OCTOBEB 13, 1905.] 



SCIENCE. 



459 



measures which should be taken to ameli- 

 orate the nature of the poorer soils, for, 

 remote as may now seem the prospects of 

 spending time and labor on bad land in 

 new countries where there is still a choice 

 of good, once the road' to improvement is 

 indicated little by little the work will be 

 done. It is hardly realized to what extent 

 the soils in England have been ' made ' ; 

 the practise of 'chalking,' previously men- 

 tioned as having doubled or trebled the 

 value of the Rothamsted land, must have 

 added between 100 tons and 200 tons of 

 chalk per acre to those soils before the end 

 of the eighteenth century, and in other 

 parts of the country marling, claying, in- 

 corporation of burnt earth and other lighter 

 material have contributed enormously to 

 render the present degree of fertility pos- 

 sible. 



The main facts of the nutrition of the 

 plant have been so long established that it 

 is not always realized how much still re- 

 mains unknown. It has become a common- 

 place of the text-books that the plant needs 

 nitrogen, phosphoric acid, potash, often in 

 excess of the quantities present in a normal 

 soil ; so that these substances alone are con- , 

 sidered of manurial value, other necessary 

 materials like lime, magnesia, iron, sul- 

 phuric acid and chlorine being practically 

 never lacking under natural conditions. 

 But the function of these substances in the 

 development of particular plants, the man- 

 ner in which the character of the crop is 

 affected by an excess or a deficit, is still 

 imperfectly apprehended. We realize the 

 dependence of vegetative development upon 

 the supply of nitrogen, and how an excess 

 defers maturity; we are also beginning to 

 gather facts as to the manner in which an 

 overplus of nitrogen causes alterations in 

 the structure of the tissues and variations 

 in composition of the cell contents that re- 

 sult in increased susceptibility to fungoid 

 attack. Again, it is clear that potash takes 



a fundamental part in the process of as- 

 similation, the production of carbohydrate 

 in all forms being dependent on the supply 

 of potash ; but of the manner or the location 

 of the action we have no knowledge. Our 

 ignorance of the function of phosphoric 

 acid is even greater; broadly speaking, it 

 hastens maturity, and is bound up with 

 such final processes in the plant's develop- 

 ment as the elaboration of seed. With this 

 we naturally correlate on a priori grounds 

 the presence of phosphorus in the nucleo- 

 proteids ; but there is no particular evi- 

 dence that excess of phosphoric acid leads 

 to increased assimilation of nitrogen. 



Some of the barley plots at Rothamsted 

 show this very clearly; where there has 

 been no phosphatic, but a nitrogenous, 

 manuring for the last fifty years, the 

 amount of nitrogen assimilated by the crop 

 is diminished, but the gross production of 

 dry matter is still further diminished. By 

 the addition of phosphoric acid the gross 

 production is increased to a greater degree 

 than the amount of proteid formed is in- 

 creased, so that the crop shows now a 

 smaller percentage of nitrogen and a lower 

 ratio of nitrogen to phosphoric acid than 

 on the plots which are experiencing phos- 

 phoric-acid starvation. In other words, 

 where an excess of nitrogen is available 

 the amount assimilated does not increase 

 pari passu with the amount of phosphoric 

 acid which the plant can obtain. 



But with these three substances all exact 

 knowledge ceases ; magnesia, sulphuric acid 

 and chlorine are invariable and necessary 

 constituents of all plants, yet their func- 

 tion and their practical effects are still 

 unknown. To take a further example, it 

 was early in the history of agricultural 

 science that silica was discovered to be the 

 chief constituent of the ash of cereals and 

 of a few other plants. Liebig's term of 

 'silica plants' still survives to show the 

 importance once attached to this body, and 



