This section is from the book "The Gardener V2", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
In nearly every garden this is used. In some it is made the most of - in others it is not used to the extent it should be; and much that might be made into manure-water, or what has actually been converted into this, is allowed to be lost in various ways. Those who do their best to make the most of their land and everything they possess, generally keep a sharp eye on the manure-water or any kind of matter good for making this; and in offering a few remarks on the subject, attention may be directed to it in two ways : firstly to its manufacture, and then to its application.
Every one who has any kind of living vegetation under his care that would be benefited with manure-water, may find ways of getting or making it. In some cases it is made of itself daily; in others it can only be had artificially. In large gardens the former plan should be followed, if possible. Wherever there is a manure-heap there should always be a manure-water tank close by. Here our manure is placed on a spot with a sharp incline, and at the bottom of this there is a large deep manure-tank. Nearly all the year round there is some manure or other kept in store, and every time it rains much of the valuable properties which are washed out of the manure are conveyed to the tank. From here it is sometimes taken as fast as it comes in, and at other times it is kept in stock, to be drawn in small quantities as required; but on no account is any allowed to run away. Such an arrangement as this gives the full benefit of every particle of the manure to the plants; and the best part is not lost, as is too often the case, by the liquid sinking into useless ground. In summer, when no rain may fall for weeks, and the manure heap and tank become quite dry, it may be filled up with water, and a quantity of pig or fowl manure be emptied in and stirred up in it.
In a very short period this will make a valuable supply, and more can be made in the same way when it is done. The next best plan to this is to have a large tub or barrel for making it up in the same way. This may be placed near the plant-houses, or in whichever part of the garden it is most wanted. Two or three manure-water tubs are always handy in all gardens, as different manures can be mixed up in them to suit any kind of crop. Pig-manure is a good material to make manure-water with, and so are fowl-droppings. Cow-manure is the next strongest, and then the manure from the stables; but the strength of all depends a good deal on how the animals have been fed. A bag of soot kept at the bottom of the water is also valuable for almost everything; and all kinds of manure-water may be made from the different kinds of artificial manures. Guano is a favourite material for this, and there are few better; and we have an impression that everything is better when mixed up in a large quantity together. Some kinds of liquid manures are very burning, if mixed up in a watering-pot and at once given to the plants; but they are not so much so if dissolved in some large holder, and allowed to remain there a little time before using.
Old barrels placed in odd corners about the sheds are always convenient for keeping liquid manure in stock, and with a number of them different kinds of manures may be had to suit different plants. Soap-suds and all kinds of slops from dwellings are also good liquid manures, if they can be emptied into some large receptacle and mixed up with other stuff. Few liquids about a garden are so valuable as soap-suds, and they would pay well to be more used than they generally are. For syringing trees affected with any kind of insect, they are more efficacious than many bought compounds; and if Carrots, Parsley, or any kind of roots are being destroyed by worms, a thorough soaking with suds will, as a rule, entirely destroy them, and leave the crop in a most healthy condition. In speaking of applying liquid manure, a caution may be given never to use it too strong, as two or three weak doses are more beneficial than one very strong one. This applies to everything, and should be remembered, as much of the success of the liquid depends on this.
In applying liquid to Vine, Peach, Fig, and other borders, some think it is only when these subjects are in leaf that they need attention in this way; but poor borders might be wonderfully enriched if some good strong liquid was thrown over them in winter, when the top-growth is dormant. In the case of manure-tanks being attached to dung-heaps, it is generally in winter that these are oftenest filled, and they cannot be too often emptied on to the borders or around trees, and even empty quarters in the kitchen-garden may have it thrown over them - that is to say, where there are not other crops in want of it, or where the supply is greater than the demand. Unless for some special purpose, few are in the habit of giving their vegetable crops regular supplies of liquid manure; but in many cases it would pay well. Peas and Celery are two crops which may always be improved with liquid, and Cauliflower and all this tribe are benefited by it, particularly at the time they are about forming heads. In all cases of giving liquid, it is a great advantage to apply it when the soil about the roots is quite moist. With outdoor crops we generally try to give it after the soil is well saturated with rain.
Then it does not run away from the roots, but lodges amongst and near them.
In watering pot-plants with liquid, how often may it not be seen running out at the bottom as fast as it is put in at the top, and passing away without doing the slightest good ! whereas, if the soil is really dry at watering-time, and clean water is given until it is reten-tively moist, and then give the liquid, little or none of it will run from the soil, but all will remain stored past, as it were, for immediate and future use. All pot - plants, including Pines, which we water with liquid, are always gone over first with clean water, to make the soil thoroughly moist, and then a good supply of liquid manure is given. This is the best way of applying it to all kinds of plants and soils, no matter whether in pot, border, or field. It is generally no loss to allow clean water to drain away at watering time; but as little as possible of this should take place with valuable liquid. When to apply liquid manure is also a matter of much importance. If weak and cool, few and tender roots may escape being injured by it; but to use strong water to plants with few roots is always dangerous.
Supposing the soil in a Pine-pot to be saturated with guano-water shortly after potting, and just as the young roots were beginning to push, ten chances to one they would never go much further, but growth would stop, and the plant assume a yellow, unhealthy colour. Subject the same plant to similar treatment when it is showing fruit, and has a pot well crammed with roots, and the advantage of it will soon be apparent. The use of liquid manure to all plants should be guided by the same considerations. Some manures, early applied, will burn; others will cause the soil to become sour and unhealthy, as there are not sufficient roots to utilise the manure; but with abundance of roots and robust top-growth the result is very different. It is not easy to say how often liquid manure should be given, so much depends on the state of the roots, the kind of plants, and the weather. In warm, dry weather, when watering must be done often, liquid may sometimes be given every other day; but when growth is going on slowly, once a-week or so is often enough to use manure; and it will generally be found safest to be very careful with it until the plants have passed their soft growth, and are on the way of gaining maturity.