Deleterious Effects of Seaweed as a Soil Amendment
Submitted by Robert D . Lucas, Ph. D, CFS, Certified Food Scientist
Recently, there has been a hue and cry over the use of Sargassum seaweed as a soil amendment locally. Mr. Joshua Forte has been touting the benefits to be derived from the use of the fertilizer. On the other hand, Ms. Deborah Hunte, President of the Farmers’ Association of Barbados, has aired her reservations about the use of the fertilizer. Ms. Hunte is on safe grounds scientifically as I will show.
Anyone who has been trained in agricultural sciences (specifically in soil science) would be aghast at the ignorance displayed locally. The deleterious effects which have been described by Ms. Hunte in St. Lucy, can be attributed to the development over time of soils that are called saline-sodic. In the case of saline soils, the difference in the osmotic pressure between soluble salts in the soil solution and added salts derived from the seaweed, results in the formation of saline soils which affect plant growth over a period of time. Eventually, the salt content of the added fertilizers becomes greater than the salt content of the soil solution; as a result, there is a flow of water from the soil solution towards the fertilizer. This results in the salt content of the soil solution becoming more concentrated. In addition there is transpiration of water by the plant: this water is adsorbed by the plant roots from the soil solution: and there is also the action of solar evaporation of water from the soil solution. These two factors which are called evapo-transpiration result in an increase in soil salinity as well. If the saline content is high enough it can negatively affect plant growth, resulting in reduced crop yields and even plant death under severe conditions.
In the case of sodic soils, a feature known as the cation exchange capacity(CEC) of the soil comes into play. This feature is best explain in terms of surface chemistry and the mono-layer adsorption (adsorption not absorption) on a surface. Soil particles and soil organic matter are negatively charged. The metallic radicals of dissolved salts in the soil solution are positively charged and are adsorbed onto the negatively charged soil particles mentioned above. Adsorption is directly proportional to the size of the charge on the metallic radical and its ionic radius/volume. For example calcium sulfate CaSO4 = Ca ++ + SO4– –. (Ca++ is the metallic radical). Ca ++ is adsorbed before a sodium Na+. since it has a charge of plus two, whereas, sodium has a charge of plus one. Calcium also has a greater ionic volume. However, if large concentrations of sodium salts are added to the soil, sodium is then adsorbed onto the surface of soil particles. If fifteen percent or more of the CEC of a soil has adsorbed sodium, the soil a sodic one. Soil sodicity causes degradation of soil structure which results in soils difficult to till, poor seed germination and restricted plant root growth.
Digressing, recently Morocco donated one million dollars for the Ministry of Agriculture to carry out a survey of soil fertility in Barbados. Vernon, K.C and Carroll, D.M (1965) “Soil and Land Use Surveys # 18, Barbados,” Imperial College of Tropical Agriculture (ICTA) have done detail surveys of soils of Barbados and the other Caribbean Islands. The fertility of the soils and the land use patterns to be adopted for Barbadian soils have been documented by Vernon and Carroll. It seems that The Ministry wants to re-invent the wheel.