Sea Lettuce as Fertilizer: Some Considerations

There has been talk of late of using sea lettuce as a fertiliser for the land, and there is a lot of suggestions on various websites about its potential. But there are relatively few studies actually made of how this would work, any potential problems, and any side-effects.

One exception is the report:

“An Investigation into the Potential use of Sea Lettuce (Ulva lactuca) as a Soil Amendment in Vegetable Gardens and Orchards Contract Report for Bay of Plenty Regional Council” (July 2013)

The Bay of Plenty is in New Zealand. It is known in Māori as Te Moana-a-Toi, and is a large indentation in the northern coast of New Zealand's North Island.

Current advice is clearly based on the study, and says:

“Sea lettuce can be used as a garden or orchard mulch if used sparingly to avoid high levels of salt building up in the soil, killing sensitive plants. It can also be used to make compost, but needs to be thoroughly mixed with other compostable materials.”

There is also a booklet produced called “Sea Lettuce and the Garden”, useful if people want to collect sea lettuce themselves, on how to use it.

http://www.boprc.govt.nz/media/395700/web-version-sea-lettuce-and-the-garden-brochure-tauranga-harbourwatch-inc-2011.pdf

For best results when using sea lettuce

In the garden or compost:

• Collect sea lettuce that is fresh, if it is already producing bad odours or has dried out, the sea lettuce will not have the same benefits for the garden.
• It is important that the sea lettuce is hosed down with fresh water to remove excess salt and sand before it is used in the garden.
• The sea lettuce must be mixed with other organic materials such as other organic plants matter to mitigate the effects of odours and carbon dioxide gas.
• The sea lettuce will decompose faster if it is chopped up first.

Benefits

• It has a relatively high nitrogen and phosphorus content.
• It decays quickly in the soil.
• It can be used fresh or composted.
• It contains strong growth hormones to stimulate plant growth and development. These include auxlns, gibberillins and cytokinens which contribute to plant growth responses.
• It increases the water retention capacity of the soil and plant resistance to water stress
• It can stimulate the root systems of plants to further growth
• It contains vitamin A, B and C, iron and iodine

Here are some of the highlights of the main report, but for the full report, with tables, photos and graphs, see:

https://www.boprc.govt.nz/media/367880/an-investigation-into-the-potential-use-of-sea-lettuce-ulva-lactuca-as-a-soil-amendment-in-vegetable-gardens-and-orchards.pdf

Sea lettuce (Ulva lactuca) is a type of marine algae that is native to New Zealand. It frequently blooms in large numbers in Tauranga Harbour and once it washes up on the foreshore it creates a public nuisance. This is due to the algae forming thick mats which blanket the foreshore and undergo anaerobic decomposition, creating a gaseous release of sulphurous compounds.

The Regional Council have been recently evaluating different methods of disposal of sea lettuce, which is currently being transported to a composting facility near Papamoa. One such proposal is the application of sea lettuce directly to kiwifruit and avocado orchards within close proximity to the harbour. The other option is to encourage home gardeners to utilise sea lettuce as a soil amendment in their vegetable gardens.

Sea lettuce has proven to be a useful soil amendment because it not only provides macro-nutrients such as nitrogen, phosphorus and potassium, but also contains many of the micronutrients required by plants (Eyras et al., 1998). Several studies have shown that seaweed may have different effects on different plants. A study in Patagonia, Argentina, showed that seaweed compost increased plant yield and plant biomass for tomato plants (Eyras et al., 2008).

To determine:

1. Any detrimental effects of sea lettuce application to orchard or garden soils
2. The effect of sea lettuce application on the growth of vegetable plants

In order to carry out objective No. 1, the trial was conducted in such as way as to provide a ‘worstcase scenario’ in terms of the potential utilisation of sea lettuce as a soil amendment.

Vegetables chosen to plant were basil, spinach, capsicum and radish, representing two leaf crops, one fruit crop and one bulb crop. Vegetables were planted two weeks after the initial sea lettuce application. In each plot three spinach seedlings and three capsicum seedlings were planted 20 cm apart. Nine basil seeds were sown at a depth of 2 cm and 15 cm apart and 25 radish seeds were planted 5 cm apart Not all plants survived and therefore additional replacement seeds were germinated in a laboratory on moist filter paper and replanted into the vegetable garden. The vegetables were irrigated twice daily using a sprinkler with an automatic tap timer for 10 minutes on each occasion. Weeds were removed weekly.

Subsequent sea lettuce applications were made on the 21st December and 20th February and were mixed into the soil around the plants.

Plants were harvested entirely, including roots, washed of excess soil and placed into labelled plastic bags for transportation to the laboratory. Once at the laboratory each plant was photographed and measurements of a number of different parameters for each crop were carried out.

Plants were sampled for nutrient content analysis on 8th February for spinach and radish and 16th March for capsicum. Leaves, roots, fruit and bulbs were analysed separately. Basil plants were not analysed for nutritional composition.

Soil from each experimental plot was analysed for a number of chemical parameters. These were pH, organic matter, total exchange capacity, exchangeable cations, anions, base saturation, major and minor nutrients, total acidity and carbon. All of these parameters give an indication of the fertility of the soil.

Levels of heavy metals in all batches of sea lettuce were below New Zealand Standard guideline levels for composts, soil conditioners and mulches and Bio-Gro Organic standards 2009

Soil chemical properties were considerably altered as a result of direct applications of sea lettuce. Sodium levels increased dramatically in all plots after each application of sea lettuce The vegetable plots had the greatest increase in sodium from the baseline level (582%).

Soil sulphur also increased markedly with sea lettuce applications. Although not directly toxic, excess sulphur can lead to a decrease in soil pH. The mean level of sulphur in soils receiving three applications of sea lettuce were significantly higher than the control soils (p < 0.05). The garden soils showed the most dramatic increase in sulphur after sea lettuce applications, increasing from a baseline level of 13ppm to 76ppm after three applications.

For calcium, no significant differences were found between treatments (p < 0.05). However, there was a noticeable reduction in calcium base saturation with sea lettuce applications, especially for the kiwifruit orchard soil, where the control plots had a value of 79% compared with 59% for the plots receiving three applications. There was also a corresponding increase in magnesium levels although the increase was not significant (p < 0.05)

Harvest data for each of the vegetable crops were analysed and a number of differences between treatments were apparent. It was also noted that control plots had many more weeds than the treatment plots.

From the results of the field trial it is evident that the direct application of sea lettuce to soil causes a significant change in the chemical properties of the soil. The dramatic increase in levels of soil sodium has most likely caused a negative impact on the growth of radish and basil plants, which had poor growth in plots receiving 2 applications of sea lettuce compared to control plots. The growth of spinach plants was not affected by sea lettuce applications and this may be due to its ability to tolerate high levels of sodium. Conversely, capsicum fruit were significantly larger in plots receiving three applications of sea lettuce, but at this application rate other vegetable plants could become negatively impacted due to the high sodium content of the soil

The heavy metal content of the Tauranga Harbour sea lettuce used in this trial were all below New Zealand Standard guideline levels for composts, soil conditioners and mulches and Bio-Gro Organic standards 2009. Therefore, use of sea lettuce as a soil amendment on orchards and home gardens shouldn’t result in soil heavy metal concentrations exceeding the Bio-Gro standard limits.

However, if frequent, long-term applications of large quantities are made then it may be advisable to periodically test soil for heavy metals which may accumulate over time.

Part of the objectives of this trial was to provide a ‘worst case scenario’ for applying sea lettuce to gardens or orchards. The unwashed material was applied directly to the soil in large amounts, up to three applications over a short period, to identify any detrimental effects to soil and plant growth.

Although soil sodium levels increased, levels of heavy metals in the soils were all below NZ Bio-grow organic guideline levels (2009) and plant growth was not severely impacted in the garden plots. There were no differences between plants on the basis of colour or other defects and overall, the plants looked healthy, and were all perfectly edible

The use of sea lettuce as a soil amendment in vegetable gardens and orchards may be viable. The apparent ability of sea lettuce to help suppress weed growth is an attribute that could be worthwhile investigating further. However, it would not be advisable to apply unwashed sea lettuce directly to soil immediately prior to seed sowing or seedling planting as it may physically suppress seed germination and seedling growth. It should also be applied sparingly so as to avoid excess salt accumulation in the soil. The best means by which to utilise sea lettuce in the garden would be to:

• Wash first to remove salt and sand 
• Prior to planting, apply directly to soil in small amounts, leave to break down and mix in well 
• Compost well with other organic material before application