Final Reply by Dr. Lucas Responds to Peter Webster’s Deleterious Effect of Seaweed on Soils Piece
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 : 618.mg.
TIC Pretested Agar Agar 100 FCC/NF Powder Sodium: 487mg
Robert D. Lucas, Ph.D.and CFS.
Certified Food Scientist.