Most garden soils can be improved by the addition of organic manures as the physical condition of it will be improved by incorporating bulky, organic materials. Not only do organic manures supply plant food, but they add substances that have the power to act on the insoluble compounds already in the soil, and so reduce these compounds to a form that plants can use.
Perhaps the only soil that does not require the addition of organic material is peaty soil, though even this will benefit from the addition of artificial manures and lime. Heavy, clay soils are improved by organic manuring as they become lighter and easier to work. Light, sandy soils are 'bulked-up' by using organic manures and this help them to retain moisture, and so it helps them to withstand drought periods.
This is the oldest and perhaps (where available) the most popular type of organic manure. It consists of the animal excrements, both liquid and solid, and of the litter (usually straw) put down for the animals to lie on. The value of these farmyard manures varies according to the way the animals have been fed and looked after. The value varies according to the way that the farmyard manures has been stored. The best farmyard manure is old manure that has been made and kept under cover.
If farmyard manures are left exposed to the action of the sun, the wind, and the rain for any length of time, its value may be reduced by half.
The litter within the farmyard manure can play an important part in altering the value of it - farmyard manure containing peat is richer in nitrogen than a similar quantity containing straw. Farmyard manures made with sawdust or wood chippings should be avoided as it is inferior to any other kind of litter - the wood itself can be harmful as it does not rot down quickly.
In comparing the manure produced by various animals, horse manure is probably more valuable than cow manure, although horse manure does lose its value if kept in the open more quickly than cow manure. Pig and sheep manures are richer in nitrogen.
If it can be obtained, any farmyard manure can be applied every year to the crops that require it. Some land will need heavier dressings than others, and a good dressing is 20 tons per acre (approximately 10lb of dung to the square yard).
In the case of heavy soils, it is a good idea to dig in farmyard manure in the autumn or winter, and in the case of light soils in spring. For heavy soils, strawy manure is better while for light soils well-rotted material is preferred.
This is available in some areas and care should be taken to know the drugs and chemicals used on the poultry flock. Manure from intensive farmed poultry should generally be avoided.
Always store it in a dry place. When applied wet, its value is only about half that of similar material stored and kept dry.
It should be used with caution, at the rate of about 1 lb to the square yard, and may be dug in, or used as a top dressing in the spring or early summer and hoed in.
Pigeon manure contains more plant food than poultry manure, and is approximately twice as valuable.
Fish manure is made from fish offal and the fish carcasses discarded during processing for human consumption. The manure is better if the oil has been removed, as then it is quick acting. Fish manure usually contains sufficient nitrogen and phosphates for most crops, but practically no potash, unless this has been added in a chemical form during the process of production.
In days not long ago, seaweed was often used by gardeners living near to the sea as Seaweed manure is nearly as good as farmyard manure, although it is lower in phosphates but richer in potash. Seaweed manure should be applied at the same rate as farmyard manure.
In these days of the early 21st century, problems with pollution of the seas around the coast have reduced the use of seaweed as a manure. Of particular concern is oil pollution which often causes small (and not so small!) lumps of thick oil attached to the seaweed found on most beaches.
Chemicals are available which will convert straw and other vegetable compost into a suitable organic manure substitute, the chemicals should always be applied in accordance to the instructions. Synthetic organic manure of this kind is normally made by sprinkling the chemical on to the compost heaps.
Other substitutes include: