PLANT FOOD IN SOILS. 205 



of potash was generally sufficient for the barley crop, a lack 

 of potash being only shown in some instances by a deficiency 

 in the straw. 1 



It is thus evident that, compared with the one per cent, 

 citric acid, the barley roots took up potash from the soil 

 much more easily than they took phosphoric acid ; or, in 

 other words, that if the action of the citric acid is to be 

 made comparable with that of the barley roots, its solvent 

 powers for soil phosphates must be reduced. We have 

 already seen that the average acidity of root sap in the 

 Graminece was found by Dr. Dyer to be equal to 0*62 per 

 cent, of citric acid. In the single specimen of barley root 

 sap examined by him the acidity was only 0*38 per cent. 

 If Schlcesing's view be correct, and it suffices merely to 

 acidify the soil in order to bring the available potash into 

 solution, a reduction in the strength of the citric acid might 

 be made without diminishing the quantity of potash dis- 

 solved, while it would probably considerably diminish the 

 quantity of phosphates brought into solution. 



As Dr. Dyer is continuing his researches, and has com- 

 mand of the best series of soils which is available for the 

 establishment of a method of determining the amount of 

 effective plant food in soils, we may be permitted perhaps 

 to point out some of the questions which seem at the present 

 time to stand in need of answers. 



Dr. Dyer has worked upon undried soils. It is very 

 important that we should know if the citric acid method 

 yields the same results before and after the soil is dried. 

 Unless a soil can be dried sufficiently to admit of powdering 

 and sifting, it is impossible to obtain a fair sample of it for 

 analysis, and thus a method which demands undried soil 

 becomes of little practical use. 



1 Dr. Dyer's explanation of the relatively low proportion of soluble 

 potash found in the soils from the fact that the soil samples were taken in 

 autumn, before the winter weathering had taken place, seems untenable. The 

 quantities of potash and phosphoric acid found in autumn must be those 

 existing in the previous spring, that is, after winter weathering, minus that 

 removed in the crop. The quantity removed in the preceding crop is thus 

 the only correction which need be applied in the case of autumn sampling, 

 and this correction must affect both the phosphoric acid and potash. 



