Seaweed contains all major and minor plant nutrients, and all trace elements, alginic acid vitamins and antibiotics. Alginic acid is a soil conditioner the remainder are plant conditioners. All are found in fresh seaweed, dried seaweed meal and liquid seaweed extract.
Seaweed products improve the water-holding characteristics of soil and help the formation of crumb structure. They do this because the alginic acid in the seaweed combines with metallic radicals in the soil to form a polymer with greatly increased molecular weight, of the type known as cross-linked. One might describe the process more simply, if less accurately, by saying that the salts formed by alginic acid with soil metals swell when wet and retain moisture tenaciously, so helping the soil to form a crumb structure.
As far as soil-conditioning is concerned bacterial activity in the presence of seaweed has two results: first the secretion of substances which further help to condition the soil, and second, an effect on the nitrogen content of the soil.
The substances secreted by soil bacteria in the presence of seaweed include organic chemicals known as polyuronides. Polyuronides are chemically similar to the soil conditioner alginic acid.The second effect of adding seaweed, the addition of seaweed leads to a temporary diminution of nitrogen available for crops, then a considerable augmentation of the nitrogen total. When seaweed is completely broken down. Total nitrogen then becomes available to the plant, and there is a corresponding upsurge in plant growth. Liquid seaweed extract is not subject to this latent period. The nutrients and other substances it contains are available to the plant at once.
At least two gibberellins (hormones which simply encourage growth, and have not, like auxins, growth-controlling properties too) have been identified in seaweed. They behave like those gibberellins which research workers have numbered A3 and A7.
Seaweed contains all known trace elements. Chelating properties are possessed by the starches, sugars and carbohydrates in seaweed and seaweed products. As a result, these constituents are in natural combination with the iron, cobalt, copper, manganese, zinc and other trace elements found naturally in seaweed. That is why these trace elements in seaweed and seaweed products do not settle out, even in alkaline soils, but remain available to plants which need them.
With liquid extract, this ability to chelate can be taken a stage further than happens naturally with seaweed and seaweed meal. Chelation can also be used, artificially, to cause extract to carry more trace elements than are found in fresh seaweed, in seaweed meal, or in ordinary hydrolized extract.
It is also possible that seaweed sprays stimulate metabolic processes in the leaf and so help the plant to exploit leaf-locked nutrients -- for it is known that trace elements won from the soil, and delivered by the plant to the leaf tissue, can become immobilized there. A 'considerable proportion' of photosynthesis is carried out by bacteria at the leaf surface, spraying with seaweed extract at this point may feed and stimulate them, and thus increase the rate of photosynthesis.
It is known that plants treated with seaweed products develop a resistance to pests and diseases, not only to sap-seeking insects such as red spider mite and aphides, but also to scab, mildew and fungi. Soil-borne diseases of plants were reduced by adding seaweed products. The reason why seaweed and seaweed products should exert some form of biological control over a number of common plant diseases is unknown. Soil fungi and bacteria are known to produce natural antibiotics which hold down the population of plant pathogens, and when these antibiotics are produced in sufficient quantities they enter the plant and help it to resist disease. The production of such antibiotics is increased in soil high in organic matter, and it may be that seaweed still further encourages this process.