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

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

Summary

Application of seaweed as a soil fertility management strategy is a traditional practice in many coastal regions, utilizing an inexpensive, abundant, nutrient-rich resource. As a practice that re-purposes waste materials, diversifies inputs, and relies on coastal resources, seaweed amendment may be an effective and inexpensive means of strengthening and supporting agriculture in coastal agroecosystems. Putatively, seaweed biomass may be a useful amendment for crop production due to provision of primary plant nutrients and micronutrients (e.g. N, P, K, Ca), effects on soil water holding capacity, and promotion of microbial activity, among other plant production benefits, but may be limited by high sulfur, salt, and heavy metal content. In this study, the effects of seaweed amendment on soil physical, chemical, and biological quality and yield of sweet corn (Zea mays L. var. rugosa) were determined in a field experiment. Soil quality analyses were completed using standard protocols and molecular techniques. Low- and high-dose seaweed amendment treatments were compared to a treatment receiving pre-formulated fertilizer. Preliminary results indicate that soil pH decreased, and electrical conductivity and nitrate levels increased, as a result of seaweed amendment, but the changes may vary in persistence. In contrast, bulk density, infiltration, earthworm abundance, and available water capacity were not initially affected. Yield of sweet corn was significantly higher in seaweed-amended treatments as compared to pre-formulated fertilizer treatments (p<0.05). Further evaluation of changes in soil quality and effects developing over time will continue to explore the feasibility and viability of adopting seaweed use in agriculture.

Objectives/Performance Targets

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 (CN) composition analysis (fall collection)

Ongoing
– CN composition analysis (spring collection)
– Trace nutrient and heavy metal analysis (fall and spring collection)

2) Evaluate the effect of seaweed amendment on the yield (hundred weight) and quality of sweet corn, an economically important crop for local agricultural production, in comparison to a conventional pre-formulated fertilizer 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 conventional 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

Ongoing
– Dry sweet corn ear, husk, and stalk CN and trace nutrient analysis
– Continued data analysis and processing

3) Evaluate seaweed amendment effects on physical, chemical and biological soil quality parameters in comparison to a pre-formulated fertilizer treatment.

Complete
– Pre-seaweed application (October 2011), pre-corn seeding (May 2012), and post-harvest (September/October 2012) sampling – Aggregate stability, bulk density, available water capacity, and infiltration
– Monthly (October-November 2011 and April-October 2012) sampling – 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
– Complete soil quality analyses (field and laboratory): bulk density, available water capacity, infiltration, soil respiration, pH, EC, and extraction for primary nutrients, trace nutrients and PMN

Ongoing
– Continued extract analysis for primary nutrients, trace nutrients and PMN
– Insect and nematode identification and counting
– Continued data analysis and processing

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

Ongoing
– Analysis of costs and potential changes in income based on yield improvements
– Synthesis of yield and soil quality results

Accomplishments/Milestones

Field plot preparation and initial soil sampling

Field treatment plots (7.6 x 7.6 m with 10 m borders) were established at the University of Rhode Island Greene H. Gardner Crops Research Center (Kingston, RI). The field was previously planted with winter squash (2011 season) and disc harrowed prior to initial soil sampling. In October 2011, initial soil sampling for physical, chemical and biological quality before application of seaweed amendment treatments was completed. Sampling procedure and analysis methods are summarized in Table 1. Preliminary results do not indicate substantial differences in physical, chemical, and biological soil quality parameters across plots prior to seaweed amendment. Plots were rototilled to an approximate depth of 25 cm after initial sampling but before seaweed application.

Seaweed biomass collection, identification and field application

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, including Ascophyllum nodosum (12.5% 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 material CN content was analyzed using a Carlo Erba EA1108 CHN analyzer at the URI Graduate School of Oceanography and determined to be 21% C and 1.6% N (C:N = 13.7).

Additionally, seaweed biomass was collected and applied in late April 2012 from Mackerel Cove (Jamestown, RI) to supplement fall application. Analysis of CN content is ongoing, as well as trace nutrient and heavy metal content analysis (for both collections). For both applications, the seaweed biomass was piled near the treatment plots for ~1 week, and received no further processing prior to application. Seaweed biomass was applied by hand. In total, the seaweed biomass applications rates were as follows: A) low-dose seaweed (~13,840 kg wet wt/ha) and B) high-dose seaweed (~27,840 kg wet wt/ha).

Soil quality sampling and analysis

For all soil quality analyses, sampling was conducted at least 1.5 m from each plot edge to allow for possible border row effects. Physical soil quality parameters (aggregate stability, bulk density, available water capacity, and infiltration) were determined at 3 sampling points: 1) Pre-seaweed application (October 2011), 2) pre-corn seeding (May 2012), and 3) post-harvest (September/October 2012). While data analysis and completion of some laboratory analyses for these parameters is ongoing, no differences among treatments are apparent based on initial data. The following biological and chemical soil quality parameters were analyzed monthly (October-November 2011 and April-October 2012) according to recommended regional protocols (Table 1): 1) Soil respiration, 2) earthworm abundance, 3) insect abundance and community composition (ID to family), 4) soil pH, 5) EC, 6) nitrate, 7) ammonium, 8) phosphate, 9) potassium, 10) active C, 11) PMN, 12) sulfate, 13) SOM, 14) nematode abundance and community composition, and 15) heavy metal content.

Initial results indicate significant differences in pH, EC, and nitrate as a function of seaweed treatment. Soil pH decreased significantly (p<0.05) after addition of seaweed biomass, but returned to control level after subsequent sampling. Electrical conductivity increased significantly (p<0.05) after addition of seaweed biomass, but EC levels were not high enough to cause negative effects (<450 µS/cm) (Bauder et al., 2011). Soil nitrate increased for all treatments over time, and was significantly higher for high-dose seaweed (p<0.05) before corn planting. However, parameters with no apparent differences among treatments according to initial data analysis include aggregate stability, bulk density, earthworm abundance, and infiltration. Analysis of remaining soil quality parameters and persistence of effects is ongoing.

Sweet corn crop production

Prior to corn seeding, pre-formulated fertilizer (Nature’s Turf 8-1-9, North Country Organics, Bradford, VT) was applied at a rate of 45 kg N/hectare for the non-seaweed amendment treatment. In May 2012, sweet corn (Zea mays L. var. rugosa, Trinity F1 se+ bicolor, 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. Throughout the growing season, weeds were maintained within the treatment plots by hand cultivation. Between-plot borders were seeded with annual ryegrass and mowed weekly. Side-dress supplemental N was applied at a plant height of ~30 cm, with an N application rate of 68 kg N/hectare for all treatments. European corn borer (Ostrinia nubilalis) was controlled by plant and ear-tip application of B. thuringiensis var. kurstaki.

Corn was harvested by hand at silk dry-down stage throughout August and September 2012. Immediately after harvest, all ears were weighed whole (kg) to determine average weight of 100 ears (hundred weight). According to this metric, corn yield was significantly higher in seaweed-amended plots than in those amended with pre-formulated fertilizer (p<0.05) but did not differ between low- and high-dose treatments (Figure 1). Additionally, 40% of the collected ears were husked and oven-dried, and 20% of the ears were analyzed for DSS (°Brix) using a field refractometer. After harvest, 33% of the remaining standing stalks (every third stalk) were cut, weighed, and dried to estimate above-ground biomass. Air-dry above ground biomass was also greater in seaweed-amended treatments, but this difference was not significant (p=0.109). Overall, preliminary corn yield data indicates that an equivalent or even improved yield could be achieved using seaweed amendment as an alternative to spring fertilizer application.

• Table 1. Summary of soil quality analysis sampling procedures, schedules, and methods.
• Figure 1. Corn yield (hundred weight in kg).

Impacts and Contributions/Outcomes

Beach-cast seaweed biomass is often removed and disposed of in landfills, by-passing the opportunity for utilization of this “misplaced” resource. Crop yield and soil quality evidence supporting the use of seaweed as a nutrient source and soil-improving amendment material may in turn support the practice as an alternative soil fertility and quality management strategy that addresses the dual problems of reliance on pre-formulated chemical fertilization and wasting of valuable, nutrient-rich biomass. In this study, initial results supporting the phenomenon of beneficial seaweed amendment effects will be added to and elucidated by continued analysis and processing of soil quality data, as well as integration of cost-benefit information and logistical concerns.

During the corn growing season, the supporting background information and general soil quality and low-input management topics related to this study were presented to a group of ~25 vegetable growers from the region through the URI Cooperative Extension Vegetable “Twilight Talk” program. Many questions and discussions followed the initial presentation, indicating that many participants were interested in the results and potential implications of the project. Interested participants may serve as a future audience for final result presentation and communication in the farming community. In addition to presentation at the summer Twilight Talk, results of this study were presented in a poster at the joint Soil Science Society of America-Crop Science Society of America-Agronomy Society of America Annual Meeting 2012 in Cincinnati, OH (Possinger et al., 2012). During the poster presentation, there were many opportunities for discussion with experts (both US and international) in the fields of soil science and crop production, particularly nutrient management for corn.

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