Assessing arsenic and soil salinization risk of Sargassum used in crop growth, agricultural compost, and upland waste piles in the US Virgin Islands

Project Overview

Project Type: On-Farm Research
Funds awarded in 2024: $29,990.00
Projected End Date: 03/31/2026
Grant Recipient: University of the Virgin Islands
Region: Southern
State: U.S. Virgin Islands
Principal Investigator:
David Hensley
University of the Virgin Islands


No commodities identified


No practices identified

Proposal abstract:

The Virgin Islands faces a
complex challenge with the annual influx of Sargassum,
necessitating a comprehensive and research-driven solution. The
proposed remedy centers on initiating a grant-funded on-farm
research project, fostering a direct collaboration between a
University of the Virgin Islands researcher (David Hensley, St.
Croix) and a local farmer (Laura Martin, St. Thomas). The primary
objective is to address uncertainties and risks linked to the use
of beach-cast Sargassum in agriculture, with a primary focus on
arsenic contamination, composting dynamics, soil type variations,
and the promotion of sustainable crop production.

This research endeavor will begin
with an analysis of beach-cast Sargassum samples to quantify As
levels. This initial step is imperative for establishing a
baseline understanding of the As content in Sargassum, though it
is expected to comport with recent literature from the Caribbean
region and unpublished data collected by University of the Virgin
Islands agricultural scientists in 2018 (approximately 50 ppm;
Hensley, unpublished data). 

In order to gather information
about the effect of Sargassum application to the varying soil
types of the Virgin Islands, the volcanic-soil hillside farms of
St. Thomas will be contrasted with the calcareous, flat plains
soils of St. Croix for a composting study. For this, the
participation of St. Thomas farmers is crucial, because farms are
the likely site of any agricultural composting effort (no
centralized or commercial effort to compost Sargassum has so far
been discussed in the territory). Pre- and post-soil sampling of
As and Na content at depths will permit a characterization of the
annual effect of composting unlined piles of Sargassum on the
range of soil types of the Virgin Islands to quantify the effects
of such a practice on soil health. Though this would primarily
benefit farmers to understand the possible effects on soil
salinization and As contamination associated with on-site
composting, the results would be informative for government
authorities who may be considering initiating large-scale dump
sites for collected Sargassum from beaches, particularly from
valuable tourist beaches. 

In tandem with soil analyses, the
research proposal extends to the examination of compost derived
entirely or primarily from Sargassum, considering its interaction
with different soil types, unlike past efforts in other Caribbean
islands that consider Sargassum composting as a minor addition to
primarily sugarcane bagasse or green-waste composting (Devault et
al., 2020). This step is crucial in assessing whether composting
effectively reduces As levels to acceptable levels as established
by the US EPA or European Union, but also is an important piece
of information to determine the assumed fraction of As leached
into groundwater stores, since about 80% of As in the original
Sargassum input not accounted for in the compost or soil fraction
will be assumed to have been leached below the sampling region
(since arsine gas volatilization is reported in As-rich
composting to be about 20%; Cao et al., 2010). Follow up soil
sampling after several months to a year of fallowing will also
permit initial estimates on any potential of As build-up in soils
if the composting process is repeated in the same location year
after year. 

Finally, to comprehensively
address the potential risks associated with Sargassum use in
agriculture, the research project includes studies on crop uptake
of As, with a focus on promoting sustainable crop production. The
choice of crops will be led by the farmers to reflect the current
agricultural reality, while realizing that the choice of species
plays a role in how much As is present in the harvested yield
(Sahoo and Mukherjee, 2014). By cultivating crops in
Sargassum-amended soil and monitoring As levels in the harvested
produce, the project aims to quantify the extent of As transfer
to food crops while establishing sustainable

Additionally, the research
project will address the salinity aspect by assessing the Na
content in Sargassum and its potential impact on soil salinity,
ensuring that sustainable soil management practices are
incorporated into the proposed solution. One important outcome of
studying these together could be to emphasize which of the two
possible problems with Sargassum use, Na or As, is more immediate
and should be emphasized in management. 

Recognizing the limited capacity
of individual farmers in the Virgin Islands for comprehensive
testing, the proposed solution emphasizes a collaborative on-farm
approach. The research project will actively engage local
farmers, ensuring a direct and hands-on collaboration with a
focus on improving the sustainability of crop production while
retaining fidelity to actual, realistic agricultural practices in
the current Virgin Islands context. This cooperative effort
involves the farmers in all phases of the research, fostering
knowledge exchange and ensuring that research findings promote
sustainable choices for on-farm practices, and improving
transparency on the topic. 

The insights gained from the
research will form the basis for the development of clear and
practical guidelines for the safe and sustainable use of
Sargassum in Virgin Islands agriculture. These guidelines will
encompass recommended application rates, frequency, and best
practices to minimize risks and maximize the potential benefits
of Sargassum utilization while promoting sustainable crop
production. The collaborative nature of the on-farm research
ensures that these guidelines are not only scientifically sound
but also reflective of the practical realities faced by local
farmers, aligning with the broader goal of sustainable
agricultural practices in the Virgin Islands.

Project objectives from proposal:

As described above, the project
will comprise essentially three major objectives or phases to
provide useful data at all stages of Sargassum application in
Virgin Islands agriculture with respect to risks associated with
arsenic and sodium: 1) the first objective is to characterize the
total As and Na content of raw beach-cast sargassum; 2) the
second is to characterize the fate over time of total As and Na
in compost piles over an area and its effect on the root zone of
the soil underneath (as well as assumed leachate) with varying
application rates and on varying soil types; and 3) the third
objective is to characterize the fate of total As and Na added to
soil from sargassum soil amendment in the root zone, and in plant
tissue, including harvested crops intended for human consumption.
Project methods are based loosely on the As uptake study of Khan
et al. (2010), who studied As uptake of wetland rice in

Objectives 1 and 2 of the project
will be carried out, at a minimum, at two sites, one on the
island of St. Thomas (rocky, steep, volcanic-origin soils) and
one on the island of St. Croix (sedimentary, gentle,
marine/calcareous-origin soils). For the first objective, during
the first year of the project (summer 2024), beach-cast Sargassum
will be collected from the beaches of St. Thomas and St. Croix,
to be tested for (at a minimum) As and Na. This is expected to
align with published literature on the range of As content in
pelagic Sargassum species
S. natans and S. fluitans found on Caribbean beaches, but will also be
necessary for characterizations in Objective 2, and will permit
comparisons between island and between years of the project
(sampling will also occur during the summertime Sargassum season
in Year 2, 2025). For all samples (tissue, soil, and compost),
samples will be collected and processed before being shipped to
laboratories to perform chemical analysis of total As species and
Na content. 

For the second objective, compost
bins will be constructed at both study locations (on-farm St.
Thomas and St. Croix) where harvested Sargassum will be piled for
aerobic composting. In keeping with typical practice, the
Sargassum will be rinsed with freshwater before composting to
remove excess surficial Na (this will also be done for samples
sent for chemical analysis in Objective 1). Samples will be taken
and dried to estimate the total dry weight of the entire pile of
raw Sargassum. Prior to the deposition of Sargassum, soil samples
at 3 depths will be collected at the location of the intended
compost pile to characterize initial soil chemical conditions,
porosity, particle size distribution, and so on. 

Sargassum will be left, with
periodic turning, to compost for the next 3-4 months, the typical
time period required for aerobic composting in the region and in
line with reported Sargassum composting in the literature
(Devault et al., 2020). Finished compost will be sampled to
estimate total finished product dry weight and for chemical
analysis (total As species and Na content). After removal of
compost from the pile location, soil will be sampled at 3 depths
below the pile to determine chemical changes in terms of As and
Na. A simple physical model of As distribution throughout this
profile and the remaining As in the compost will be used to
determine what fraction, if any, is assumed to have been leached
beyond the sampling domain. Results can be compared across both
island/soil sites, and at different rates of composting (i.e.,
depth of pile). This process will be repeated the following year
(2025) in the same locations, with soil pre-sampling in the same
locations to estimate the fate over time of soil As, and allowing
an estimate of the potential for As accumulation in the soil. The
same analysis will be carried out for Na to determine any soil
salinization effect. 

For the third objective, in Year
1, when finished Sargassum compost has been collected, it will be
applied by St. Thomas farmers as a soil amendment with measured
rates to annual crops of their choosing for the winter growing
season. Soil will be sampled and chemically tested prior to
amendment, and at the time of harvest, to detect change over
time. Plant tissues (i.e., root, stem, leaf, and fruit) will be
sampled and chemically analyzed for total As, which will include
harvested yield. This will permit an As-balance similar to that
in Objective 2 that allows for assumed leachate, and also
provides a final estimate of any potential health risk posed by
As content in harvested crops from one year of Sargassum compost
use at the measured As content rates. These rates, both for
compost and crop tissues, can be compared to safe limits for As
established by public authorities. Though only crops selected by
the farmers will be tested, this is considered to be beneficial
since it will provide data on crops actually normally grown by
farmers in the Virgin Islands, and will provide a needed starting
point to determine the feasibility, if any, of safely using
Sargassum compost in local sustainable agriculture.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.