To recapitulate, then, nitrification is effected through the agency of micro-organisms, which are present to a greater or less extent in all soils. It requires for its favourable development air, warmth, moisture, absence of strong light, presence of a salifiable base--viz., carbonate of lime--the presence of certain mineral food-constituents, such as phosphates, and a certain amount of alkalinity. It consequently takes place to the least extent in barren sandy soils. Soils rich, light, well ventilated, uniformly moist, warm, and chalky, are best suited for its development. Other things being equal, it develops better in a fine-grained soil than in a coarse-grained soil, because, in the case of the former, aeration and uniform moistening of the soil are best secured. _Absence of Nitrification in Forest-soils._ A point of considerable interest is the practical absence of the process in forest-soils. The absence, or occurrence in the most minute traces, of nitrates in forest-soils has been accounted for by the lowness of the normal temperature of such soils and their extreme dryness. This latter condition is accounted for by the enormous transpiration of water which takes place through the trees, especially in summer-time, which is such as to render the soil almost air-dry. Lastly, it may be accounted for by the want of mineral food ingredients. _Important Bearing of Nitrification on Agricultural Practice._ Before concluding this chapter, it may be well to draw attention to the important bearing which nitrification has on agricultural practice. The light which our present knowledge--imperfect as it is--of this most interesting process throws on the theory of the rotation of crops is very striking, for it shows how the adoption of a skilful rotation may be made to prevent the loss of enormous quantities of the most valuable of all our soil-constituents,--the one on the presence of which fertility may be said most to depend--viz., nitrogen. _Desirable to have Soil covered with Vegetation._ The constant production of nitrates going on in the soil, the inability of the soil to retain them, and the consequent risk of their being removed in drainage, furnish a strong argument in favour of keeping our soils as constantly covered with vegetation as possible. _Permanent Pasture most Economical Condition of Soil._