17 



Determination of Phosphoric Oxide. 500 c.c. of the filtered soil extract 

 (corresponding to 50 grammes of true soil) are evaporated to dryness in 

 a platinum dish, carefully ignited, and taken up with hydrochloric acid, 

 again evaporated, ignited, and extracted with hydrochloric acid; the ex- 

 tract is filtered and washed, and the filtrate and washings concentrated. 

 The concentrated solution is allowed to cool, and then 200 c.c. of a solu- 

 tion of ammonium molybdate in nitric acid prepared as already de- 

 scribed are added, and the rest of the procedure outlined under the 

 method of determining phosphoric oxide in the " fine earth " (see method 

 I.) is followed. 



It now becomes a matter of interest and importance to agree upon 

 the interpretations that are to be placed upon the figures obtained by 

 analysis from a soil that has been extracted according to one of the fore- 

 going methods. In this Colony absolutely no investigations have yet been 

 made to show what quantities of plant food are necessary in order to ren- 

 der fertile a soil that is in other respects well circumstanced ; for it must 

 be remembered that a brack or arid soil, or one physically unfit for culti- 

 vation, cannot be otherwise than unproductive even when it is amply 

 supplied with the needful chemical constituents. 



Maercker, of the Halle Experiment Station, graded soils, on the plant 

 food basis, as follows, the extraction being made with strong acid: 



*> P oS h ;f i clay soil^sandy soils. "-* 



Poor ...... < '05 < -05 <-10 < '05 < '05 



Medium ... -05 '15 -05 '10 -10 '25 -05 -15 -05 '10 



Normal ... -15 -25 -10 '15 '25 '50 '15 '20 '10 '15 



Good ...... -25 -40 -15 -25 '50 I'OO '20 '30 -15 '25 



> -40 >'25 >1'00 > '30 > '25 



This classification Professor Hilgard declares to be in remarkable 

 agreement with his own, with the proviso that, in the presence of high 

 lime percentages, relatively low proportions of phosphoric oxide and 

 potash may, nevertheless, prove adequate. Many of the results tabulated 

 in the following pages may incidentally throw light upon the subject, but 

 until it has been directly investigated, and sufficiently so to enable limits 

 to be laid down for different parts of this Colony, it will probably prove 

 most convenient and satisfactory provisionally to judge of soils by 

 Maercker's limits, and such, in fact, has been my practice hitherto, nor 

 has it, as already observed, been found to lead to conclusions inconsistent 

 with practical experience. 



According to Dyer's investigations, a soil extracted with his citric 

 acid solution is sufficiently supplied with potash and phosphoric o-xide for 

 immediate purposes when the former shows over '005 and the latter over 

 '010 per cent. The investigations upon which these conclusions were 

 based, it should, however, be said, had been conducted upon well-studied 

 and productive soils at Rothamsted. 



Just here one point calls for special emphasis : Liebig enunciated the 

 law that the growth and development of plants is regulated by the amount 

 of that particular plant food constituent which is present in the soil in 

 smallest proportion: if one element of plant food is deficient in the soil, 

 no excess, however great, of any of the others will atone for the defect. 

 If a soil contain abundance of lime, potash, and nitrogen, but lacks phos- 

 phoric oxide, no crop can reach perfection. Liebig's law of the minimum, 

 thus briefly set forth, must now be extended so as to fill our widened 



