_Indestructible Elements in the Soil._--Land in the economic sense is indestructible because the natural effect of use is not to destroy it. This does not mean that it is not physically possible to destroy land to the extent of making it forever impracticable to use it in the ways in which land is commonly utilized. Nature may do this by sinking it beneath the ocean, and man can, if he will, do something akin to this; but he does not naturally destroy what is truly land in the using. It is impossible to use a plow, a spade, or a reaping machine without injuring it and, in the end, wearing it out. It is also impossible to draw the nutritive constituents out of the superficial loam and convert them into crops without exhausting the supply of these sources of fertility and so spoiling that which is commonly called the land, though it is not so in the economic sense. What is really land in this sense is not affected. Nitrates and phosphoric acid that lie in the topmost stratum of the soil are among the destructible instruments of agriculture. The supply of them has to be renewed, if cultivation is continued, and they are therefore in the class with the plows, spades, and reaping machines which also wear out. But whatever there is in the soil that suffers no deterioration from any amount of use is the land with which political economy has to deal. _The Gross and the Net Rent of Land Identical._--As land does not wear out and require renewal, all that it adds to the products of the labor and capital that are used in connection with it may be taken by the landlord as an income without reducing the amount of his property. Whatever land produces at all is a net addition to the general income of society. _Net Rent of Artificial Instruments Smaller than Gross Rent._--It is not safe, on the other hand, for the owner of buildings, tools, or live stock to take for his own consumption all that these produce. If he were to use up their gross produce as he gets it, he would find, in due time, that a considerable part of his property had vanished. Such instruments wear out and become worthless, and if no part of what they produce is set aside as a sinking fund with which to purchase other instruments to take their places, one whole genus of capital must go altogether out of existence.