One of the oldest manures known is seaweed, which is widely used for improving the soils of gardens and farms in coastal districts. The different weeds vary in plant food content; the long broad-leaved species, which is usually found just below the low water mark, is richer than the bladder wrack (Focus) found between low and high tides. It is gathered all the year round but the richest harvest is thrown up by spring tides, or during storms.
About three-quarters of seaweed is water, the remainder being humus-forming material. It contains about 1/2 percent nitrogen and up to 11/2 percent phosphates and about 1-11/2 percent potash. Since seaweeds have no roots they obtain all their nutrients from the dissolved substances in the sea, which is constantly being enriched by drainage from the land. So they absorb very large quantities of nutrients and are, therefore, an excellent source of trace elements.
The actual organic matter consists very largely of alginic acid which, unlike the cellulose which you get in farmyard manure, rots readily in the soil and is an excellent soil conditioner. The other carbohydrates and simple sugars found in seaweed also decompose readily in the soil.
It is best dug into the soil immediately after spreading to prevent it from drying out into a hard, woody mess. The usual rate of application is 5kg (10lb) per square meter (yard). Seaweed is particularly suitable for sandy soils in view of its comparative freedom from fibre, thus allowing rapid humus formation. Freedom from weed seeds and disease organisms is an additional advantage of this manure. Its content of common salt is not usually harmful to plants when the manure is used at the normal rates of application.
Dried seaweed products
Whole seaweed dried and ground is now available under brand names. Thus prepared the natural product can be transported inland economically and is four to five times as concentrated as wet seaweed or farmyard manure. The product is dry and pleasant to handle and is sold in small packs. Some manufacturers reinforce their products with fertilizers to give them a higher plant food content and to overcome the tendency of dried seaweed temporarily to tie-up nitrogen in the soil.
The best results are obtained from dried products to which fertilizer has been added when they are applied to the surface of the soil during winter cultivation and left for a few weeks to break down; this time varies according to weather and soil conditions, but about four weeks is sufficient in most instances. A reasonable dressing is 60-120g (2-4oz) per square meter (yard).
Products reinforced with NPK fertilizers are sold by some manufacturers and are intended to overcome the initial `tie-up’ of nitrogen in the soil. These can be used in the spring and summer on growing plants, at the rate of 60-120g (2-4oz) per square meter (yard) for all crops.
Dried seaweeds are also used in soil composts and in topdressings for lawns. The usual rates of application already quoted may seem to be rather low for a bulky organic manure whose main function is to supply humus, but seaweed preparations contain alginates which stimulate the soil organisms to greater activity that results in better tilth formation in heavy soils and greater water-holding properties in sandy soils.
Their trace element content is most valuable and makes this class of manure one of the best and safest means of providing soils with nutrients that are needed in very small amounts.