Final Reply by Dr. Lucas Responds to Peter Webster’s Deleterious Effect of Seaweed on Soils Piece

The Editor

Barbados Underground

Bridgetown, Barbados

West Indies

Dear Sir/Madam,

In a blog (4th. August) in reply to my last article: Webster had this to say: “The only thing obvious about this statement is the contradiction i.e. that the sodium is leached and yet there is an accumulation. Barbados limestone soils are well drained and there can be no accumulation of sodium ions which are readily leached”.

My reply is as follows: “Webster is obviously out of his depth when it comes to surface chemistry and West Indian soils. Let me explain again for his benefit that since sodium has a smaller ionic volume/radius it not adsorbed as readily as potassium and therefore is leached down the soil profile. The black earth soils of Barbados have been classified by Vernon And Carroll(1965, ICTA) as soil type 30. Soil type 30 is what is called a montmorillonite soil ( and has what is known as a two -in one lattice structure). This soil type has a high clay content as can be readily attested after a rainfall by walking through it. The soil adheres to one shoes and is difficult to handle. Indeed after downpours pools of water are often seen dotting the surface of these soils. I will also now explain what is meant by capillary action that I alluded to, that Webster again selectively does not cite. After soluble salts have leached down the soil profile, when a period of prolong dry weather occurs, the soil pore-space acts like what happens when blotting paper is dipped in water. In the latter case water rises up the blotting paper by capillary action. A similar event occurs in the soil. Since there is a water deficit between the water content of the surface (lacks water) and the soil solution down the profile that contains salts (sodium, since I am dealing seaweeds), there is upward movement of salt containing water. When this water reaches the surface, solar evaporation takes place and the soil becomes saline.

I want to disabuse Webster of the view that by washing seaweed one can remove the salt it contains. According to TIC Gums product data the sodium content of various purified seaweed products are as follows per 100 grams. There substances have under gone several washing to meet food-grade standards.

Ticagel 121-AFG Powder Sodium: 619 mg.

Agaroid RS-507 Powder Sodium :

TIC Pretested Agar Agar 100 FCC/NF Powder Sodium: 487mg


Robert D. Lucas, Ph.D.and CFS.

Certified Food Scientist.

2 thoughts on “Final Reply by Dr. Lucas Responds to Peter Webster’s Deleterious Effect of Seaweed on Soils Piece

  1. In any discussion/argument a personal attack in the form of insulting, put down labels like: “ignorance”; “out of his depth” and “the need to disabuse” is an indirect indication of insecurity and lack of confidence in the argument. Let us look beyond these labels and highfalutin brambling to the facts about sodium in seaweed and in Barbados soils.
    1. My original submission referred to the need to wash the seaweed to remove any remaining sea salt from the surface of the weed before composting. There was never any suggestion of washing the inherent/intrinsic sodium out of the seaweed;
    2. The inherent sodium composition of the seaweed is less than 1%. Do not be misled by the large numbers of milligrams as there are 1000 milligrams to a gram and the sample referred was 100 grams;
    3. The series 30 black soils classified by Vernon and Carrol amount to less than 20 % of all the soils in Barbados and to use these as an example is meaningless;
    4. Most soils World over will have puddles of water during and after heavy rains simply because the accumulation is faster than the drainage. This does not mean that there is no drainage and where there is drainage there is leaching;
    5. Barbados farmers have actually applied sodium nitrate fertilizer in the past with no deleterious effects. There is no accumulation of sodium. It is leached;
    6. Recent soil tests (2016) at River in St Phillip (which is a series 30 black soil and probably the poorest draining in the island) found no accumulation of sodium. All organic matter has sodium and other salts and these soils have had organic matter compost for 100s of years and the sodium and other salts have been leached from the soil;
    7. For there to be an accumulation of sodium or other salts there would need to be an impervious layer in the soil that would stop the water movement/leaching. There are no such layers in our limestone soils;
    8. Capillary action can only bring salt or sodium back to the soil surface if there is salt or sodium to bring back;
    9. In every wet and dry cycle capillary action will bring some salts back to the soil surface but then they are leached again in the next cycle. There is no accumulation simply because there is 100 times more drainage and leaching than there is capillary action during the dry cycle.

  2. What about this article that was published in The Nation where a one year trial by farmers in St. Lucy showed initial success but severe salt problems after a while. There has been some research at UWI which showed that simply washing the seaweed does not remove significant amounts of salt. Although not in that UWI presentation, it was also found that old sargassum stored at SBRC for months and rain washed over that period of time still posed a salt problem. The sodium that it contains rather than that simply on its surface is the main problem.

    With Barbados being ranked high in water scarcity would washing/treating significant amounts of seaweed be the best use of this water in the future or should we be investigating new ways of treating or using Sargassum where the salt content does not present such an potential issue such as bio-gas production?

    The sodium content of sargassum is not that high relatively speaking in terms of total composition but you apply significantly more seaweed to the soil that you would a chemical fertiliser so the sodium does add up.

    “An analysis done by the Ministry of Agriculture found that sargassum seaweed had a nutrient content about of about one to 1.5 per cent nitrogen, 0.5 to 1.5 per cent phosphorous, and one to two per cent potassium. It was also pointed out in a Government Information Service (GIS) report that that since these nutrient concentrations were so small large amounts of the seaweed would be needed to “experience meaningful results”. It was also important to note that seaweed has a high salt content, so that if it were applied directly to the soil, the soil’s alkalinity would increase.”

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