Nutrient composition and content of various biological active compounds of three South African-based commercial seaweed biostimulants

Positive results reported in commercial agriculture following the application of seaweed (kelp) products vary from root growth stimulation to increased disease resistance. The described impact of seaweed applications is mostly reliant on the specific species of seaweed used, the prevalent environmental growing conditions, together with the extraction and formulation protocol implemented in the production of the commercial extracts. These possible variables alone or in combination may lead to inconsistencies in efficacy between different commercial seaweed products, especially under field conditions. In this paper, we quantify selected active components in three different, but apparently similar, commercial seaweed products manufactured from Ecklonia maxima which could impact on biological activity, following application as evaluated for mung bean root growth. Significantly higher P and N concentrations were found both in Afrikelp® and Basfoliar® Kelp compared to Kelpak®. These elevated mineral nutrient concentrations indicate possible enrichment of these products, as they exceed the natural nutrient element concentrations in the freshly milled E. maxima prior to formulation. Kelpak® showed higher concentrations of Ca, Mg and K compared to Afrikelp® and Basfoliar® Kelp, but lower levels were reported to occur naturally in freshly minced seaweed from E. maxima. Concentrations of mannitol, uronic acid and neutral sugars together with alginic acid content were significantly higher in Kelpak® than in Afrikelp® and Basfoliar® Kelp. Similarly, mung bean root growth stimulation was significantly higher in Kelpak® than in Afrikelp® and Basfoliar® Kelp. This study showed that commercial seaweed products, manufactured from the same seaweed source such as E. maxima, and thus generally marketed as equivalent products, may vary significantly in product composition and thus in efficacy to induce specific plant responses following application, when manufactured by different companies. Distinct differences in harvesting and manufacturing protocols place an obligation on the end user to ensure that the composition of the product of choice is well aligned with the specific plant response required.