THE MANURING OF THE CANE 103 



nearly free from carbon dioxide gas, practically the whole of- the lime salts are 

 deposited as calcium carbonate, while the water Is being concentrated to one-third 

 ot its original bulk, and the remaining water becomes a saline one, containing large 

 quantities of magnesium salts as chlorides, sulphates and caibonates in solution. 

 The calcium salts, which are known to exercise a profound influence in reducing 

 the highly toxic action of the magnesium chloride and carbonate on plants, are 

 almost wholly removed from solution and the soil water becomes in a condition 

 which is poisonous to vegetation ; this 's probably what takes place during pro- 

 longed periods of dn,' weather on more or less worn-out cane soils, in which bv 

 injudicious cultivation and especially by long-continued destruction of the trash 

 by burning the normal proportion of organic matter has been largelv reduced. 

 ^\^len, on the other hand, the evaporation takes place in an atmosphere heavilv 

 charged ^\-ith carbon dioxide, as in the air present in soils containing the proportion 

 of organic matter normal to good soils, the calcium salts remain for a long time in 

 solution until the fiquid commences to become a saturated, brine, and this for a 

 prolonged period continues to modify the toxic action of the magnesium salts. 

 It is possible on such land that the soil water during drought mav become con- 

 centrated in the upper layers of the soil, without anj' material injury- to the plant, 

 until by concentration of the soil water the toxic action of the magnesium salts 

 exerts itself." 



It used to be one of the boasts of German a^icultural economists that, 

 in exchanging white sugar for cereals, they robbed America of much of its 

 potential fertility, while offering nothing in exchange, since the susjar was 

 composed entirely of materials supplied by water and carbon dioxide. 

 There is much truth in this boast, and it is to be noted that the policy of 

 the German Empire was to retain the molasses at home, and use it as one 

 of the means to build up a great alcohol industry. If such a policy were to 

 be followed in the tropics there should be no such thing as exhausted soils, 

 and on the contrary the lands should with continued cultivation become 

 more and more fertile, and it should even be possible to grow heavy crops 

 continuously without resort to supplies of readily available nitrogen, although 

 wth this additional stimulus there is nothing to indicate that the profits 

 might not be even- yet increased. It is finally to be noted that white sugar 

 manufacture alone in place of raw does not result in the retention of the 

 greater part of the plant food unless the molasses also are reserved, for the 

 manufacture of white sugar only results in transferring a small proportion 

 of the plant food from the raw sugar to the molasses, and if these are removed 

 the total loss remains the same. 



Rotations. — Different crops have a predilection for different forms of 

 mineral matter, and thus remove from the soil very different amoimts of the 

 different constituents of plant food, so much so that the ash of a crop may 

 consist in general of one predominant constituent. By gro\nng continually 

 one and the same crop on the same piece of land there is then a tendency to 

 exhaust one particular constituent. If, however, different crops be grown 

 in rotation, an element of plant food which was removed in large quantities 

 in one \-ear is not absorbed to such an extent by the succeeding crop, and by 

 the time the crop first in rotation is planted a second time a sufficiency of the 

 particular material exhausted by this crop will have become available, due 

 to the natural process of disintegration which soils are continually under 

 going. As an example of such a rotation, the Norfolk system may be quoted. 

 This is wheat, roots, barley, clover ; the roots are consumers of potash, the 

 wheat takes up phosphates, the barley absorbs silica, and the clover feeds 

 largeh' on hme and magnesia. 



It is especialh^ to be noted in this rotation that the wheat follows the 

 leguminous crop of clover ; wheat is a crop that responds to a supply of 



