± 0.1% for salinity levels of 0.01–5%, but ± 1% for salinity levels of 5.10–25%. The accuracy of EC and TDS readings was ± 2% of the reading. Moreover, a second meter (Portable Mini Digital Soil EC Meter Salinity Tester LCD) was used to measure electrical conductivity percentage from a soil slurry that was one part soil and two parts demineralized water (McCullough et al., 1999). The accuracy of the second meter for EC percentage was ± 2%.

In addition, the funnel method by Govindasamy et al. (2022) was modified to derive soil water holding capacity (SWHC) using 25 grams of soil and 50 mL of water. From the SWHC obtained, the soil water retention percentage was calculated. Loss on ignition (LOI) tests were performed to determine organic matter percentage of the soil samples as soil weights were recorded after oven drying at 105°C for 24 hours in a Fisher Scientific Isotemp Oven, and after ignition (Heiri et al., 2001). An SH Scientific Muffle Furnace (version 2016) was used to combust the organic matter of the soil. The ignition conditions were 550°C for 3 hours (Hoogsteen et al., 2015).

Data collection

Growth performance parameters of the cherry tomato plants including plant height (cm), number of leaves, number of buds, and number of flowers were recorded every 3 days. The plant height in centimeters was measured from the surface of the soil to the tip of fully opened leaves of the plant (Dineshkumar et al., 2020). Sixty days after transplanting, the plants were uprooted, washed, and dissected into roots and shoots. Root to shoot ratios were calculated from the dry biomass weights (Kumar & Nikhil, 2016). To determine the dry weight of the roots and shoots, the plant material was weighed, wrapped in foil, oven dried at 60°C for 72 hours in a Fisher Scientific Isotemp Oven and re-weighed after drying (Hussain et al., 2010; Sultana et al., 2015).

Data Analysis and Statistical Analysis

The growth parameters and soil analysis data (including soil pH, nitrate nitrogen, nitrite nitrogen, potassium, and phosphorus levels, organic matter percentage, water retention percentage, salinity concentrations, TDS value, EC value, salinity level percentage and EC percentage) were analyzed via the software Microsoft Excel. The reported values for growth parameters are the means and standard error (means ± SE) of five replicates for treatments that did not experience plant death. The reported values for soil analysis data prior to the addition of treatment are the means and standard error (means ± SE) of three pseudo replicates. The reported values for soil analysis data after the experiment and root and shoot dry weights are the means and standard error (means ± SE) of three of five replicates. The app “Tiny Decisions” was used to randomly choose the three of the five replicates for treatments that had more than 3 plants.

Soil analysis

The soils collected were alkaline prior to and after the addition of fertilizer treatments. The soil from North Long Island remained at a pH of 8.4 and the soils from South and Central Long Island remained at a pH of 8. The soils acquired from North, Central, and South Long Island were classified as loamy sand, silt loam, and silt loam, respectively. The percentage of water retained in the soils from North, Central, and South Long Island were 28.7% ± 2.91, 42.7% ± 1.76, and 45.3% ± 1.76. Prior to the 60-day pot trial, soils collected from North, Central, and South Long Island had an organic matter percentage of 2.04% ± 0.07, 14.79% ± 2.24, and 15.78% ± 0.45 respectively. After the 60-day pot trial, most treatments experienced an increase in soil organic matter (SOM) (Table S1). Preceding and following treatment, the soil obtained from Central Long Island had the highest salinity concentrations, TDS values, and EC values (Table S2 and S3).

The nutrient composition of each soil varied before and after the pot trial. The nitrate nitrogen levels for North, Central, and South Long Island soils before the pot trial were 5 ± 0 mg kg^− 1, 26.7 ± 3.33 mg kg^− 1, and 66.7 ± 8.33 mg kg^− 1 respectively. The nitrite nitrogen levels for North, Central, and South Long Island soils were below detection, 5 ± 0 mg kg^− 1, and 1 ± 0 mg kg^− 1 respectively. The phosphorus concentration levels for North, Central, and South Long Island soils were 18.3 ± 6.67 mg kg^− 1, 50 ± 0 mg kg^− 1, and 5 ± 0 mg kg^− 1 respectively. The potassium concentration levels were below detection for all soils collected. After the 60-day pot trial, nitrite nitrogen and potassium were below detection in all soils and all treatments including NPK fertilizer treatment. However, nitrate nitrogen and phosphorus levels varied among treatments (Fig. 3).

Growth performance parameters

Plant height

During the 60-day pot trial, there were fluctuations in plant height (Fig. S1). The difference in initial and final plant height for the treatments in each soil varied; however, those plants in soil from North Long Island had the greatest change in height for most treatments (Fig. 1). All plants in soil from Central Long Island, treated with 10% algae, died.

Number of leaves

Analogous to plant height, during the 60-day pot trial, there were fluctuations in the number of leaves (Fig. S2). The difference in initial and final number of leaves for the treatments in each soil varied; however, those plants in soil from North Long Island had the greatest change in number of leaves for most treatments (Fig. 2).

Number of buds and flowers

Buds and flowers did not appear until the end of the pot trial experiment. The five treatments that produced buds were the control groups for tomatoes planted in soil from both North and Central Long Island, and the tomatoes under NPK fertilizer treatment planted in soils from North, Central, and South Long Island. During the last 24 days of the experiment, the control group for tomatoes planted in soil from North Long Island produced buds. The average number of buds recorded every third day were 3, 3, 5, 3, 3.33, 7, 7.67 and 7.5. During the last 15 days of the experiment, the control group for tomatoes planted in soil from Central Long Island produced buds. The average number of buds recorded every third day were 4, 5, 6, 4, and 3. During the last 15 days of the experiment, the tomatoes treated with NPK fertilizer planted in soil from North Long Island produced buds. The average number of buds recorded every third day were 3, 3, 4.5, 5.5, and 6. During the last 15 days of the experiment, the tomatoes treated with NPK fertilizer planted in soil from Central Long Island produced buds. The average number of buds recorded every third day were 3, 4, 4, 7 and 4. During the last 6 days of the experiment, the tomatoes treated with NPK fertilizer planted in soil from South Long Island produced buds. The average number of buds recorded every third day were 3 and 3.

The four treatments that produced flowers were the control groups for tomatoes planted in soil from both North and Central Long Island, and the tomatoes under NPK fertilizer treatment planted in soils from North and Central Long Island. During the last 15 days of the experiment, the control group for tomatoes planted in soil from North Long Island produced flowers. The average number of flowers recorded every third day were 4, 4, 4, 4 and 2.5. During the last 9 days of the experiment, the control group for tomatoes planted in soil from Central Long Island produced flowers. The average number of flowers recorded every third day were 2, 4 and 5. During the last day of the experiment, the tomatoes treated with NPK fertilizer planted in soil from both North and Central Long Island produced flowers. The average number of flowers recorded for both treatments was 3.

Root to shoot biomass

The root and shoot dry weights varied among all treatments; however, there is a clear depiction of the treatments that did the best for each soil location (Table 1). For the plants in soil from North Long Island, the highest dry root and shoot weights were seen in the control group. The treatment that produced the plants with the highest dry biomass weights, in soil from Central Long Island, was the 5% algae treatment. Soil from South Long Island treated with NPK fertilizer produced the plants with the highest dry root and shoot weights.

Table 1

Root and Shoot Dry Weights and Root to Shoot Ratios for each treatment
Treatment	Root Dry Weight (g)	Shoot Dry Weight (g)	Root to Shoot Ratio
North Control	0.93 ± 0.19	4.29 ± 0.15	1:4.61
North Algae 1%	0.12 ± 0.03	0.69 ± 0.21	1:5.75
North Algae 5%	0.08 ± 0.03	0.41 ± 0.22	1:5.13
North Algae 10%	0.02 ± 0.02	0.14 ± 0.08	1:7.00
North NPK	0.39 ± 0.08	2.68 ± 0.62	1:6.87
Central Control	0.23 ± 0.13	1.53 ± 0.69	1:6.65
Central Algae 1%	0.02	0.23	1:11.5
Central Algae 5%	0.30 ± 0.20	1.63 ± 0.97	1:5.43
Central Algae 10%	-	-	-
Central NPK	0.10 ± 0.09	0.57 ± 0.47	1:5.70
South Control	0.04 ± 0.03	0.27 ± 0.12	1:6.75
South Algae 1%	0.10 ± 0.10	0.47 ± 0.40	1:4.70
South Algae 5%	0.01 ± 0.01	0.04 ± 0.02	1:4.00
South Algae 10%	0.01 ± 0.01	0.06 ± 0.01	1:6.00
South NPK	0.26 ± 0.15	1.64 ± 1.02	1:6.31

The Bahamas has alkaline, oolitic and calcareous soils that are derived from dissolved limestone which is primarily calcium carbonate (CaCO₃). Soil pH is an excellent indicator of the relative availability of nutrients. Maximal availability of nutrients is seen in soils with a pH range of 6 to 7 (Taylor et al., 2017). Due to the alkalinity of soils in Long Island, nutrient bioavailability was reduced. The presence of carbonate ions affects the soil chemistry. Soils with a pH greater than 7.5 result in deficiencies in iron (Fe), phosphorus (P), and zinc (Zn). In addition, there is a nutrient imbalance of the cations calcium (Ca), magnesium (Mg), and potassium (K) (Msimbira & Smith, 2020). In this study, the addition of Sargassum sp. did not affect soil pH as a result of the strong pH buffer capacity of soils containing clay (Senbayram et al., 2019). The Sargassum treatments also decreased plant growth; conversely, data from other studies are contrasting (Table S4). However, there was an increase in SOM, nutrient concentration (nitrate nitrogen and phosphorus), and salinity levels. Similar results are seen in a study conducted by Izzati et al. (2019) as there was an increase in SOM, and nitrogen levels. Contrarily, the addition of the macroalgae reduced the pH to 7, thus increasing nutrient bioavailability. Additionally, Muarif et al. (2022) articulates that the addition of seaweed increases SOM, nitrogen, phosphorus, potassium, and sodium (when used in large quantities) levels.

Soil pH, organic matter content, nutrient concentrations and salinity levels have a significant role in plant growth and development. Alkalinity stress on crop plants such as tomatoes is similar to salt stress. Alkalinity stress results in stunted plant growth due to poor nutrient uptake as seen in most Sargassum sp. treatments of the 60-day pot trial when compared to the control groups (Fig. 1). Furthermore, tomatoes are classified as glycophytes: plants that are sensitive to salt and cannot tolerate salt stress despite their ability to adapt by developing protective measures (Safdar et al., 2019). Soil salinity levels become toxic for glycophytes between 50 and 100 mM NaCl (Guo et al., 2021). However, Tola et al. (2023) states that tomatoes have a salinity threshold of 1600 mg kg ^− 1. Although this salinity threshold was not obtained in any of the soils treated with Sargassum treatments, there were treatments within soil from Central Long Island that surpassed 1000 mg kg^− 1 (Fig. 4). The incredibly high salinity concentration of soil collected from Central Long Island prior to the Sargassum treatments is attributable to the nearby brackish bodies of water. During previous hurricanes, this area has experienced storm surge and settlement of water. Wei et al. (2021) corroborates that nearby brackish water can induce an accumulation of salt in soil.

Salinity is an abiotic stress that affects the growth of tomato plants at all stages and ultimately restricts fruit production. It interferes with the uptake of nitrogen thus minimizing plant growth, development, and reproduction (Devkar et al., 2020). Salinity reduces plant uptake of phosphate as it causes phosphate ions to precipitate with calcium ions; thus, limiting water uptake (Shrivastava & Kumar, 2015). In addition, high soil sodium (Na) levels increase water lost as stomatal closure is affected (Msimbira & Smith, 2020). This aggregation of sodium imposes osmotic stress, ion toxicity, and nutrient deficiency (of N, P, K, Ca, Fe, and Zn). Oxidative stress, resulting in reactive oxygen species, is also a consequence of sodium accumulation within the plant (Shrivastava & Kumar, 2015).

Root and shoot weights are also affected by soil alkalinity and salinity. Carbohydrates produced during the process of photosynthesis are allocated by plants to support growth, maintenance, development, and reproduction (Hartmann et al., 2020). The dry biomass, including root and shoot dry weights, show carbon allocation. Leaf number assists in increasing shoot biomass. Therefore, the more leaves, the higher the shoot biomass. Increased numbers of leaves allow for a greater surface area for photosynthesis to occur. Plants can detect changes in resources in the soil and respond accordingly by optimizing growth, development, and biomass allocation (Kudoyarova et al., 2015). In addition, carbon allocation to the roots is done in response to shortage of nutrients including nitrate and phosphorus (Kudoyarova et al., 2015). Larger shoot biomass, as compared to root biomass, is an indication that water and nutrients are readily available and more carbon is allocated to the shoot to allow for growth and development of leaves, fruits, etc. There are variations in the dry root to shoot biomass for the treatments of this study (Table 1). Increased salinity levels reduce root growth and development and result in a decline in the shoot biomass (Tang et al., 2021). Moreover, alkaline soils reduce root development by impairing the water supply to the plant (Msimbira & Smith, 2020).

Consequently, the macroalgae treatments negatively impacted the growth performance parameters of the cherry tomato plants as the control groups produced plants with greater changes in plant height and number of leaves (from day 1 to day 60) and reduced their total biomass (Figs. 2 and 3; Table 1). Additionally, the first plants to start reproducing were those in the control groups. Although the addition of Sargassum increased nutrient content, the bioavailability of the nutrients did not increase due to the alkalinity of the soil. As a result of stress from soil salinity and alkalinity, the survivability rate for tomato plants varied among treatments (Table S5).

To conclude, all the aims and objectives of this study were met. The soil quality of Long Island is poor as the soils lack sufficient concentrations of the basic macronutrients (nitrogen, phosphorus, and potassium). Sargassum sp. can be used as a biofertilizer for soils in Long Island with specific recommendations such as waiting on the soil to age, allowing decomposition and release of nutrients. Sargassum sp. has a positive effect on soil health and fertility as it increases nutrient concentration and organic matter; however, with the application rates used, it did not enhance growth performance. Lastly, based on results obtained, none of the application rates are ideal for tomato cultivation.

If this research was to be repeated, it is recommended that another species of algae be used, categorically a freshwater species, to prevent the increase in soil salinity, TDS and EC. However, if Sargassum sp. is used, the soil should be allowed to age so that nutrients can be released upon decomposition of the macroalgae and become more bio-readily available. Additionally, if time is limited, a halophyte can be used as these plants are tolerant to salt. Other recommendations include composting or bio-digesting the Sargassum, using different application rates, and conducting a field experiment instead of utilizing a pot trial method to limit the ‘pot effect.’

Although Bahamian soils are characterized by alkalinity and low fertility, the 60-day pot trial has provided data that will benefit Bahamians in the agricultural field, especially sustainable farmers. The addition of Sargassum sp. enhanced soil fertility by increasing organic matter content, nitrate nitrogen levels, and phosphorus levels. Prior research portrays that algae including various Sargassum species has improved soil health and increased plant growth performance parameters including plant height and number of leaves, buds, flowers, and fruits. However, the results obtained in this research show that Sargassum sp. treatments negatively affected plant growth performance parameters when compared to the control groups, and increased the soil salinity concentrations. For this primary reason, farmers should take caution when cultivating Sargassum sp. as a biofertilizer. Overall, this study provides significant insight into the effects of Sargassum sp. as a soil amendment for cultivating tomatoes in alkaline soil. How Sargassum sp. affects other species and different soil types are questions worthy of further investigation.

Ca Calcium

EC Electrical conductivity

Fe Iron

K Potassium

Mg Magnesium

NPK nitrogen, phosphorus, and potassium

P Phosphorus

TDS Total dissolved solids

Zn Zinc

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Availability of data and material

All data generated or analysed during this study are included in this published article and its supplementary information files.

Competing interests

The authors report there are no competing interests to declare.

Funding

Not applicable.

Authors' contributions

AA, WG and SW performed the measurements. CW, CB and WG were involved in planning and supervised the work. AA, JF and DS processed the experimental data and performed the analysis. AA, WG, and CW drafted the manuscript. AA, WG, SW, DS, CB, and CW discussed the results and commented on the manuscript. All authors have read and approved the manuscript.

Acknowledgements

The authors acknowledge The University of The Bahamas’ Small Island Sustainability Department for providing the equipment for analysing soil and plant parameters. The authors thank Dr. Mark Stephens and Dr. Jelena Andrejic for insightful comments and confirming the genus of the macroalgae, respectively. We also are grateful to Ms. Ernestia Fraser for her assistance in proofreading the manuscript. Lastly, we thank Mr. Ethan Knowles for his emotional support, and his assistance in analyzing plant growth parameters.

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Sargassum sp. as a Bio-Fertilizer: Is It Really a Key Towards Sustainable Agriculture for the Bahamas?

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Abstract

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Highlights

Background

Methods

Algae collection and preparation

Experimental design

Soil collection and analysis

Data collection

Data Analysis and Statistical Analysis

Results

Soil analysis

Growth performance parameters

Plant height

Number of leaves

Number of buds and flowers

Root to shoot biomass

Discussion

Conclusions

Abbreviations

Declarations

References

Supplementary Files

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