Seaweed and Plant Growth
Seaweed contains all major and minor plant nutrients, and all trace elements; alginic acid; vitamins; auxins; at least two gibberellins; and antibiotics.
Of the seaweed contents listed after nutrients and trace elements, the first, alginic acid, is a soil conditioner; the remainder, if the word may be forgiven in this context, are plant conditioners. All are found in fresh seaweed, dried seaweed meal and liquid seaweed extract -- with the one exception of vitamins: these, while present in both fresh seaweed and dried seaweed meal, are absent from the extract.
We will deal first with alginic acid as a soil conditioner. It is a matter of common experience that seaweed, and 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.
These brief notes cover two examples: one of the way in which seaweed helps to produce a crumb structure in the soil, another of the way in which it helps soil to retain moisture.
As far as soil-conditioning is concerned -- and that is all we are to consider for the moment -- 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. We will deal with these in turn.
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, whose direct effect on the soil we have already noticed, and themselves have soil-stabilizing properties. This means that to the soil-conditioning agent which the soil derives from undecomposed seaweed -- alginic acid -- other conditioning agents are later added: the polyuronides, which result from the decomposition of seaweed.
The second effect of adding seaweed, or seaweed meal, to a soil well populated with bacteria, has already been mentioned briefly. It is a more complex matter, and requires consideration in some detail. Basically, the addition of seaweed leads to a temporary diminution of nitrogen available for crops, then a considerable augmentation of the nitrogen total.
When seaweed, or indeed any undecomposed organic matter, is put into the soil, it is attacked by bacteria which break the material down into simpler units -- in a word, decompose it. To do this effectively the bacteria need nitrogen, and this they take from the first available source, the soil. This means that after seaweed has been added to the soil, there is a period during which the amount of soil nitrogen available to plants is reduced. During this period seed germination, and the feeding and growth of plants, can be inhibited to greater or lesser degree. This temporary nitrogen deficiency is brought about when any undecomposed vegetable matter is added to the soil. In the case of straw, for example, which is ploughed in after harvest, bacteria use up soil nitrogen in breaking down its cellulose, so that a 'latent' period follows. Farmers burn stubble after harvest to avoid this latent period, and the short-term loss of available nitrogen which causes it. But such stubble-burning is done at the cost of soil structure, soil fertility, and long-term supplies of nitrogen which ultimately would have been released from the decomposed straw.
It has been said by one authority that the latent period following the application of seaweed to the soil is one of fifteen weeks. But during this period, while there is a temporary shortage of available nitrogen, total nitrogen in the soil is being increased. This increase makes itself felt after the seaweed is completely broken down. Total nitrogen then becomes available to the plant, and there is a corresponding upsurge in plant growth.
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.