_ 210 Ad. Mayer_, Das Düngerkapital und der Raubbau (Heidelberg, 1869), sees the only conditions of production which man cannot increase at will exclusively in the sun’s rays, the employment of which also depends on the quantity of land. Thus would he explain _Senior’s_ law. 211 See the tables of increase in _Cotta_, Anweisung zum Waldbau, p. 228. _Count Buquoy_, Theorie der N. Wirthschaft, p. 54, ridicules the absurd procedure of a great many farmers, as if by forcing the ploughshare deeper into the soil, they could compel it to produce a double return, and asks: if one should dig a square foot of land to the center of the earth and manure it, who would take it off his hands? As to the effect of manure, _Kuhlmann’s_ investigations have shown that 300 kilogrammes of guano produced in three years an increase per _hectare_ in the yield, of 2,469 kilogrammes of hay; while 600 kilogrammes produced an increase of only 2,870 kilogrammes. _Schübler_, found that where salt had been used for manuring purposes, 40 kilogrammes produced a maximum of fertility from which point forward every increase in the amount of salt was attended by diminished returns, and finally led to complete barrenness. See _Wolff_, Naturgesetzliche Grundlagen, I, 408, 412, 502. Constantly increased irrigation would convert the land into a swamp instead of indefinitely adding to its fertility. Nor can abundant sowing be of any use when it reaches such a point that the plants stand so closely together as to interfere with their proper development. 212 These differences correspond with the differences in the kinds of deterioration to which land is liable from rivers, floods, lava, etc., soil-exhaustion, and the growing wild of the land. 213 From a technic point of view, it would, perhaps, be practicable, in most instances, to obtain the phosphoric acid immediately from the land and transfer it to other land; but the relation of the cost to the result makes it impossible from an economical point of view. 214 It most certainly is always an uncommon advantage that certain kinds of soil, rich in kali and decayed vegetable matter, yield a long series of harvests without the addition of manure, provided, always, that a short interval is allowed to the process of decay to replace the exhausted plant-food. Thus in many volcanic regions. Compare on similar districts in the Deccan: _Rilter_, Erdkunde, V, 714.