From the point of view of conservation of soil-nitrates, permanent pasture may be said to be the most economical condition for the soil to be in. In such a case the nitrates are assimilated as they are formed, and, by being converted in the plant into organic nitrogen, they are at once removed from all risk of loss. A consideration, therefore, of the process of nitrification furnishes many arguments in favour of laying down land in permanent pasture--a practice which of late years has been increasingly followed in many parts of the country. As, however, it is not possible or desirable to carry out this practice beyond certain limits, the rotation which most nearly conforms to the condition of keeping the soil covered with vegetation, and most approximates in this respect to permanent pasture, is most to be recommended. _Nitrification and Rotation of Crops._ The chief risk of loss of nitrates is in connection with a cereal crop such as wheat. Where turnips follow wheat, there is a period during which the soil is left uncovered, and during which most serious loss of nitrates is apt to ensue. The risk of loss is enhanced by the fact that the assimilation of nitrates by cereals ceases before the season of their maximum production in the soil. The soil is then left bare of vegetation during the autumn, which is the most critical period of all, and the result must be serious loss. In order to minimise this loss, the practice of growing catch-crops has been had recourse to. As, however, this practice will be dealt with elsewhere, nothing further need here be said. FOOTNOTES: [97] As the formation of nitrites is a stage in the process, the term _nitrification_ includes the formation of nitrites as well as nitrates. [98] Nitre seems to have been known as early as the thirteenth century. [99] Lawes and Gilbert, for example, have shown that in the Rothamsted soils it only amounts to a few parts per million of soil. [100] See Appendix, Note I., p. 196. [101] The artificial production of nitre seems to have been first effected by Glauber in the seventeenth century. [102] The lime-rubbish from old buildings, especially those parts which have come in contact with the earth, or plastering from the walls of damp cellars, barns, stables, &c., have been found to be rich in nitrate of lime, and, as has been long well known, constitute by themselves a valuable manure. The formation of the nitrate of lime can be accounted for by the contact of the lime with nitrogenous matter of different kinds. [103] As much of the nitric acid in this solution was present as nitrate of lime, it was usually treated with a solution of potassium carbonate, the result being the precipitation of the lime as carbonate, pure saltpetre being left in solution, according to the following equation-- K_{2}CO_{3} + Ca(NO_{3})_{2} = 2 KNO_{3} + CaCO_{3}.