In the first place it may be asked, How far can what we may call the permanent fertility of a field be influenced by the application of manures? And to this question the answer must be made, that the influence of manuring in increasing soil-fertility is very slight and only very gradually felt. This is illustrated by the difficulty experienced in attempting to restore to a fertile condition a soil which has long been treated by an exhaustive system of cultivation. In such a case it will be found impossible to restore the fertility of the soil, except very gradually. Farmers who farm in new countries, and in rich virgin soils, little realise sometimes how quickly they may impoverish the fertility of their soils by exhaustive treatment, and how slow the process of restoration is. Nor is this strange when we reflect on the relatively small quantities of fertilising ingredients we are in the habit of adding to the soil by the application of manures, and the nature of their action. The small rate at which they are applied, and the impossibility of distributing them equally in the soil, explain how comparatively limited their action must necessarily be. Some manures, it is true--viz., those which are soluble--are more equally distributed; but then such manures, from their very nature, are little likely to affect the permanent fertility of the soil. _Influence of Farmyard Manure on the Soil._ Of manures which have the best effect in improving a soil's permanent fertility, farmyard manure is undoubtedly the most important. This is owing partly to the fact that it is applied in such large quantities, and partly on account of its composition. Liberal manuring with farmyard manure, systematically carried out, will in time do much to build up a soil's fertility. But liberal manuring with artificial manures will also effect the same end. This it does in an indirect manner by means of the increased crop residues obtained under such treatment. Indeed one of the speediest methods of bringing a soil into good condition is by heavily manuring certain green crops, and then ploughing them in. _Farmyard Manure v. Artificials._