Seaweed Extract Application Rates: Setting a Dose

Every buyer eventually asks for “the rate”. The honest answer is that a rate depends on four variables, and any supplier who quotes a single number for all crops and all routes is guessing. What a supplier can give you is the declared content of the product, a defensible starting point, and the arithmetic to convert between units — which is what this page sets out.

Why there is no single rate

Seaweed extract is not a nutrient salt with a fixed dose-response curve. The active fractions — alginic acid, mannitol, betaines and natural plant hormones — act at low concentrations, and the response flattens or reverses above a certain point. Doubling the rate does not double the effect; in a hormone-driven response it can suppress it.

Four things move the number:

• Declared content. A product at ≥ 35% alginic acid is not dosed like one at ≥ 18%. The same gram per litre delivers roughly half the active ingredient.
• Application route. A foliar spray is dosed per litre of spray volume; a soil application is dosed per hectare of ground. These are different denominators and cannot be interchanged.
• Spray volume. The same product at the same concentration delivers twice as much per hectare if the grower sprays 600 litres instead of 300.
• Crop stage and target. A rooting application at transplant, a stress-recovery application and a pre-harvest quality application are three different jobs, often at three different rates.

The two conversions worth knowing

From concentration to rate per hectare. If a programme specifies 1.5 g per litre and the sprayer applies 600 litres per hectare, the applied rate is 1.5 × 600 = 900 g per hectare, or 0.9 kg/ha. At 300 litres per hectare the same concentration delivers 0.45 kg/ha. This is why a rate quoted without a spray volume is not a rate.

From product to active ingredient. A granule declared at ≥ 25% alginic acid carries about 250 g of alginic acid per kilogram of product. At ≥ 35% it carries about 350 g. When comparing two quotations, divide the price per kilogram by the declared percentage before comparing anything else.

What we publish, and what we do not

One grade in our range carries explicit published rates rather than only a method: Seaweed Polysaccharide Grade X, at 1–2 g per litre of water by foliar spray every 10–15 days, or 1–2 kg per hectare through irrigation, adjusted to crop demand. The same grade is used in cosmetic formulations at typically 1–5% concentration.

The other grades describe the application route without publishing a rate. That is deliberate. A rate we can defend for one crop and one soil type is not a rate we can defend for twenty, and a published number that is wrong for most of a catalogue is worse than no number. Where a buyer gives us the crop, the growth stage and the application method, we propose a rate to trial rather than a rate to apply blindly.

Running a dose-response trial

A dose-response trial on a single field takes one cycle and settles the question for that field. The design that works:

1. Four strips, one variable. An untreated control, 0.5×, 1× and 1.5× the proposed rate. Same product, same spray volume, same timing, same block.
2. Randomise the strips. Do not run them in a line along the headland; soil and drainage gradients will confound the result.
3. Fix the spray volume across all four. If the volume changes between strips, you are testing two variables.
4. Record what you can measure. Root mass at a fixed date, leaf colour score, flower count, fruit set, or a simple weight at harvest. A photograph from the same height on the same day is worth more than a subjective note.
5. Repeat once. A single strip is an observation. A repeated strip is data.

Three mistakes that cost more than the product

• Overdosing a hormone-forward product. The most common error with liquids. More is not better, and the cost of the excess is paid twice — in product and in the response.
• Mixing with the wrong tank partner. Our solid grades are alkaline (pH 9.0–11.0) and our fulvic acid is acidic (pH 4.0–5.0). Neither should go into a tank with the other, or with acidic actives, without a jar test.
• Calculating per hectare when the product is dosed per litre. A foliar concentration applied at the wrong spray volume silently changes the rate by a factor of two.

What to ask a supplier for

Before setting a rate, ask for the declared content of the specific grade (not the family), the batch certificate of analysis, the pH, and the solubility behaviour. Those four items tell you whether the rate you have been quoted belongs to the product in the bag. If a supplier cannot state the declared content of the grade they are shipping, the rate is not verifiable.

The grades with published rates are in the seaweed polysaccharide range. For what the active fractions do inside the plant, see the active ingredients guide; for how the evidence behind the response has been measured, the application efficacy guide.

Seadom has manufactured seaweed extract from Quebec Ascophyllum nodosum for more than 35 years and exports to more than 40 countries. Send the crop, the stage and the application method and we will propose a rate to trial.