Using green seaweed (Ulva spp.) as a soil amendment: Effects on soil quality and yield of sweet corn (Zea mays L.)

Final Report for GNE11-026

Project Type: Graduate Student
Funds awarded in 2011: $13,853.00
Projected End Date: 12/31/2013
Grant Recipient: University of Rhode Island
Region: Northeast
State: Rhode Island
Graduate Student:
Faculty Advisor:
Dr. Steven Alm
University of Rhode Island
Faculty Advisor:
Dr. Jose Amador
University of Rhode Island
Dr. Rebecca Brown
University of Rhode Island
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Project Information


As a practice that re-purposes waste materials, diversifies inputs, and relies on coastal resources, application of seaweed to manage soil fertility may be an effective and inexpensive agricultural practice in coastal agroecosystems. Seaweed may be a useful amendment for crop production and soil quality improvement due to provision of plant nutrients (e.g. N, P, K, Ca), and promotion of microbial activity, but may be limited by high S, salt, and heavy metal content. The objectives of this field study were to: (1) evaluate the effects of seaweed biomass application on soil physical, biological, and chemical properties; and (2) assess the sweet corn (Zea mays L.) yield obtained using seaweed amendment for soil fertility management. Low-dose seaweed (LDS), high-dose seaweed (HDS), and pre-formulated 8-1-9 (N-P-K) organic fertilizer (PFF) fertilization treatments were employed at 110, 152, and 113 kg total N/ha, with seaweed-derived N accounting for 40% and 55% of total N for LDS and HDS, respectively. Soil properties were assessed prior to seaweed application (October 2011) and throughout the growing season (April – September 2012) using recommended national and regional protocols. Physical properties, extractable Ca2+, total heavy metals, total nutrient elements, soil organic matter, earthworm abundance, nitrate, and phosphate did not differ among fertilizer treatments. Extractable K+, electrical conductivity, sulfate, and active C levels were increased with seaweed addition, and potentially mineralizable N decreased, though these effects varied in persistence. Yield, above-ground biomass, and dissolved soluble solids did not differ among fertilizer treatments, but average fresh ear weight was greater for LDS compared to PFF. Overall, yield results suggest that seaweed application, as a means of partially replacing total nutrient supply, may be a viable agricultural practice, but must be considered in light of potential yield enhancement value, persistence and magnitude of soil quality changes, and labor and transportation costs.


Soil amendment with organic materials is a common component of soil fertility management for crop production, with the aim of providing essential plant nutrients and improving overall soil physical, chemical, and biological quality (Diacano and Montemurro, 2010). Marine macroalgae, or seaweed, has been historically used as a soil amendment material, and may have application for modern agriculture as a low- cost source of nutrient-rich biomass (Angus and Dargie, 2002; Cuomo et al., 1995). While seaweed compost and extract products have been widely evaluated for agricultural applications (Woznitza and Barrantes, 2005; Jaulneau et al., 2010; Khan et al., 2010), evaluation of unprocessed seaweed biomass as an amendment material is limited, particularly with regard to soil quality. Application of seaweed material may uniquely affect soil quality parameters as a result of its chemical characteristics, including carbon (C) and nitrogen (N) composition, and salt, sulfur (S), heavy metal, and trace element content. In this study, the putative benefits of seaweed amendment for crop growth and production were assessed in a sweet corn (Zea mays L.) field experiment, including analysis of soil physical, biological, and chemical properties.

Historical use of seaweed in agriculture

In coastal regions, collection and application of seaweed is a traditional soil fertility management strategy, especially where agriculture relies on use of local resources (Cuomo et al., 1995). As a readily-available, low-cost material to supplement soil fertility, application of seaweed biomass is often an integral component of traditional, small-scale, diversified agriculture (Angus and Dargie, 2002). For instance, agriculture in the Machair region of the Scottish Outer Hebrides Islands involves a rotation-intensive system that integrates the application of locally- available seaweed biomass, primarily the brown alga Laminaria digitata (Angus and Dargie, 2002; Kent et al. 2003). Promotion of seaweed application as a part of sustaining small-scale, diversified agriculture is supported by Scottish Natural Heritage, a governmental conservation organization, as well as local conservation group efforts (Angus and Dargie, 2002). In addition to the Machair region, historical accounts of seaweed use in agriculture range from the British Isles, to coastal mainland Europe, to the northeastern region of the United States, including New York, Maine, and Rhode Island (Fussel, 1973; Smith et al., 1989; Cuomo et al., 1995).

Seaweed in the modern agricultural context

In organic or reduced-input cropping systems, both in the U.S. and worldwide, seaweed-based agricultural products (e.g. extracts for foliar application and composts) are commonly employed (Khan et al., 2009). However, application of unprocessed biomass is less prevalent. To reduce dependence on application of inorganic fertilizers, make use of an abundant (sometimes over-abundant) resource, and improve soil quality, the traditional practice of seaweed application may have modern application in coastal regions. Because adding seaweed to soil can increase plant macro- and micronutrients, and may improve soil biological, chemical and physical properties (Khan et al., 2009), the practice may be an additional strategy to manage soil fertility and quality that addresses the dual problems of reliance on inorganic chemical fertilization and wasting of valuable, nutrient-rich biomass. Inorganic fertilizer inputs account for a large fraction of conventional farm expenses, energy consumption, and carbon emissions (Lal, 2004). Application of inorganic fertilizers without addition of organic amendments, cover crop use, or use of alternative tillage practices can result in depletion of soil organic matter (SOM), with concomitant negative effects on many soil properties important for crop productivity (e.g. nutrient retention, moisture-holding capacity, aggregate formation, and microbial activity) (Brock et al., 2012; Franzluebbers, 2012). Furthermore, levels of nutrient elements other than N, P, and K (e.g. Ca, Mg, Mo, B, and S) are generally low in inorganic fertilizers, and are of increasing concern for crop quality and nutritional value (Welch and Graham, 2012). Consequently, reliance on inorganic fertilizer as a sole source of fertility is often questioned as a sustainable management strategy, and diversification of inputs is encouraged, particularly inputs that provide not only primary nutrients (i.e. N, P and K), but also organic matter and trace elements (Lal, 2004). Organic amendments used to improve soil fertility include traditional (e.g. animal manure) and non-traditional (e.g. industrial by-products) materials (Power et al., 2000). Seaweed, which contains primary nutrients, organic C, and other nutrient elements, is thus a good candidate organic amendment material as part of a diversified soil fertility management strategy. In addition to the potential crop nutrition benefits of seaweed amendment, the prevalence of seaweed biomass in coastal areas as a result of both natural phenomena and anthropogenic impacts may allow for use of seaweed with minimal cost. Nutrient (N and P) enrichment of coastal waters – sometimes attributed to fertilizer runoff from agriculture and home use – can cause excessive seaweed growth (Morand and Merceron, 2005). In addition to detrimental ecological impacts (e.g. oxygen depletion), the accumulation of seaweed biomass on beaches can have negative economic consequences (RI DEM, 2010). For instance, in the summer of 2012, accumulation of the red seaweed Polysiphonia sp. on Massachusetts beaches required mechanical removal and disposal in order to maintain beaches for public use, costing money for equipment use and labor, as well as preventing beach use. Beach-cast biomass is often removed and disposed of in landfills. Although the species composition and properties of beach-cast seaweed varies based on location and environment (e.g. estuarine vs. marine), the coordination of accumulated seaweed biomass removal with agricultural application may provide a low-cost, locally- available resource for soil fertility management. To initiate this arrangement for coastal regions, characterization of seaweed biomass in terms of location and abundance, species composition, and chemical characteristics relevant to soil quality and plant nutrition is required. Additionally, quantification of seaweed biomass effects on soil quality and crop production is required to validate putative benefits or negative effects of seaweed amendment practices.

Seaweed effects on agronomic factors

Previous research has addressed the potential for re-purposing of problematic seaweed biomass, usually through production and evaluation of composted seaweed products (Cuomo et al., 1995). Characteristics of seaweed-derived composts vary greatly depending on other ingredients (e.g. inclusion of high C materials such as wood chips) and the properties of the seaweed material (e.g. C:N ratio). Composting processes may be a means to generate a consistent seaweed-derived product in terms of chemical and biological characteristics. However, the relatively low C:N ratio of seaweed biomass (C:N = 18:1) in comparison to terrestrial plant biomass (C:N = ~20 to 100:1) (Lobban and Harrison, 1997) favors N mineralization, so transformation of fresh seaweed biomass to a compost material may result in loss of N, with concomitant reduction in crop yield in comparison to non-seaweed composts (Cuomo et al., 1995; Wosnitza and Barrantes, 2005). Seaweed-based extracts have been widely evaluated for various agricultural purposes, including the stimulation of plant growth and defense response, soil nutrient enrichment, and promotion of microbial activity and mycorrhizal fungi (Khan et al., 2009). However, most evaluations have been laboratory-based, and the effects of amendment with unprocessed or composted seaweed on soil quality and crop productivity in the field remain understudied. The seaweed properties with most potential relevance to crop production include (1) elemental composition (e.g. primary plant nutrients, other nutrient elements, heavy metals, S and salts) and (2) organic compound composition (e.g. energy sources for microbial processes). In comparison to terrestrial plants, seaweed generally has higher concentration of Ca, K, Mg, Na, Cu, Fe, I, and Zn (MacArtain et al., 2007). However, as dynamic accumulators of contaminants in the marine environment, seaweed biomass may also be a source of toxic trace metals and metalloids (e.g. Pb, Cd, Cr, and As) when collected from an environment with high levels of these elements (Castlehouse et al., 2003; Woznitza and Barrantes, 2005). Additionally, high S content in seaweed may result in increased S application, which after decomposition and subsequent oxidation to SO42- may lower soil solution pH. However, S is also a plant nutrient, and in soils with limited S supply, its addition in seaweed may be beneficial for crop production (Brady and Weil, 2008). Seaweed may also have a high salt content, which may increase its plant nutrient content (e.g. K+) (Rupérez, 2002), but can also contribute to development of saline soil conditions from long-term application. Consequently, historical application of seaweed biomass generally includes a period of rinsing by rain for the purpose of decreasing salt content (Angus and Dargie, 2002). Inorganic ions present in seaweed include Na+ and Cl-, the most prevalent ions in seawater, as well K+, Ca2+, and Mg2+ (Rupérez, 2002). Increases in soil salinity could result in negative effects on the soil biotic community and crop production through effects on water balance and toxicity of salt ions (primarily Na+ and Cl-), particularly for salt-sensitive crops such as legumes, which have a salinity threshold of ~1000 microsiemens (μS)/cm (Maas, 1990).

Study description

In light of possible beneficial and/or negative effects of seaweed on soil and crop agricultural production factors, the practice of amendment with un-processed seaweed biomass requires evaluation prior to widespread recommendations for adoption can be made, particularly with regards to: (1) yield enhancement capacity, and (2) effects on integrated physical, chemical, and biological soil properties. In this study, the sweet corn (Zea mays L.) yield obtained by supplying a fraction of total nutrient supply from seaweed biomass was determined, and changes in soil properties after seaweed addition and throughout the growing season were measured. Sweet corn, an economically valuable crop in the northeast ($2,000,000 USD in Rhode Island in 2007), has relatively high nutrient requirements, and typically receives both broadcast and side-dress fertilization (USDA NASS, 2013; UMass Cooperative Extension, 2013). Effects on soil were assessed using integrated soil quality properties, including both standard soil test measures (e.g. primary nutrients and pH) and less-common parameters developed as indicators of overall soil health and potential for crop production (e.g. active C and potentially mineralizable N) (Gugino et al., 2009). Such soil quality indicators are supported by evidence of correlation with increased yield, and reflect both rapid and long-term changes in soil quality as a result of management practice (Weil et al., 2003; Gugino et al., 2009). In this study, the effects of using seaweed as an amendment material on soil quality and yield of sweet corn were determined in a field study, with low-dose seaweed (LDS), high-dose seaweed (HDS), and pre-formulated fertilizer (PFF) fertility treatments. The hypothesized effects of seaweed amendment on crop yield and soil quality properties were developed by drawing on previous studies evaluating seaweed compost quality and putative qualities of seaweed extract products. In seaweed-amended plots, sweet corn yield and quality is hypothesized to be at least equal to plots fertilized with pre-formulated organic 8-1-9 (N-P-K) fertilizer due to the provision of plant nutrients and improvement of diverse soil quality parameters. Hypothesized positive effects of seaweed amendment (i.e. improved soil quality parameters) include increased aggregate stability, infiltration, available water capacity (AWC), SOM, active C, PMN, primary and trace elements, and decreased bulk density (Table 1). Potential negative effects include increased heavy metal content, electrical conductivity (EC), sulfur/sulfate concentration, and decreased pH and earthworm abundance (Table 1).

Project Objectives:

In order to evaluate the application of seaweed biomass as a soil fertility management strategy, the overall objective of this study is to determine the effects of seaweed amendment on soil quality parameters and the yield of sweet corn (Zea mays) in a field experiment. The specific objectives and progress steps associated with each objective are listed below. 1) Characterize and prepare suitable amendment material from raw green seaweed biomass for field application Complete Fall and spring seaweed biomass collection Species identification and specimen preservation Field biomass application Carbon and nitrogen (C and N) composition analysis Trace nutrient and heavy metal analysis Notes In Rhode Island bays or estuaries (e.g. Warwick Bay), seaweed biomass accumulating on beaches is generally dominated by green seaweed species (e.g. Ulva spp.), and removal is often required. Consequently, the original study proposed use of green seaweed biomass. However, the abundance of beach-cast seaweed biomass, particularly in estuarine environments, is often affected by variation in factors such as temperature, wave activity, and wind strength (Merceron et al., 2007). Due to seasonal variability, beach-cast green seaweed from bays or estuarine sites in Rhode Island was limited during September and October 2011. In contrast, seaweed proliferation closer to the open ocean is generally less dependent on seasonal variability. Consequently, seaweed biomass for fall application was collected by hand from Watch Hill, Westerly, RI, on November 2, 2011. This seaweed biomass was largely composed of brown and red seaweed species. Additionally, seaweed biomass was collected and applied in late April 2012 from Mackerel Cove (Jamestown, RI) to supplement fall application. 2) Evaluate the effect of seaweed amendment on the yield (bushel/acre and biomass/cob) and quality of sweet corn, an economically important crop for local agricultural production, in comparison to a conventional inorganic fertilization treatment Complete Fall tillage for sweet corn plot preparation and weed management Seaweed biomass application and side-dress fertilizer application for seaweed amendment treatments (fall and spring) Pre-planting and side-dress fertilizer application for PFF treatment Sweet corn seeding and weed management Sweet corn pest control (Bacillus thuringiensis var. kurstaki application for corn borer) Sweet corn ear harvest (silk dry down), weighing, drying, and dissolved soluble solids (DSS) analysis Above-ground corn biomass collection, weighing and drying Data processing and analysis Notes The research proposal included a comparison between seaweed and inorganic chemical fertilizer. However, a pre-formulated, granulated organic 8-1-9 (N-P-K) fertilizer composed of poultry litter, peanut meal, and feather meal was used as the sole (PFF treatment) or supplemental (LDS and HDS treatments) nutrient supply. The reason for this substitution was to present results with application to organic or low-input sweet corn growers, a target audience generally interested in alternative organic amendment materials and practices. 3) Evaluate seaweed amendment effects on physical, chemical and biological soil quality parameters in comparison to a conventional inorganic fertilization treatment Complete Soil quality sampling Pre-seaweed application (October 2011), pre-corn seeding (May 2012), and post-harvest (September/October 2012) – Aggregate stability, bulk density, available water capacity, and infiltration Monthly (October-November 2011 and April-October 2012) – Soil respiration and earthworm abundance (in-field analysis); insect collection; bulk sample collection for soil pH, electrical conductivity (EC), nitrate (NO3-), ammonium (NH4+), phosphate (HnPO4n-3), potassium (K), active C, potentially mineralizable N (PMN), trace nutrients, sulfate (SO42-), soil organic matter (SOM), nematode abundance and community composition, and heavy metals Soil quality analyses (field and laboratory) Bulk density, available water capacity, infiltration, soil respiration, pH, EC, extraction for primary nutrients, trace nutrients and PMN, and total element/heavy metals 4) Assess the economic and practical feasibility of seaweed amendment for sustainable agriculture in coastal New England through synthesis of experimental findings, both from this and previous studies, and through discussions with local agriculturalists, Extension agents and agricultural economists Complete Documentation of time and cost requirements for seaweed location, collection, preparation and application Determination of potential yield improvement benefits Analysis of costs and potential changes in income based on yield improvements Informal assessment of farmer interest and current/prospective adoption Notes Estimated values for quantifiable expenses and benefits (e.g. fuel and labor requirements, crop yield enhancement) were used to generate an overall assessment of economic viability, but the difficulty in assigning monetary value to less quantifiable factors and externalities did not allow for comprehensive assessment of many complexities involved in the practice of seaweed amendment (e.g. fertilizer manufacture environmental impacts, improvement in soil quality).


Click linked name(s) to expand/collapse or show everyone's info
  • Dr. Steven Alm
  • Dr. Jose Amador
  • Dr. Rebecca Brown
  • Andrew Giguere


Materials and methods:
Experiment overview

I conducted a field experiment was conducted over one growing season to evaluate the effect of seaweed addition on soil quality properties and yield of sweet corn. Soil properties evaluated were: (1) physical (aggregate stability, bulk density, infiltration, and AWC); (2) biological (active C, SOM, soil respiration, PMN, and earthworm abundance); and (3) chemical (sulfur/sulfate, heavy metals, primary and other plant nutrients, pH, and EC). Seaweed material of mixed composition (red, green, and brown species) was collected from Rhode Island beaches in fall 2011 and spring 2012, and applied at two levels: low-dose seaweed (LDS) (42 kg total N/ha) and high-dose seaweed (HDS) (84 kg total N/ha), to replace broadcast fertilization. The seaweed fertilizer treatments were compared to an organic pre-formulated fertilizer (PFF) treatment (45 kg total N/ha). Sweet corn was seeded in May 2012, and side-dress PFF (68 kg total N/ha) was applied to all treatments. At the end of the growing season, corn was harvested and yield determined. Sweet corn quality was assessed by determination of dissolved soluble solids. Soil quality parameters were analyzed for time, overall fertilizer treatments, and interaction effects using Repeated Measures ANOVA, followed by Univariate ANOVA to determine differences among fertilizer treatments at each sampling date. Sweet corn parameters at harvest were analyzed statistically using Univariate ANOVA.

Site description

Twelve field treatment plots were established in October 2011 at the University of Rhode Island’s Greene H. Gardner Crops Research Center in Kingston, RI. The field was previously planted with butternut winter squash (Cucurbita moschata) in 2011 and disc harrowed prior to initial soil sampling. The soil at the site is in the Enfield series (coarse-silty over sandy or sandy-skeletal, mixed, active, mesic Typic Dystrudepts) (Soil Survey Staff, 2003). Mean annual temperature is 7 to 11°C and mean annual precipitation is 102 to 127 cm (Soil Survey Staff, 2003). Across the site, average (n=3) soil particle size distribution was 40% sand, 49% silt, and 11% clay-sized particles.


In order to evaluate the effect of pre-seeding seaweed biomass application in comparison to pre-formulated fertilizer application on the yield of sweet corn (Zea mays L. var. rugosa) and soil quality parameters, 3 fertilizer treatments were employed: A) Low-dose seaweed (~13,840 kg wet wt/ha) (LDS) B) High-dose seaweed (~27,680 kg wet wt/ha) (HDS) C) Organic pre-formulated fertilizer (8-1-9 N-P-K) (PFF) Fertilizer treatments differed in the form of nutrient addition prior to crop production (broadcast fertilization). For the PFF fertilizer treatment, prior to corn seeding, granulated organic fertilizer (Nature’s Turf 8-1-9, North Country Organics, Bradford, VT) was applied at a rate of 45 kg total N/ha. Of the total N in the PFF treatment, readily-available NO3--N composes 27%, with the remaining 73% N in organic forms (e.g. peanut meal, pasteurized poultry litter, and feather meal). Likewise, seaweed biomass was applied at a rate of 42 and 84 kg total N/ha for the LDS and HDS treatments, respectively, but with the majority of the N in organic forms (0.1% and 0.06% of total N as NO3- and NH4+, respectively). Consequently, the amount of N available is dependent on N mineralization rate. For the PFF treatment, 27% of total N is readily available; of the remaining organic N, ~60% N mineralization is expected within the growing season (Hartz and Johnstone, 2006). Available N from broadcast PFF application is thus estimated as 32 kg N/ha. While no field mineralization values are available for seaweed biomass, N availability for subsequent crops over the growing season for materials with similar N content and low lignin content (e.g. common vetch, Vicia sativa L.) are near 50% (Sattell et al., 1998), corresponding with a broadcast N application rate of 21 and 52 kg N/ha for LDS and HDS seaweed treatments, respectively. For each treatment, 4 replicates were employed and arranged in a randomized block design. All crop production and soil quality sampling was conducted within the inner 4.6 x 4.6 m of each plot, excluding border rows.

Seaweed collection and characterization

Seaweed biomass for fall application was collected by hand from Watch Hill Beach (WHB), Westerly, RI (41°18'30.27"N, 71°51'48.08"W) in November 2011. Seaweed material was separated by major species groups and identified using a dichotomous key (Villalard-Bohnsack, 2003). Additionally, seaweed biomass was collected in late April 2012 from Mackerel Cove, Jamestown, RI, (41°29'18.55"N, 71°23'0.28"W) to supplement fall application. For both applications, the seaweed biomass was piled (~1 m3) near the treatment plots for ~1 week, and received no further processing prior to application. Seaweed biomass was applied by hand. Randomly selected biomass sub-samples (~1000 g, n=3 for each collection date) were air-dried, ground, and sieved (0.25-mm-mesh) prior to C, N, heavy metal, and trace element analysis. Seaweed C and N content was analyzed using a Carlo Erba EA1108 CHN analyzer (CE Instruments, Inc., Wigan, Ireland). NO3- and NH4+ levels were determined in dry, ground seaweed biomass for both collection dates by extraction with 2 M KCl at a 1:5 seaweed-to-extractant ratio. Extracts were shaken for 1 h and gravity filtered with Whatman #42 paper. The filtrate was analyzed for NO3- and NH4+ colorimetrically (Weatherburn, 1967; Doane and Harwath, 2003).

Soil quality

The sampling schedule, sampling methods, and analysis methods for the determination of soil chemical, biological, and physical properties are summarized in Table 2.

Sweet corn production

In May 2012, sweet corn (Zea mays L. cv. “Trinity,” Johnny’s Selected Seeds, Winslow, ME) was seeded by hand at a depth of 2.5 cm and a rate of ~2-4 seeds/20 cm, and later thinned to a final linear plant density of 1 plant/30 cm. Due to uneven germination, corn was re-seeded as necessary through June 5, 2012. Between-plot borders were seeded with perrenial ryegrass (Lolium perenne) and mowed weekly. Following standard sweet corn management (UMass Cooperative Extension, 2013), side-dress supplemental N was applied using Nature’s Turf 8-1-9 pre-formulated fertilizer at a plant height of ~30 cm, at a rate of 68 kg total N/ha for all treatments. Throughout the growing season, weeds were removed within the treatment plots by hand cultivation. European corn borer (Ostrinia nubilalis) was controlled by plant and ear-tip application of B. thuringiensis var. kurstaki (Johnny’s Selected Seeds, Winslow, ME). Corn was harvested by hand at silk dry-down stage in August and September 2012. Immediately after harvest, ears were weighed whole to determine average fresh weight and yield (hundredweight and bushels/ha). Additionally, 20% of the fresh ears were analyzed for dissolved soluble solids (°Brix) using a field refractometer. After harvest, 33% of the remaining standing stalks (every third stalk) were cut, weighed, and dried (24 h at 60°C) to estimate above-ground biomass.

Statistical analyses

Sweet corn yield and soil quality properties were analyzed statistically using Univariate or Repeated Measures Analysis of Variance (ANOVA) as appropriate (α=0.05) followed by post-hoc multiple comparisons using Tukey’s Test.

Economic evaluation

In order to evaluate the economic implications of seaweed application, expenses throughout the sweet corn production process were recorded for both seaweed and pre-formulated fertilizer treatments (e.g. seed, labor, agricultural chemicals, transportation, and fertilizer costs). Additionally, the difference in expected income was assessed by estimation of corn market value based on data from the USDA National Agricultural Statistics Service (NASS).

Research results and discussion:
Seaweed collection

In this study, fall and spring seaweed collection was completed at local sites with frequent and reliable accumulation of beach-cast seaweed biomass. Watch Hill Beach and Mackerel Cove are relatively protected inlets, which often supports high beach deposition of seaweed by natural currents and wave action. However, seaweed proliferation as a result of anthropogenic nutrient inputs may be less at these sites than sites located further from the open ocean. At the time of collection for this study, sites with excessive seaweed accumulation presumably due to anthropogenic causes were scarce, due in part to the season during which collection took place, since seaweed biomass usually reaches the highest levels in July and August (Thornber et al., 2008), as well as beach-clearing weather events, such as Hurricane Rita in September 2011. Consequently, the seasonal variation of excess seaweed accumulation may require monitoring and communication among beach managers and farmers in order to make optimal use of this resource. In lieu of using often hard-to-predict “problematic” seaweed biomass, beach sites with high seaweed accumulation due to inherent geographic or environmental conditions may offer a more consistent and reliable source.

Seaweed characterization

Seaweed biomass collected in fall 2011 from Westerly, RI, was largely composed of brown and red seaweed species, including Ascophyllum nodosum (12.5% dry weight (DW), Laminaria digitata (2% DW), Chondrus crispus (15.2% DW), Fucus vesiculus (8.2% DW)), assorted filamentous red algae (10.5% DW), and mixed, non-algal plant material (e.g. eelgrass, 51.5% DW). Seaweed collected in spring 2012 from Jamestown, RI included Saccharina saccharina (0.5% DW), A. nodosum (9.5% DW), Fucus sp. (11.6% DW), Grinellia americana (2.5% DW), C. crispus (3.3% DW), Ulva sp. (0.95% DW), assorted filamentous red algae (69.7% DW), and mixed non-algal plant material (1.7% DW). In comparison to terrestrial plant biomass, the seaweed collected for this study can be characterized as a high-moisture material with relatively high N content (15.5 g/kg DW) and low C:N ratio (10:1 to 13:1). For instance, values of N content and C:N ratios of common crop residue materials are in the range of 4-5 g N/kg plant DW and C:N ratio of ~100:1 for stem biomass, and 12-15 g N/kg plant DW and C:N ratio of ~30:1 for leaf biomass, based on values for soybean (Glycine max), corn (Zea mays), and switchgrass (Panicum virgatum) stems and leaves (Johnson et al., 2007). Additionally, the total calcium (Ca) and potassium (K) content of the seaweed material was higher than many terrestrial plants, on the order of 4 times greater Ca content and 2 times greater K content (Tian et al., 1992). For comparison, biomass of the cover crop velvet bean (Mucuna pruriens) contains approximately 5-7 g Ca/kg DW and 18 g K/kg DW (Tian et al., 1992), whereas levels in the seaweed biomass in this study were 21-38 g Ca/kg DW and 31-50 g K/kg DW. Elements of concern (e.g. heavy metals) may also be found at higher levels in seaweed relative to terrestrial plant biomass; however, in the context of field application guidelines, these were not sufficiently high to raise concerns in this study. For example, As was present in seaweed biomass (maximum 9.8 ± 1.8 mg/kg DM), but at much lower levels than those for F. vesiculosus and L. digitata biomass collected from coastal Scotland (25 ± 7 and 74 ± 2 mg/kg DM, respectively) (Castlehouse et al., 2003). For sewage sludge, an agricultural amendment with particular risk of heavy metal contamination, the US EPA regulatory limit for As for soil application is 75 mg/kg (USDA NRCS, 2000). Although the seaweed biomass collected in this study does not pose a concern in terms of exceeding regulatory limits for As application, consideration of As is warranted for seaweed application, particularly when brown algal species such as F. vesiculosus and L. digitata constitute a higher percentage of the biomass applied. US EPA yearly application limits and long-term maximum cumulative loading also provide guidelines and perspective for potential As application risks. In the present study, the As level in seaweed biomass would result in an annual loading of 0.03 kg/ha/yr for the HDS treatment, well below the EPA regulatory limit of 2 kg/ha/yr. Additionally, the total maximum cumulative loading for As is 41 kg/ha, which would require ~1,300 years of seaweed application at the HDS rate to reach the maximum load, assuming no losses of As from the soil. Cadmium (Cd), chromium (Cr), mercury (Hg), and copper (Cu) were not present above the limit of detection. Pb and Zn were detected, but at concentrations substantially lower than the US EPA regulations for sewage sludge application. For Pb and Zn, the maximum levels present in seaweed (14.2 ± 2.8 and 68.4 ± 11.0 mg/kg, respectively), are ~30 times lower than the US EPA limits of 420 and 2500 mg/kg for Pb and Zn, respectively, in sewage sludge. Consequently, the heavy metal content of the seaweed used in this study does not pose a concern in terms of long-term accumulation of heavy metals. Since many seaweed species or ecotypes are tolerant of high levels of heavy metals (e.g. Cu in anti-fouling paint) (Reed and Gadd, 1990), amendment with seaweed biomass collected from areas likely to be affected by heavy metal contamination may require pre-application analysis. Seasonal variation in heavy metal levels in seaweed may also be important for timing of collection, since higher concentrations are generally found in winter and early spring, and lower concentrations in summer and autumn (Caliceti et al., 2002).

Soil physical properties

For all physical properties, no statistically significant differences (p≥0.05) were detected among the fertilizer treatments (Table 3). Consequently, the hypothesized positive effects of seaweed amendment on soil physical quality (increased aggregate stability, infiltration and available water capacity, and reduced bulk density) were not supported in this study. Overall, aggregate stability of soil at the study site is rated as moderate to poor, ranging from ~30-40% (Gugino et al., 2009), a value that could be expected for an agricultural soil with repeated, frequent cultivation. Available water capacity, at 0.224 g/g, is rated as medium to good (Gugino et al., 2009). Many soil physical properties, including aggregate stability, require several years after management changes before appreciable improvements are observed (Islam and Weil, 2000). The duration of this study was likely insufficient for development of uniform observable, significant improvements in aggregate stability across the study site. In order to provide a better perspective for adoption of alternative management strategies, including amendment with seaweed, it is important to recognize that many changes in soil physical quality may not be observed immediately after implementation of a management change. In contrast to aggregate stability, AWC, bulk density, and infiltration rate are more ephemeral properties, and may be expected to respond within days of change in management (Islam and Weil, 2000). Rapid changes in these parameters are generally associated with tillage (i.e. mechanical disruption and aeration), although they may also be affected by substantial inputs of organic matter (Gugino et al., 2009). In this study, the amount of seaweed biomass applied resulted in a layer approximately 0.25 – 1 cm thick, a small amount of biomass relative to the volume of soil in the plow layer, which is on the order of 15 cm. Additionally, the treatments were uniformly subject to mechanical tillage following seaweed application, potentially masking any changes in AWC, bulk density or infiltration as a result of organic matter addition. High variability in infiltration rates across the treatment replicates may also have precluded the development or detection of significant differences.

Soil chemical properties

Significant (p<0.05) increases in SO42- (Figure 1), EC (Figure 2), and exchangeable K+ (Figure 3) with seaweed addition support hypothesized effects of seaweed amendment. Changes in these properties were transient, with increases observed soon after seaweed addition, returning to PFF fertilizer treatment levels by the end of the growing season. Additionally, a reduction in pH was hypothesized in response to seaweed amendment, and this effect was observed for a short period (~ 1 month) after addition (Figure 4). By contrast, NO3- (Figure 5) and PO43- (Figure 6) levels did not increase in seaweed treatments (LDS and HDS) compared to the PFF treatment, supporting the hypothesis of equivalent provision of primary nutrients. In contrast, hypothesized increases in total trace elements (Ca, Mn, Fe) and toxic metals and metalloids (Pb, Cd, Cr, Zn, Hg, Cu, As) in soil were not observed (Table 4, 5). Equivalent or increased provision of primary nutrients (in this case, NO3-, PO43-, and K+) is an essential factor for successful adoption of an alternative fertilizer management practice, because limitation of these essential nutrients generally results in the most recognizable, quantifiable differences in crop growth, yield, and ultimately, economic viability. Prior to seaweed or PFF addition, soil levels of both PO43- and extractable K+ were relatively high (~60 and 150 μg/g dry soil for PO43- and K+, respectively). Based on the Cornell Soil Health Guide, extractable K+ at the site is rated as very good, exceeding the published rating chart (Gugino et al., 2009). Potassium does not pose a leaching or toxicity risk, and does not contribute to poor soil quality at high concentrations. In contrast, PO43- and NO3- can be a concern at excessive levels due to leaching risk, and improvements in soil quality decrease above maximum concentrations, reaching “poor” rating at concentrations above 30 μg/g dry soil for both PO43--P and NO3--N (Gugino et al., 2009; Marx et al., 1999; Heckman, 2003). For the soils at the study site, high natural abundance of Fe oxides and hydroxides generally allows for substantial retention of phosphate by metal-P complex formation, and the amount of PO43- in the soil solution (i.e. water-extractable PO43-) was negligible (N. Winkler, unpublished data), suggesting that the majority of PO43- extracted with NaHCO3 was previously loosely sorbed to Fe and Al oxide surfaces (Schoenau and Karamanos, 1993). In contrast to PO43- and K+, NO3- was consistently low (close to 0) across fertilizer treatments prior to seaweed application in October 2011, and early in the growing season (April and May 2012). Presumably, as availability of N from the seaweed biomass and PFF increased, NO3--N levels increased to moderate (10-15 μg N/g dry soil), but leaching risk was likely minimal due to rapid crop uptake. Addition and subsequent decomposition of seaweed biomass high in total S (~23 g/kg DM) resulted in significant differences in SO42-, but returned to control levels by the end of the growing season. Increases in SO42- could have conflicting effects on soil quality and crop production, influencing both pH and plant nutrition. For example, microbial S oxidation to sulfate results in the production of hydrogen ions (H+), reducing soil pH, with the sulfate contributing to the soluble salt content (Janzen, 1993; Germida, 2005). Alternatively, as a component of amino acids (cysteine, cystine, and methionine) and vitamins (e.g. vitamin A), S is also be a plant nutrient, and may have positive effects on crop production. Reduction in pH was observed in this study soon after addition of seaweed biomass. In addition to S oxidation as a potential influence on pH, other microbial processes contributing to reduced pH as a result of organic matter addition include (1) C mineralization and carbonic acid (H2CO3) production (Simunek and Suarez, 1993), and (2) ammonia oxidation via nitrification (Myrold, 2005). With respect to plant growth, the pH at the site was initially low, rated as “poor”, with an average pH ~5.3 in October 2011, and increased to “moderate” in the spring, with an average pH ~6.0 in April 2012 (Gugino et al., 2009). pH is a critical variable for soil quality, particularly in relation to nutrient availability; consequently, with a low initial pH, acidification as a result of seaweed addition may be of particular concern (Gugino et al., 2009). Distinction between pH reduction specifically related to seaweed addition (i.e. S oxidation) and acidification as a result of organic matter addition and decomposition is necessary for elucidation of potential pH effects of seaweed amendment. As hypothesized, EC was significantly higher in seaweed-amended plots, with higher values in HDS than in LDS. EC, which represents the ability of the soil solution to conduct an electrical charge, has a well-supported relationship with soil salinity and crop growth (Janzen, 1993). While Na+ and Cl- are the predominant ions accounting for soil salinity, Ca2+, Mg2+, and SO42- are also important components of total soil salinity (Janzen, 1993), and are considered plant nutrients. However, the negative effects of increased inorganic ions on plant and microorganism physiology are generally of greatest concern, with negative effects observed at levels above 2000 μS/cm (Janzen, 1993). In this study, EC reached a maximum of ~350 μS/cm as a result of seaweed addition, well below the risk for crop damage, even for especially salt-sensitive crops. For instance, beans (Phaseolus vulgaris), have an EC threshold of 1000 μS/cm (Maas, 1990), and sweet corn is moderately salt-sensitive, with a threshold of 1700 μS/cm (Maas, 1990). Thus, although significant EC increases were observed with seaweed addition, they did not reach levels known to have a negative effect on crop physiology. In temperate climates, dissolved salts are generally mobile in the soil, as evidenced by EC returning to PFF fertilizer treatment levels at conclusion of the growing season. Consequently, long-term accumulation may not be of concern, but increased EC remains a potential short-term negative effect.

Soil biological properties

Among the biological properties analyzed, only active carbon (C) was affected by seaweed application (Figure 7). Soil respiration (Figure 8), earthworm abundance (Figure 9) and total SOM content (Figure 10) varied over the growing season, but the same trends over time were observed for all fertilizer treatments. PMN was consistent among the fertilizer treatments throughout the growing season, with the exception of a significantly greater value in the PFF treatment in July 2012 (Figure 11). Soil respiration, a measure of overall microbial activity, was hypothesized to increase as a result of seaweed addition due to the provision of C substrates for microbial metabolism, some of which may be new to the microbial community and readily-degradable. In some cases (e.g. May, June, and July 2012), average soil respiration was greater in the HDS fertilizer treatment, but these changes were not consistent over time, and high variability within treatment replicates precluded identification of significant differences among fertilizer treatments. Consequently, the hypothesis of increased soil respiration in response to seaweed amendment was not supported. Similarly, earthworm abundance – hypothesized to decrease due to changes in soil EC – was highly variable across treatment replicates, and overall abundance of earthworms was very low across the study area. Although significant differences in earthworm abundance were not identified, some changes in chemical properties (e.g. increased EC, reduced pH, and increased heavy metal content) would be expected to affect earthworm abundance, particularly through effects on osmotic balance. Earthworms, with high surface area exposed to the soil-water environment, are particularly sensitive to changes in soil EC and other chemical properties (Lee, 1985), and their population density and biomass would be a useful continued indicator of the soil environment’s suitability for macrofauna. Potentially mineralizable N is a measure of the microbial community capacity to mineralize organic N to NH4+, a plant-available form. This process is dependent on the presence of microorganisms involved in N mineralization and availability of organic N for mineralization (Myrold, 2005). With addition of N-rich organic matter, PMN levels would be expected to increase with seaweed amendment, but hypothesized increases in PMN were not observed. The opposite effect (decreased PMN in comparison to the PFF fertilizer treatment) was observed on one sampling date (July 2012), wherein PMN in both seaweed treatments was significantly reduced. Overall, PMN was very low for all fertilizer treatments (5-10 μg N/g dry soil/week), corresponding to a rating of “poor” to “moderate” (Gugino et al., 2009), except for the sampling date following the addition of side-dress N, with values increasing to “moderate” to “good” in this treatment. In general, PMN may be correlated with factors such as organic matter, active C, and aggregate stability (Gugino et al., 2009); given moderate to low ratings in these properties in this study, low levels of PMN are a reasonable finding. In some cases, negative values of PMN were observed, indicating net immobilization of NH4+ resulting from an N-limited environment. The hypothesized increases in active C were observed on sampling dates closer to the end of the growing season. However, while increases in active C were observed with seaweed addition, these did not correspond with seaweed biomass quantity. The LDS fertilizer treatment had either equivalent or greater values than the HDS treatment on the dates when differences between seaweed and non-seaweed treatments were observed. The time required for disintegration processes (e.g. physical reduction of seaweed particle size) may be a factor in the lag in active C changes, with increases apparent approximately 3 months after seaweed biomass application (July, August, and September 2012). The average active C level at the study site across all fertilizer treatments (~600 mg/kg dry soil) is rated as “moderate” and follows the general trend of other related biological properties (e.g. SOM and PMN) (Gugino et al., 2009). In comparison to other measures of soil C (e.g. total SOM), active C responds relatively quickly to changes in management (Weil et al., 2003), representing the fraction of soil C oxidizable by dilute KMnO4. This fraction includes C in both living and dead microbial biomass, as well as C in compounds readily available to the microbial community (e.g. C in functional groups at the edge of complex organic molecules) (Weil et al., 2003). Active C is generally correlated with increased microbial activity, and is well-supported as an indicator variable of potential crop yield improvement (Weil et al., 2003). Consequently, changes in active C may represent a substantial benefit for overall soil quality and potential yield improvement as a result of seaweed amendment.

Sweet corn production

As a soil fertilizer management practice, replacing a part of the total N supply with seaweed was equally effective in terms of yield (Figures 12, 13) and above-ground biomass production (Figure 14) as using only pre-formulated fertilizer. In this study, seaweed biomass was used to replace fertilizer added at the pre-seeding, or broadcast, fertilization step. At this step, PFF fertilizer was applied at a rate of 45 kg N/ha. Due to high N requirements of sweet corn, an additional side-dress N application is usually required based on soil test results (UMass Cooperative Extension, 2013). Since NO3- and NH4+ levels were uniformly low - indicating N deficiency - prior to the sidedress stage (plant height ~30 cm), an additional 68 kg N/ha was applied to all fertilizer treatments, according to standard recommended management practices (UMass Cooperative Extension, 2013). Thus, sweet corn and soil quality production results must be viewed with the perspective of seaweed as providing a portion of overall N supply. Sweet corn yield was either equivalent or significantly greater for seaweed treatments compared to the PFF fertilizer treatment for all yield measures (average ear biomass, hundredweight, bushels/ha, and above-ground biomass). For average ear biomass, the LDS fertilizer treatment was significantly higher than either the HDS or PFF treatments (Figure 15). The remaining yield measures, as well as ear quality (dissolved soluble solids) (Figure 16), were statistically equivalent across all fertilizer treatments. While differences in yield were not statistically significant, a trend towards higher yield in seaweed-amended plots was observed for all yield measures, which may have economic relevance. For instance, the average increase in hundredweight on a per- hectare basis is equivalent to an additional $575 in income. Regardless of potential economic impact, the equivalent yield supported by seaweed amendments may be sufficient for consideration of adopting this as an alternative management practice, assuming additional costs are not restrictive. Yield across all fertilizer treatments in this study was lower than average yield for sweet corn in Rhode Island in 2007 (~45 vs. 60 hundred weight) (USDA NASS, 2013). Consequently, it is likely that supply of nutrients was equally limited for all treatments. Since extractable PO43- and K+ levels were generally high across treatments, NO3- is more likely the limiting nutrient. K+ was significantly higher in seaweed-amended treatments after spring application, but returned to PFF treatment levels at the conclusion of the growing season. At an initial (October 2011) average K level of ~150 μg/g dry soil across all fertilizer treatments, K+ was unlikely to be initially limiting, with an Cornell Soil Health overall soil quality rating of over 100% (Gugino et al., 2009). In contrast, the timing of N availability is critical for sweet corn growth, and initial values of NO3- were uniformly low across the study site. While no obvious signs of N limitation were observed in the corn crop, addition of seaweed biomass and PFF may have not been sufficient to maximize crop growth. The potential for N limitation underscores the importance of reliable predictions of N availability, based on material decomposition rates, nitrification and denitrification processes, leaching losses, volatilization, and other components of N cycling relevant to plant-available N supply. A large body of research regarding prediction of N supply as a function of material composition and climatic factors has been developed over several decades, including laboratory and field evaluations (De Neve and Hofman, 1996; Trinsoutrot et al., 2000). For seaweed biomass, the prediction of N mineralization in the field is largely unknown. For this study, a general value of mineralized N of 50% of total N was assumed based on mineralization for high-N legumes (Fox et al., 1990; Sattell et al., 1998), but this value can differ greatly depending on climatic variables (e.g. precipitation, temperature) and does not take into account losses due to leaching. As a means of strengthening N availability predictions for seaweed biomass, stable isotope techniques (e.g. enrichment of seaweed biomass with 15N) could be implemented in the field to trace the fate and mineralization rate of applied organic N.


Seaweed biomass collected for this study was composed of a mixture of red, brown, and green algal species, and in comparison to terrestrial plant biomass, was relatively high in elements important for crop growth (e.g. N, K, Ca, and Fe). Primary plant nutrients were either equivalent (N and P) or greater (K) with seaweed amendment, so the potential for realizing target crop nutrition requirements is supported. No effects, either negative or positive, were observed for the soil physical properties evaluated, which may be a function of time required for physical property change to be observed. For chemical properties, hypothesized effects on pH, EC, and SO42- were detected, with short-term decreased pH and increased EC and SO42- after seaweed addition. Decreased pH, as a critical variable for soil productivity, may be of concern for farm application, but the observed decrease is also associated with decomposition of any organic amendment. Increased EC, which at high levels may negatively affect crop growth, did not reach levels of concern for this season. Finally, SO42- production may play a part in decreasing pH, but S is also a plant nutrient, so effects may be contradictory. No effects on the biological properties of soil respiration, SOM, or earthworm abundance were observed. PMN was significantly higher in the PFF treatment in July 2012, while at the same date seaweed treatments had net N immobilization, so the prediction and consistency of N supply may be a limitation of seaweed application, as is the case for most organic amendment materials. However, a positive biological quality effect was increased active C, a soil quality indicator with good correlation to plant productivity and crop yield. Overall, effects on soil quality are both negative (e.g. decreased pH and increased EC) and positive (e.g. increased active C), but should be viewed in light of the persistence of effects, as well as the distinction between seaweed-specific and general organic matter addition effects. When applied as a fraction of overall N crop requirements, no significant differences in above-ground biomass or yield were observed, indicating that equivalent crop productivity could be obtained by implementing seaweed amendment. Additionally, while no differences in dissolved soluble solids were observed, the average weight of corn ears was greater in seaweed-amended treatments, so potential for improved ear quality and marketability may be a positive benefit of seaweed amendment. However, labor requirements and transportation costs may limit the economic viability, especially when balanced with the limited financial benefit of increases in yield. The balance of potential for increased crop success and material costs requires facilitation of improved methods and timing of collection.

Research conclusions:

In general, the practice of seaweed amendment in an optimized system uses locally-available, low-cost, otherwise wasted nutrient-rich biomass, and may impact agricultural sustainability in several ways: (1) diversifying inputs, thereby decreasing the need for exogenous nutrient supply; (2) introducing plant nutrients beyond those found in most fertilizers, (3) promoting microbial activity as a result of relatively readily-decomposable organic matter, among other putative benefits (e.g. plant growth promotion through seaweed plant hormone content). As a field trial over one growing season, this study, in particular, best represents the result that would be initially obtained by a farmer implementing seaweed amendment. This initial evaluation indicates that yield and quality of sweet corn obtained could be equivalent to sole use of pre-formulated fertilizers, and that some improvement of soil quality properties with relatively fast response to change in management (e.g. active C) could be achieved over one season. In order to make broad recommendations for implementation, the evaluation of changes over multiple seasons is required, as well as optimization of factors such as collection efficiency and seaweed application quantity. Nonetheless, interest in implementation of and experimentation with seaweed amendment was apparent in Cooperative Extension farm visits, poster presentation discussions, and Southeastern Mass/RI Young and Beginning Farmer social events. As a result of established connections with RI farmers and home gardeners, future studies and refinement of seaweed amendment recommendations would have a willing audience and likely participants in suggested management techniques.

Participation Summary

Education & Outreach Activities and Participation Summary

Participation Summary:

Education/outreach description:

Completed outreach activities: (1) Poster presentations: “Using Seaweed As a Soil Amendment: Effects On Soil Quality and Yield of Sweet Corn (Zea mays L.)” at academic conferences: ASA, CSSA & SSSA International Annual Meetings 2012, Cincinnati, OH, Poster Number 2531, abstract available at: Biennial Meeting of the Soil Ecology Society 2013, Camden, NJ Conversation with ~30 attendees at each conference; effective in communicating study background and primary results with experts in the fields of agronomy, crop science, soil science, and soil ecology (2) URI Cooperative Extension outreach: URI “Twilight Talks” vegetable grower program (Summer 2012) Field visit and discussion of preliminary results with ~40 attendees; discussion and questions suggest substantial interest and possible implementation, particularly for small-scale growers Follow-up – returning attendees for summer 2013 Twilight Talks; (3) Narragansett Times newspaper article (Narragansett, RI) – “Researchers at URI using seaweed for crops,” Wednesday, Jan 23, 2013, Narragansett Times Article in local newspaper (circulation ~6500 readers) highlighting participation of undergraduate Coastal Fellow Nathan Winkler, as well as description of project background, objectives, and preliminary results (4) URI College of the Environment and Life Sciences – website story featuring Nathan Winkler as part of the Coastal Fellow program (5) Oral presentation of proposed research (Spring 2012), research update (Spring 2013), and final thesis defense (Summer 2013) in URI Department of Natural Resources Science (6) Publication of defended Master’s Thesis (open access) Ongoing outreach activities: (1) Manuscript preparation for submission to the Journal of Sustainable Agriculture (2) Oral presentation: “Using Seaweed As a Soil Amendment: Effects On Soil Quality and Yield of Sweet Corn (Zea mays L.)” to be given at the ASA, CSSA, & SSSA International Annual Meeting 2013 in Tampa, FL (3) Video interview – Field visit, demonstration, and discussion with myself, PI Jose Amador, and Nathan Winkler, to be posted on the URI Laboratory of Soil Ecology and Microbiology website ( (4) Northeast Organic Farming Association – Summer conference 2014, workshop on seaweed use in agriculture

Project Outcomes

Project outcomes:

Examination of quantifiable differences in costs and benefits between seaweed and non-seaweed treatments resulted in net expenses 3.5 and 0.5 times greater than PFF treatments for LDS and HDS seaweed treatments, respectively (Table 6). These estimates take into account both mileage and wear-and-tear for transportation ($0.55/mile), assume Rhode Island minimum wage ($7.75/hr) for collection labor, and are based on the area of each amendment treatment (0.01 ha) employed in this study. Consequently, the transportation and labor involved in collecting seaweed as a fraction of overall nutrient supply on the scale utilized in this study does not support the practice based solely on basic, quantifiable costs. However, several factors should be considered in application of economic findings to seaweed amendment on a larger scale. First, the increase in income associated with increased harvest area may not be proportional to the increase in cost for application on a larger scale, following the principles of “economies of scale.” Second, this economic evaluation does not account for externalities associated with either seaweed or PFF application, such as environmental impacts (e.g. energy requirements for fertilizer production). These externalities can be included as monetary factors in life-cycle assessment procedures, a style of evaluation with increasing use in the agricultural sciences (Haas et al., 2000). For a more inclusive estimation of total costs and benefits of seaweed amendment, future studies would benefit from life-cycle assessment or a similar analysis procedure. Finally, the monetary value of increased yield is based on Rhode Island wholesale value for sweet corn on a hundredweight basis, a value that may increase for direct farm sales, especially of organically-certified sweet corn.

Farmer Adoption

This project did not directly involve farmer partners for on-farm research or aim to immediately initiate adoption of seaweed amendment as a management practice, but general farmer response via outlets such as the URI Cooperative Extension Twilight Talks and the Southeastern MA/RI Beginning and Young Farmer Network events was positive. Many farmers, as well as home gardeners, noted that seaweed was being already being used as a component of compost or mulch. For instance, Alex Houtzager of Kettle Pond Farm (Berkley, MA) uses seaweed collected in the fall as mulch for garlic. A limitation noted for large-scale collection of seaweed from beaches was trash contamination, noted by Mike Merner of Earthcare Farm (Charlestown, RI). With support of the practice in terms of soil quality improvements, particularly those related to micronutrients, many farmers expressed interest in implementing, increasing, or improving existing seaweed amendment practices, including Stephen Murray of Kettle Pond Farm (Berkley, MA). Stephen Murray mentioned one concern, in particular: depth of seaweed mulch preventing adequate emergence of onions. This concern underscores the need for refinement of seaweed application practices for optimal nutrient provision.

Assessment of Project Approach and Areas of Further Study:

Areas needing additional study

As a short-term study, intended to evaluate initial changes in soil quality and the capacity for production of sweet corn, a longer-term evaluation of seaweed amendment would add useful information. Additionally, exploration of the mechanisms underpinning soil and plant growth effects would be useful not only for improved refinement of the agronomic practice, but also improved understanding of the biological, chemical, and physical processes that support decomposition and nutrient cycling in agroecosystems. Some factors for future study include: (1) Changes in soil physical properties that develop over longer timescales (e.g. aggregate stability) (2) Magnitude and duration of changes in EC, as well as monitoring of long-term accumulation (dependent on climate) (3) Distinction between pH reduction specifically related to seaweed addition (e.g. S oxidation) and acidification as a result of organic matter addition and decomposition (4) Capabilities to predict nutrient supply (N and P mineralization) and tracing C and N cycling through decomposition processes.

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