1. That fermentation takes place more quickly.
2. That it results in a more equable distribution of the manurial constituents in the dung, by gradually and thoroughly incorporating the liquid portion of the manure with the soil-particles.
Against, however, these undoubted advantages, one serious disadvantage may be urged—viz., that the manure, before being ploughed in, becomes robbed to a large extent of its soluble nitrogenous compounds, which, as we have repeatedly observed, are so necessary for fermentation; and that, therefore, when it is ploughed in, it does not so readily ferment. This being so, it is highly advisable, in the case of light or sandy soils, not to follow such a practice, but to plough the manure directly in.
As to the depth to which it is advisable to plough the manure in, it may be here noticed that it should not be too deep, so as to permit of the access of sufficient moisture to ensure proper fermentation, and to prevent rapid washing down of nitrates to the drains. Lastly, it need scarcely be pointed out that it is highly important to have the manure evenly and thoroughly incorporated with the soil-particles. Where the manure is permitted to cake together in lumps, it may successfully resist the action of fermentation for several years.
Value and Function of Farmyard Manure.
Practical experience has long demonstrated the fact that farmyard manure is, taking it all round, the most valuable, and admits of the most universal application, of all manures; and science has done much to explain the reason of this. The influence of farmyard manure is so many-sided that it is difficult even to enumerate its different functions. As has already been pointed out, its indirect value as a manure is probably as great as, if indeed even not greater than, its direct value. In concluding our study of farmyard manure, we shall endeavour to summarise, in as brief a manner as possible, its chief properties.
First, as to its value as a supplier of the necessary elements of plant-food. This, there can be little doubt, has been, and still is, grossly exaggerated by the ordinary farmer. Much has been claimed for it as a "general" manure. How far it merits pre-eminence on this score among other manures will be seen in the sequel. It is true that, since it is composed of vegetable matter, it contains all the necessary plant ingredients.[172] As has been shown in the Introduction, there is practically in the case of most soils no necessity to add to a manure any more than the three ingredients, nitrogen, phosphoric acid, and potash. Its value, then, as a direct manure, must depend on the quantity and proportion in which these three ingredients are present. These substances, as we have already seen, it contains only in very small quantities. It is, judged from this point of view, a comparatively poor manure. Furthermore, only a certain percentage of these substances is in a soluble or immediately available condition,—in this respect the rotten manure being very much more valuable than the fresh manure.
Again, a point of great importance in a universal manure is the proportion in which the necessary plant-foods are present. If it be asked, Are the nitrogen, phosphoric acid, and potash in farmyard manure present in the proportion in which crops require these constituents? the answer must be in the negative. Heiden[173] has very strikingly illustrated this point, in so far as the relations between the two ash ingredients are concerned, by some computations as to the amount which would be removed from the soil in the course of different rotations.[174] In the case of five different rotations it was found that the ratio between the potash and phosphoric acid removed was as follows:[175] (1) 2.96 to 1; (2) 2.76 to 1; (3) 2.95 to 1; (4) 4.13 to 1; (5) 3.78 to 1. This would give a mean of 3.32 to 1. This is not the ratio in which these ingredients are generally present in farmyard manure. Farmyard manure may be said to be much richer in the mineral constituents of plants than in nitrogen. Professor Heiden found that in the case of a farm at Waldau, the crops in the course of ten years removed from a morgen (.631 of an acre) the following quantities:—
| lb. | |
| Nitrogen | 329 |
| Potash | 263 |
| Phosphoric acid | 121 |
In order to supply these amounts the following quantities of manure would require to be supplied:—