The value and use of guano are now so well understood, that it is scarcely necessary to enlarge on the mode of its application. Peruvian guano owes its chief value to its ammonia and phosphates, but it also contains potash, soda, and all the other constituents of plants in small quantity, although in a readily available condition, as is seen in the detailed analysis given in page 205.

In other guanos which have undergone more complete decomposition, and from which the soluble matters have been more or less completely exhausted by rain, the alkaline salts, or at least the potash they originally contained, have almost entirely disappeared. Hence an important difference between Peruvian guano and most other varieties. The former can be used as a complete substitute for farm-yard manure, and excellent crops of turnips and potatoes can be raised by means of it alone, and at a less cost than with ordinary dung. But though this may be done, and in many cases is attended with great economic advantages, it is a practice that cannot be recommended for general use, because the quantity of valuable matters contained in the usual application of guano is much smaller than in farm-yard manure, and the probability is that it would not, if used alone during a succession of years, be sufficient to maintain the soil permanently in a high state of fertility. Five cwt. of Peruvian guano, which is a liberal application per acre, contains about 95 lbs. of ammonia, and 130 of phosphates, while 20 tons of good farm-yard manure contain 312 of ammonia, and about the same quantity of phosphates, and when the other constituents, such as potash and soda, are compared with those in guano, the difference is still more striking. On the other hand, guano is a rapidly acting manure; its constituents are in a condition in which they are more immediately accessible by the plant, and its immediate effect is far more marked, as it is chiefly expended on the crop to which it is applied. It has indeed been alleged that it produces no effects on the subsequent crops, but this opinion can scarcely be considered as well founded. In no case does the crop raised by means of it contain the whole of the ammonia or phosphates present in the manure, and the unappropriated quantity, though it may, and probably does, escape from the lighter soils, must be retained and preserved for the use of subsequent crops by heavy and retentive clay soils. The general inference is, that though guano may at an emergency be used as an entire substitute for farm-yard manure, the practice is one to be generally avoided. When, however, as occasionally happens after a long continued use of farm-yard manure, organic matters have accumulated in the soil, and passed into an inert condition, then Peruvian guano may be used alone with very great advantage. In all cases the rapidity of the action of guano makes it an important auxiliary of farm-yard manure, and it is in this way that it may be most advantageously employed. Experience has shewn that one-half the farm-yard manure may be replaced by guano with the production of a larger crop than by the former alone in its full quantity. The proportion of guano usually employed is from three to five cwt., and it is alleged that a much larger quantity produces prejudicial effects on the subsequent crops, although it is not very easy to see on what this depends.

The variety of guano to be selected must depend to a great extent on the use to which it is to be put. Peruvian guano is most advantageously applied as a top-dressing to young corn and particularly to oats. For the turnip, the ammoniacal guanos were formerly preferred, and on strong soils, under good cultivation, their effects are excellent, but on light soils they are less applicable, their soluble salts being more rapidly washed out, and their effects lost, and in these cases they are surpassed by the phosphatic guanos.

No definite rules can be given for determining the soils on which these different varieties are most applicable, but each individual must determine by experiment that which best suits his own farm; and the inquiry is of much importance to him, as, of course, if the phosphatic guanos will answer as well as the ammoniacal, there is a large saving in the cost of the manure. A very excellent practice is to employ a mixture of equal parts of the two sorts of guano.

Pigeons' Dung.—The dung of all birds, which more or less closely resembles guano, may be employed with much advantage as a manure, but that of the pigeon and the common fowl are the only ones which can be got in quantity. Pigeons' dung, according to Boussingault, contains 8·3 per cent of nitrogen, equivalent to 10·0 of ammonia. Its value, therefore, will be more than half that of guano, but it varies greatly, and a sample imported from Egypt into this country, and analysed by Professor Johnston, contained only 5·4 per cent of ammonia. Hens' dung has not been accurately analysed, but its value must be about the same as pigeons'.

Urate and Sulphated Urine.—We have already discussed the urine of animals, in reference to farm-yard manure. But human urine, the composition of which was then stated, is of much higher value than that of the lower animals, and many attempts have been made to preserve and convert it into a dry manure. Urate is prepared by adding gypsum to urine, and collecting and drying the precipitate produced. It contains a considerable quantity of the phosphoric acid of the urine, but very little of its ammonia; and as the principal value of urine depends on the latter, it is necessarily a very inefficient method of turning it to account. A better method has been proposed by Dr. Stenhouse, who adds lime-water to the urine, and collects the precipitate, which, when dried in the air, contains 1·91 per cent of nitrogen, and about 41 per cent of phosphates. This method is subject to the same objection as that by which urate is made, namely, that the greater part of the ammonia is not precipitated. This might probably be got over to some extent by the addition of sulphate of magnesia, or, still better, of chloride of magnesium, which would throw down the phosphate of magnesia and ammonia. By much the best mode of employing urine is in the form of sulphated urine, which is made by adding to it a sufficient quantity of sulphuric acid to neutralize its ammonia, and evaporating to dryness. In this form all the valuable constituents are retained, and excellent results are obtained from it. Its effects, though mainly attributable to its ammonia, are also in part dependent on the phosphates and alkaline salts which it contains; and it is therefore capable of supplying to the plant a larger number of its constituents than the animal matters already mentioned.

Night-Soil and Poudrette.—The value of night-soil, which is well known, depends partly on the urine, and partly on the fæces of which it is formed. Its disagreeable odour has prevented its general use, and various methods have been contrived both for deodorising and converting it into a solid and portable form. The same difficulties which beset the conversion of urine into the solid form occur here, and in most of the methods employed the loss of ammonia is great. It is sometimes mixed with lime or gypsum, and dried with heat, and sometimes with animal charcoal or peat charcoal. The manufacture of a manure from night-soil, called "poudrette," has long been practised in the neighbourhood of Paris and other continental towns. The process employed at Montfauçon and at Bondy is very simple. The contents of the cesspools are conveyed to the work in large barrels, which are then emptied into tanks capable of containing the accumulation of several months. When filled they are allowed to stand for some time, during which the smell diminishes and the contents become nearly dry. The residue is then dug out and mixed with ashes, dry loam, charcoal powder, peat, peat-charcoal, saw-dust, and other matters, so as to deodorize it, and render it sufficiently dry for transport. Its general composition may be judged of from the subjoined analyses of samples from different places:—

Montfauçon.Bondy.Dresden.American.
Water28·0013·6019·5039·97
Organic matters29·0024·1020·8020·57
Phosphates7·654·965·401·88
Carbonates of lime and Magnesia, alkaline salts, etc.7·3514·1411·307·63
Sand28·0043·2043·0029·95
————————————
100·00100·00100·00100·00
Ammonia1·541·982·601·23

These analyses shew sufficiently the extent to which the animal matters have been mixed with valueless driers, the second and third samples containing considerably more than half their weight of worthless matters.

Hair, Skin, and Horn.—The refuse of manufactories in which these substances are employed, are frequently used as manures. They are highly nitrogenous substances, and owe their entire value to the nitrogen they contain, their inorganic constituents being in too small quantity to be of any importance, wool and hair having only 2 per cent, and horn 0·7 per cent of ash. In the pure and dry state, and after subtraction of the ash, their composition is,—