Elemental uptake in above-ground plant material of perennial ley after high application of seaweed materials

High rates of seven seaweed materials, corresponding to 1.6–4 kg DM m−2, were applied to a second-year grass-clover ley in spring to study the plant growth response, and potential relationships between applied elements and the uptake of elements in above-ground plant material (APM). The rates were close to the maximum permitted over 10 years by Norwegian regulation due to the concentration of cadmium (Cd) in the seaweed products. With one fermented material (liquid, acidic), only perennial weeds survived. With solid amendments, the plants grew vigorously. The plant growth was measured over two seasons. The rate of potassium (K) applied varied from 45 to 256 g m−2 and hence was well above recommended values; e.g., of 7 g m−2 in spring for grazing of ewes. The balance between monovalent K and divalent calcium (Ca) and magnesium (Mg) in APM exceeded critical levels for ruminant animals, especially in the 1st growing season. No relationship was found between the applied rates and the uptake of Cd in APM. For arsenic (As), the uptake in APM increased with higher rates of applied As, and the As concentrations in APM as well as the uptake of As were related to the applied rates of As. The total uptake over two growing seasons was also closely related with the applied rate (R2 = 0.72) and ranged from 0.1–0.3% of the applied As. Hence, high rates of seaweed application may not hamper plant growth, but rates should be restricted to avoid risk of tetany disease in farm animals, and plant uptake of As.