MANURE 



209 



' The striking effect of phosphoric acid upon the growth of the turnip, indeed, is a 

 fact so well known to every intelligent agriculturist in Great Britain, that it would 

 seem quite superfluous to attempt to illustrate it by any direct experiments of our 

 own. However, as Professor Liebig has again, in the recent edition of his 'Letters,' 

 expressed an opinion entirely inconsistent with such a result, we will refer to one or 

 two of the results obtained in our experimental turnip-field, which bear on the opinion 

 he has reiterated as follows : thus, speaking of the exhaustion of phosphate of lime 

 and alkaline phosphates by the sale of flour, cattle, &c., he says : " It is certain that 

 this incessant removal of the phosphates must tend to exhaust the land and diminish 

 its capability of producing grain. The fields of Great Britain are in a state of pro- 

 gressive exhaustion from this cause, as is proved by the rapid extension of the cultiva- 

 tion of turnips and mangold-wurzel, plants which contain the least amount of the 

 phosphates, AND THEREFORE REQUIRE THE SMALLEST QUANTITY FOR THEIH DEVELOP- 

 MKNT." Now we do not hesitate to say that, however small the quantity of phosphates 

 contained in the turnip, the successful cultivation of it is more dependent upon a large 

 sxipply of phosphoric acid in the manure than that of any other crop. 



'In the following Table, then, is given the amount of bulb, from 1843 to 1850. 



First, the continuously unmanured plot : 



Secondly, that with a large amount of the superphosphate of lime alone each year ; and 

 Thirdly, that with a very liberal supply of potash, with some soda and magnesia also 

 in addition to superphosphate of lime. 



'It is seen, then, that in the third season, viz. 1845, the produce of the unmanured 

 plot is reduced to a few hundredweights, and since that period the size of the bulbs had 

 been such that they had not been considered worth weighing. On the other hand, ou 

 the plot with superphosphate of lime alone for eight successive years, we have an aver- 

 age produce of about 8 tons of bulb ! varying however exceedingly year by year, ac- 

 cording to the season. We see, too, that by the addition to superphosphate of lime of 

 a large quantity of the alkalis, much greater than could be taken off in the crop, the 

 average produce is not so great by nearly half a ton as by the superphosphate of lime 

 alone. It must be admitted that this extraordinary effect of superphosphate ot hme 

 cannot be accounted for by the idea of merely supplying in it the actual constituents 

 of the crop, but that it is due to some special agency in developing the assimilative 

 processes of the plant. The opinion is favoured by the fact that in the case where 

 superphosphate of Hme is at once neutralised by alkalis artificially supplied, the emcaey 

 of the manure would seem to be thereby reduced. And, from this again, we would 

 gather that the effect of the phosphoric acid, as such, cannot be due merely tc 

 liberation within the soil of its alkalis, or we should suppose that the artit 

 ply of these would at least have been attended with some increase of prod 

 this is not the case, notwithstanding that by means of superphosphate of lime alone 

 there has been taken from the land more of the alkalis in which the ash of the turnip 

 so peculiarly abounds, than would have been lost to it in a century under tb 

 course of rotation and home manuring ! Collateral experiments also clearly prove the 

 importance of a liberal supply of organic substance rich in carbon which always c 

 tains a considerable quantity of nitrogen also-if we would in practical agriculture 

 increase the yield much beyond the amount which can be obtained by mineral manures 

 alone, and these conditions being fulfilled, the direct supply of nitrogen, on the otl 

 hand, is by no means so generally essential. And it is where we have provided a libei 



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