Salsola Soda - Soda Ash and The Biology of Sodium Accumulation

Soda Ash and The Biology of Sodium Accumulation

The ashes obtained by the burning of Salsola soda can be refined to make a product called soda ash, which is one of the alkali materials essential to making soda-lime glass, soap, and many other products. The principal active ingredient is sodium carbonate, with which the term "soda ash" is now nearly synonymous. The processed ashes of Salsola soda contain as much as 30% sodium carbonate.

A high concentration of sodium carbonate in the ashes of Salsola soda occurs if the plant is grown in highly saline soils (i.e. in soils with a high concentration of sodium chloride), so that the plant's tissues contain a fairly high concentration of sodium ions. Salsola soda can be irrigated with sea water, which contains about 40 grams per liter of dissolved sodium chloride and other salts. When these sodium-rich plants are burned, the carbon dioxide that is produced presumably reacts with this sodium to form sodium carbonate.

It is surprising to find a higher concentration of sodium than of potassium in plant tissues; the former element is usually toxic, and the latter element is essential, to the metabolic processes of plants. Thus most plants, and especially most crop plants, are "glycophytes", and suffer damage when planted in saline soils. Salsola soda, and the other plants that were cultivated for soda ash, are "halophytes" that tolerate much more saline soils than do glycophytes, and that can thrive with much larger densities of sodium in their tissues than can glycophytes.

The biochemical processes within the cells of halophytes are typically as sensitive to sodium as are the processes in glycophytes. Sodium ions from a plant's soil or irrigation water are toxic primarily because they interfere with biochemical processes within a plant's cells that require potassium, which is a chemically similar alkali metal element. The cell of a halophyte such as Salsola soda has a molecular transport mechanism that sequesters sodium ions into a compartment within the plant cell called a "vacuole." The vacuole of a plant cell can occupy 80% of the cell's volume; most of a halophyte plant cell's sodium can be sequestered in the vacuole, leaving the rest of the cell with a tolerable ratio of sodium to potassium ions.

In addition to Salsola soda, soda ash has also been produced from the ashes of Salsola kali (another saltwort plant), of glasswort plants, and of kelp, a type of seaweed. The sodium carbonate, which is water soluble, is "lixiviated" from the ashes (extracted with water), and the resulting solution is boiled dry to obtain the finished soda ash product. A very similar process is used to obtain potash (mainly potassium carbonate) from the ashes of hardwood trees. Because halophytes must also have potassium ions in their tissues, even the best soda ash derived from them also contains some potash (potassium carbonate), as was known by the 19th century.

Plants were a very important source of soda ash until the early 19th century. In the 18th century, Spain had an enormous industry producing "barilla" (one type of plant-derived soda ash) from saltwort plants (barrilla in Spanish). Similarly, Scotland had a large 18th century industry producing soda ash from kelp; this industry was so lucrative that it led to overpopulation in the Western Isles of Scotland, and one estimate is that 100,000 people were occupied with "kelping" during the summer months. The commercialization of the Leblanc process for synthesizing sodium carbonate (from salt, limestone, and sulfuric acid) brought an end to the era of farming for soda ash in the first half of the 19th century.