For the present, however, we shall leave this point concerning location out of consideration and confine ourselves to natural fertility. Aside from climatic factors, etc., the difference in natural fertility depends on the chemical composition of the top soil, that is, on its different plant nutrition content. However, assuming the chemical composition and natural fertility in this respect to be the same for two plots of land, the actual effective fertility differs depending on whether these elements of plant nutrition are in a form which may be more or less easily assimilated and immediately utilised for nourishing the crops. Hence, it will depend partly upon chemical and partly upon mechanical developments in agriculture to what extent the same natural fertility may be made available on plots of land of similar natural fertility. Fertility, although an objective property of the soil, always implies an economic relation, a relation to the existing chemical and mechanical level of development in agriculture, and, therefore, changes with this level of development. Whether by chemical means (such as the use of certain liquid fertilisers on stiff clay soil and calcination of heavy clayey soils) or mechanical means (such as special ploughs for heavy soils), the obstacles which made a soil of equal fertility actually less fertile can be eliminated (drainage also belongs under this head). Or even the sequence in types of soils taken under cultivation may be changed thereby, as was the case, for instance, with light sandy soil and heavy clayey soil at a certain period of development in English agriculture. This shows once again that historically, in the sequence of soils taken under cultivation, one may pass over from more fertile to less fertile soils as well as vice versa. The same results may be obtained by an artificially created improvement in soil composition or by a mere change in agricultural methods. Finally, the same result may be brought about by a change in the hierarchical arrangement of the soil types due to different conditions of the subsoil, as soon as the latter likewise begins to be tilled and turned over into top layers. This is in part dependent on the employment of new agricultural methods (such as the cultivation of fodder-grass) and in part on the employment of mechanical means which either turn the subsoil over into top layers, mix it with top soil, or cultivate the subsoil without turning it up.