Seaweed tonic or liquid nitrogen fertilisers?


Robin Gale-Baker, from Sustainable Macleod, discusses which liquid solutions to use when. This is one of a series of articles she has written about growing techniques (see right hand sidebar). She has also written a number of articles about growing various vegetables, herbs and fruit trees.

I’m often asked questions about liquid solutions such as: Are all liquid solutions the same? Can I use them interchangeably? What’s the difference? How would I know which one to use? The short answer is that they are not all the same and they each have different purposes. To further complicate things, you might use different types on the one plant at different times in the growth cycle.

Liquid solutions are designed to be used diluted as sprays or diluted in a watering can or the container is attached to a hose for spraying. They are valuable because plant roots can only absorb nutrients in liquid form.

Liquid solutions are valuable because plant roots can only absorb nutrients in liquid form. Essentially, there are two types of liquid solutions:

  • Foliar sprays that contains trace elements (also known as micronutrients) such as seaweed sprays e.g. Seasol and Maxicrop. Trace elements are mineral elements needed by plants in minute amounts, and are essential for plant health. Foliar sprays are sometimes called ‘tonics’. Note that it is important to apply foliar nutrients by spraying rather than using a watering can because spraying produces a mist of tiny droplets that adhere to the leaves and allow the plant to absorb maximum trace elements and minerals.
  • Nitrogen-based sprays made from fish e.g. Charlie Carp and Powerfeed. Nitrogen is an essential element in many plant functions including creating chlorophyll, which is vital for photosynthesis in plants. Photosynthesis converts the sun’s energy, water and carbon dioxide from the air into sugars for plants to consume. Nitrogen sprays are sometimes called ‘fertilisers’.

To illustrate the different usage on the one plant, let’s take garlic. First of all, the soil needs to be well prepared and fertilised before planting in March-April with such things as compost, blood and bone and well decomposed manures all of which act as a slow release food source. After the shortest day in June until the end of August, garlic may need additional nitrogen as it uses up what is present in the soil. This will feed and strengthen the leaves and roots. However, if nitrogen is applied later than August, it will result in the growth of the leaves at the expense of the bulbs. Applying liquid seaweed on the other hand will result in swelling of the bulb and should be applied every 2-3 weeks from September until harvest in December. So, apply a nitrogen spray in July and August followed by a foliar spray from September onwards.

Seaweed tonics contain complex carbohydrates, plant hormones and trace elements in an available form. When applied appropriately, they result in:

  • Early flowering and fruit bearing and heavier crops.
  • Prevention of transplant shock.
  • Strengthening of the cells in plant walls which allows for better water delivery from the roots to the whole plant including flowers and fruit.
  • Production of stronger roots.
  • Protection against heat and frost stress, diseases (especially soil borne diseases) and pests.
  • Activation of compost heaps.

Nitrogen-based fish fertilisers also provide some of these benefits, namely:

  • Early flowering and fruit bearing.
  • Production of stronger roots.
  • Protection against stress, diseases (especially soil borne diseases) and pests.

In addition, they make lawns and plants greener. But overuse produces leaf at the expense of fruit.

Both are excellent soil conditioners and increase beneficial microbial life in the soil. They do this more effectively than powdered tonics and fertilisers because their liquid nature allows immediate uptake of nutrients through the root system.

Both are also sustainable products. Seaweed is sourced from beaches (especially bull kelp) where it washes up having broken from the main plant, and fish is sourced from European carp, an invasive fish in Australian waterways that also threatens native fish species.

One of the problems in trying to sort out what to use is that manufacturers don’t make it easy. They often include all ingredients on the label but with no weighting of these (and most of us would not know how to assess the relative amounts for example, of trace elements or nitrogen). They also tend to present the widest range of benefits again with no weighting about the degree of effectiveness e.g., does a particular product produce a lot of root growth or just a little?

Most of the major manufacturers make both seaweed and fish based products so, for example Seasol, which sounds like a seaweed company, also makes Powerfeed, a fish based product as well as seaweed tonic.

So, in summary, my best advice is:

  • Work out what you are trying to achieve through a liquid application; then
  • Decide whether you want to use a product which is seaweed-based or fish-based; then
  • Choose accordingly; and then
  • Follow the instructions on the label.

  3 Responses to “Seaweed tonic or liquid nitrogen fertilisers?”

  1. Thank you for all the excellent advice… I only just found the site, and have only recently started my potted vegetable garden. All articles read so far have helped clarify the details. Great ideas and help.

  2. I found this to be helpful. Thank you for taking the time to write it.

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