Using Seaweed as a Soil Amendment: Effects on Soil Quality and Yield of Sweet Corn ( Zea mays L.)

Application of seaweed to manage soil fertility is a traditional practice in many coastal regions, utilizing an inexpensive, abundant, and 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 and soil quality improvement due to provision of plant nutrients (e.g. N, P, K, Ca), and promotion of microbial activity, among other benefits. However, limitations of seaweed application include high sulfur (S), salt, and heavy metal content. The objectives of this study were to: (1) evaluate the effects of seaweed biomass application on soil physical, biological, and chemical properties important for agricultural productivity, maintenance of soil quality, and conservation of soil resources; (2) determine the sweet corn (Zea mays L.) yield obtained by implementing seaweed amendment as a soil fertility management practice; and (3) assess the economic feasibility of seaweed amendment for sustainable agriculture through synthesis of experimental findings and cost-benefit comparison between seaweed application and pre-formulated fertilizer use. Low-dose seaweed (LDS), high-dose seaweed (HDS), and pre-formulated 8-19 (N-P-K) organic fertilizer (PFF) fertilizer treatments were employed in a sweet corn production field experiment from October 2011 to November 2012. Pre-seeding N application rates were 42, 84, and 45 kg total N/ha for LDS, HDS, and PFF, respectively. All fertilizer treatments received side-dress N at a rate of 68 kg total N/ha. Seaweed was collected and applied in November 2011 and May 2012, and was analyzed for carbon (C), nitrogen (N), heavy metal, and nutrient (e.g. K) content. To determine seaweed effects on soil quality, soil properties were assessed prior to seaweed application (October 2011), and repeatedly throughout the 2012 sweet corn growing season. Soil properties evaluated were aggregate stability, bulk density, infiltration, available water capacity (AWC), nitrate (NO3 ), phosphate (PO4 3), extractable potassium (K + ), extractable calcium (Ca 2+ ), heavy metals (Pb, Cd, Cr, Zn, and As), total K, Fe, Mn, and Ca, electrical conductivity (EC), pH, sulfate (SO4 2), soil organic matter (SOM), active C, potentially mineralizable N (PMN), and earthworm abundance. Soil properties were determined using recommended national and regional protocols. To assess the effects of seaweed amendment on crop production, the yield and quality of sweet corn was determined by measurement of yield (hundredweight/ha and bushels/ha), above-ground biomass, average ear weight, and dissolved soluble solids (DSS) content. Seaweed amendment had no significant effects on soil physical properties. No significant differences in NO3 and PO4 2were observed in response to seaweed addition. Extractable K + levels were higher, indicating that primary nutrient provision was equivalent or improved with seaweed addition. In contrast, soil pH decreased and EC and SO4 2increased significantly as a result of seaweed amendment, and these effects varied in persistence. For instance, in May 2012, pH decreased from 6.0 to 5.6, EC increased from 42 to 329 microsiemens (μS)/cm, and SO4 2increased from 1.4 to 8.7 ppm between the PFF and HDS fertilizer treatments, respectively, but these values returned to PFF levels at the end of the growing season. No effects were observed in extractable Ca 2+ , total heavy metals, or total K, Fe, Mn, and Ca. Significant increases in active C in both seaweed treatments were observed in the later part of the growing season, with average active C of 608 mg C/kg dry soil in LDS and HDS in August 2012, compared to 492 mg C/kg dry soil in the PFF fertilizer treatment. In contrast, PMN decreased in seaweed treatments compared to PFF in July 2012. Soil organic matter and earthworm abundance did not differ significantly as a result of seaweed amendment. The average yield (45 hundredweight/ha), above-ground biomass (0.5 kg dry weight/plant), and DSS (15 °Brix) did not differ among fertilizer treatments, but the average weight of fresh corn ears was significantly greater in the LDS fertilizer treatment (0.22 kg) compared to the PFF treatment (0.19 kg). Overall, these results suggest that seaweed amendment as a means of partially replacing total N supply (38% and 55% for LDS and HDS, respectively) may be a viable agricultural practice. However, the implementation of this practice must be viewed in light of financial requirements (e.g. labor and transportation) and potential yield enhancement, as well as persistence and magnitude of soil quality changes. A preliminary analysis showed that the additional costs of labor and transportation may not be offset by increases in yield and decreases in fertilizer cost. For improved financial viability of seaweed amendment, these expenses may be reduced by improved coordination of seaweed collection and application. With improvements in collection efficiency and prediction of nutrient supply, seaweed amendment is recommended, primarily due to improvements in soil biological quality (active C) and sweet corn quality (average ear weight).