APPENDIX TO CHAPTER XXIII.
NOTE I. (p. 526).
Experiments on the Manuring of Beans.
Experiments with beans carried out at the Highland and Agricultural Society's Experiment Station at Pumpherston, showing the effect of potash:—
| No. of | Bushels dressed | |
| plots. | Kind of manure. | grain, per acre. |
| 27. | No manure | 2-1/2 |
| 12. | Phosphate (bone-ash) | 5-1/6 |
| 18. | Nitrate | 6-1/4 |
| 21. | Phosphate and nitrate | 5-1/3 |
| 22. | Potash | 26-1/2 |
| 17. | Potash and phosphate | 42-1/3 |
| 10. | Potash, phosphate, and nitrate | 45-1/2 |
| 38. | Potash, phosphate, nitrate, and gypsum | 51 |
CHAPTER XXIV.
ON THE METHOD OF APPLICATION AND
ON THE MIXING OF MANURES.
Having considered the manuring of the different crops, we may now pass on to the consideration of some points in the method of application and on the mixing of manures.
Equal Distribution of Manures.
A most important object in applying manures is to effect equal distribution of the manure in the soil. This is often, however, unusually difficult to do, especially in the case of artificial manures, where the quantity to be spread over a large area of the soil is extremely small. The difficulty in the case of farmyard or other very bulky manure is not so great. In order to overcome this difficulty in the case of artificial manures, it is often advisable to mix them with some such substance as sand, ashes, loam, peat, or salt. The manure is thus diluted in strength, and a very much larger bulk of substance is obtained to work with. Circumstances must decide which of these substances to use. If the soil be a heavy clay, the addition of sand or ashes may have an important mechanical effect in improving its texture; while, on the other hand, if it be a light soil, the addition of peat may improve its mechanical condition. It must also be remembered that peat itself contains a large amount of nitrogen, and thus forms a manure of some value. In using loam or peat to mix with artificial manures, they should be first dried and then riddled; while if ashes be used, they should be previously reduced to a fine state. Wood-ashes, however, must be used with caution, and ought not to be mixed with ammoniacal manures, as they are apt to contain caustic alkali, which would tend to drive off the ammonia in a volatile state.
It has been recommended, in order to save trouble and effect equal distribution, that the manure to be applied should always be made up to the same amount, so that the farmer by experience may ascertain the rate at which to apply it. And here it may be well to say a word or two on the subject of mixing manures—a subject with which the farmer is not always so conversant as it is desirable in the interests of his own pocket he should be.
Mixing Manures.
It is to be feared that not unfrequently indiscriminate mixing may cause very serious loss in the most valuable constituent of a manure. It may be well, therefore, to point out one or two of the causes of the loss that is apt to ensue on the mixing of different kinds of manures together.
As the subject depends for its clear comprehension on certain chemical elementary principles, it may be well for the benefit of non-chemical readers to state these pretty fully.
Risks of Loss in Mixtures.
The risks of loss which may occur from the mixing of artificial manures together may be of different kinds. One is the risk of actual loss of a valuable ingredient through volatilisation; another is the risk of the deterioration of the value of a mixture through change of the chemical state of a valuable ingredient. Undoubtedly the most common and most serious source of loss is the former. Of the three valuable manurial ingredients—nitrogen, phosphoric acid, and potash—only the first is liable to loss by volatilisation, and this generally only when the nitrogen is either in the form of ammonia or nitric acid.
Loss of Ammonia.
Ammonia, when uncombined, is a very volatile gas with a pungent smell, a property which enables its escape from a manure mixture to be very easily detected. It belongs to a class of substances which are known chemically as bases, and which have the power of combining with acids and forming salts. Sulphate of ammonia is a salt formed—as its name indicates—by the union of the base, ammonia, with the acid, sulphuric acid. Now when ammonia unites with sulphuric acid and forms sulphate of ammonia, it is no longer volatile and liable to escape as a gas, but becomes "fixed," as it is called.
Although most salts are more or less stable bodies—not liable to change—if left alone, and not submitted to a high temperature or chemical action, they can be easily decomposed if they are heated or brought into contact with some other substance which will give rise to chemical action. Sulphate of ammonia is a salt that is very easily decomposed. This is due to the fact that its base, ammonia, is very volatile, and not capable of being held very firmly by an acid, even by sulphuric, which is among the least volatile of all the common acids. If, therefore, sulphate of ammonia be heated above the boiling-point of water, or brought in contact with any other substance which will give rise to chemical action, it is easily decomposed. Now a salt may be acted upon by a base or an acid or another salt. When it is brought in contact with a base, if the base with which it is brought in contact be a stronger base than the base of the salt, the salt is decomposed, and a new salt is formed. The acid, in short, exchanges its old base for the new one.
Effect of Lime on Ammonia Salt.
This is exactly what takes place when the base lime comes in contact with an ammonium salt, such as sulphate of ammonia. The sulphuric acid exchanges its old base, ammonia, for the stronger base, lime, and sulphate of lime is formed, and ammonia is set free as a gas, and escapes and is lost. Sulphate of ammonia, or any substance in which there is an ammonia salt, must never be brought in contact with free lime, otherwise the ammonia will be lost, and should be harrowed in on chalky soils for this reason.
It is different entirely with gypsum—which is sulphate of lime—or phosphate of lime, both of which may be safely mixed with sulphate of ammonia without any danger of escape of ammonia. It follows from the above that a mixture which must on no account be tried is slag phosphate and sulphate of ammonia. This is because the slag phosphate contains a large percentage of free lime, which would at once, on being brought in contact with the sulphate of ammonia, decompose it, and cause the ammonia to be lost. For this same reason guano must not be mixed with slag. It is perhaps unnecessary, however, to warn one against so doing, as it is not likely such a mixture would be made, as the ratio of phosphoric acid to nitrogen in guanos is generally greater than is required. If it be desired to mix the slag with a quickly available form of nitrogen, nitrate of soda is not liable to loss; although for other reasons it is not desirable to apply nitrate of soda along with the slag, as the former manure should be applied almost always as a top-dressing.
Loss of Nitric Acid.
The risks of the loss of nitrogen in the form of nitric acid, although not so great as they are in the case of ammonia, are still considerable. As nitric acid is not a base but an acid, what is to be avoided in mixing nitrates is bringing them in contact with any other manure which contains another free and stronger acid—as, for example, superphosphate. The free acid present in superphosphate has the tendency to drive out the nitric acid from the nitrate and usurp its place. The risk of loss of expulsion in the above cases is always augmented by the rise of temperature which invariably accompanies chemical action of any kind; and although the loss of nitrogen, in the form of nitric acid, caused by mixing superphosphate and nitrate of soda, might, under ordinary circumstances, amount to very little, yet, if the mixture were to be allowed to stand any time, and the temperature of the mass to be heightened, the loss which would undoubtedly then ensue would be considerable.
The nitrogen salt which it is safe to mix with superphosphate is sulphate of ammonia.
But, as has already been mentioned, there is another loss which may result from the mixing of manures. This is the deterioration of the value of an ingredient by reason of change of chemical condition. This is a source of loss that was little suspected a number of years ago, but it is now well known that superphosphate of lime, under certain conditions, is changed from its soluble to an insoluble form. We have already referred to the reversion of phosphate in the chapter on the Manufacture of Superphosphates.[247] It was there pointed out that reversion is often caused by the presence of iron and alumina or undissolved phosphate, and that the risk of reversion is therefore very much less in a well-made article, made from pure raw material, than in one made from a raw phosphate containing much iron and alumina. Superphosphates containing a large percentage of insoluble phosphates ought not to be kept too long before being used as a manure, otherwise much of the labour and expense involved in their manufacture will be lost by the reversion of their soluble phosphate. Further, it is highly inadvisable to mix superphosphates with basic slag, which contains a large percentage of both iron and free lime. Lastly, if it is desired to mix superphosphate with insoluble phosphate, the mixture ought to be made just previous to application.
Manurial Ingredients should be applied separately.
The question of applying manure in mixtures is one on which considerable difference of opinion may exist. For many reasons manures are often better applied in the unmixed condition. For example, a mixture of a quickly acting nitrogenous manure with a slowly acting phosphatic manure is not suitable. In such a case either the nitrogenous manure will be applied too long before it is required by the plant, and thus suffer from risk of loss, or the phosphatic manure will not be applied long enough before it is likely to be used. By applying manures in an unmixed condition the chances are that a more economical use of them is made than would otherwise be the case. On the other hand, while the application of the separate constituents may be desirable from the scientific point of view, it involves a considerable amount of extra trouble. Of course a further consideration is the desirability in many cases of having a complete manure. The above hints, therefore, on the risks of loss which exist in mixing manures, may be of service to the agricultural student.