It is consequently not as a direct chemical manure that farmyard manure is pre-eminently valuable. We must seek for perhaps its most valuable properties in its indirect influence.
It adds to the soil a large quantity of organic matter. Most soils are improved by the addition of humus. The water-absorbing and retaining powers of a soil are increased by this addition of humus, while it enables the soil to attract an increased amount of moisture from the air. This is often of great importance, as in the period of germination of seed.[177] The influence it exerts on the texture of the soil in the process of fermentation is also very great. This is especially so in soils whose texture is too close, such as heavy clayey soils. It opens up their pores to the air, and renders them more friable. Where such an influence is most required, as in clayey soils, the manure ought to be applied in a fresh condition, so that the maximum influence exerted by the manure in this direction may be experienced. On light soils, on the contrary, whose friability and openness are already too great, and which do not require to be increased, the manure will be best applied in a rotten condition. It adds, further, greatly to the heat of the soils by its decomposition. Thus on cold damp soils it effects one very marked benefit. The influence it exerts in its decomposition upon the fertilising ingredients present in the soil is also by no means inconsiderable. In the process of its fermentation large quantities of carbonic acid gas are generated. This carbonic acid probably acts in a double capacity. It will, in the first place, greatly increase the solvent power of the soil-water, and thus enable it to set free an increased amount of mineral plant-food; and secondly, it will help to conserve a certain quantity of the soil-nitrogen, by preventing its conversion into nitrates.
As its indirect and mechanical properties are greatest when in its fresh condition, it will be better to apply it in that condition to soils most lacking in these mechanical properties. We may therefore say that farmyard manure is best applied in a rotted condition to light sandy soils, and to soils in a high state of cultivation, where its mechanical properties are not so much required.
An important point still remains to be discussed—viz., the rate at which the farmyard manure should be applied. This, of course, should naturally depend on a variety of circumstances—the amount of artificial manures used as supplementary to the farmyard manure, the frequency of its application, and the nature of the soil.
These considerations naturally vary so much, that the quantities of farmyard manure it is advisable to apply in different cases are widely different. There is a strong probability that the rate at which farmyard manure has been applied in the past has been grossly in excess of what could be profitably employed. Opinion is gaining ground among practical farmers, that smaller and more frequent applications of farmyard manure to the soil would be fraught with better results than the older custom of applying a large dressing at a time. This is an opinion in the support of which science can urge strong arguments. It is only of late years that we have come to recognise sufficiently the various risks which all fertilisers are subject to in the soil, and the importance, therefore, of minimising these risks as much as possible by putting into the soil at one time only as much manure as it is safely able to retain.
"The famous old German writer Thaer regarded 17 or 18 tons as an abundant dressing; 14 tons he called good, and 8 or 9 tons light. Other German authorities speak of 7 to 10 tons as light, 12 to 18 tons as usual, 20 or more tons as heavy, and 30 tons as a very heavy application."[178]
In the new edition of Stephens' 'Book of the Farm,'[179] from 8 to 12 tons per acre for roots, and from 15 to 20 tons for potatoes, along with artificials, which may cost from 25s. to 60s. per acre additional, are quoted as general dressings.
The majority of recent experiments with farmyard manure would seem to indicate that, even in the case of what are considered small dressings, the extra return in crop the first year after application is not such as to cover the expense of the manure. Of course, as is commonly pointed out, the effect of farmyard manure is of a lasting nature, and is probably felt throughout the whole rotation, or even longer. This, to a certain extent, is no doubt true; still it may be strongly doubted whether farmyard manure is, after all, an economical manure, as compared with artificial manures. The desirability of manuring the soil and not the crop is, in this age of keen competition, no longer believed in; and the Rothamsted experiments have shown that it is highly doubtful whether even the soil benefits to anything like a commensurate extent by the application of large quantities of farmyard manure. This is of course assuming for farmyard manure the value that it would fetch when sold, or, to put it rather differently, the price it would cost if the farmer had to purchase it. Farmyard manure is a necessary bye-product of the farm, and can scarcely be regarded, therefore, in the same light as the artificial manures which the farmer buys.[180]
FOOTNOTES:
[131] See Appendix, Note I., p. 279.