94 



THE ROTHAMSTED EXPERIMENTS. 



Silica in 

 the crop. 



Available 

 mineral 

 plant-food 

 in the soil. 



Soil-analy- 

 sis unreli- 

 able. 



Liebig's 

 analyses of 

 Rotham- 

 sted soils. 



the differences in growth due to the differences in the supply 

 of other constituents. 



Lastly, as to silica ; the chief point of interest to remark 

 is that, as the figures show, its percentage in these barley- 

 grain-ashes ranges from under 17 to more than 20, whereas 

 in wheat-grain-ash it ranges only from about 0.5 to about 1.5 

 per cent ; or, if we take the proportion of silica to 1000 dry 

 substance of grain, in barley it ranges from 4 to 5 parts, and 

 in wheat only from about 0.1 to about 0.3 parts. This differ- 

 ence is obviously due to the chaff being adherent in the case 

 of barley and not in that of wheat; and the figures afford 

 clear illustration of the material degree in which the composi- 

 tion of barley -grain-ash is influenced by the inclusion in it of 

 what is, in a sense, extraneous matter. It is indeed obvious 

 that under such circumstances we should expect, as we find, 

 less definiteness in the mineral composition of the grain of 

 barley than in that of wheat. 



In reference to the foregoing results showing the influence 

 of exhaustion and of supply, of certain mineral constituents 

 within the soil on the mineral composition of the produce 

 grown, it is obviously of interest to consider, as far as exist- 

 ing evidence will permit, the amount, and the condition of 

 availability, especially of the potash and the phosphoric 

 acid, within the soil. Unfortunately, results obtained by the 

 generally adopted methods of soil-analysis do not enable 

 us to discriminate between the total and the immediately 

 or approximately available constituents. The difficulty was 

 recognised and pointed out at Eothamsted very early in the 

 course of our investigations. From time to time the subject 

 has also been discussed by others ; and in recent years several 

 experimenters have approached it from various points of view, 

 with the object of fixing upon some useful modification of 

 method. 



More than twenty years ago, Hermann von Liebig having 

 asked for samples of some of the plots of the Eothamsted 

 experimental wheat- field, samples from five plots, to three 

 depths of 9 inches each in each case, were supplied to 

 him. He determined in them, besides other constituents, the 

 potash and the phosphoric acid, the former in a dilute acetic 

 acid extract, and the latter in a dilute nitric acid extract. 

 The results unmistakably showed differences in the amounts 

 of potash and phosphoric acid in the soils, according to the 

 manures employed. They further brought out the interesting 

 fact, that comparatively very little of the applied potash or 

 phosphoric acid had gone below the first 9 inches of soil, 

 and that certainly none had gone into the third depth. 



In our own country, for some years past, Dr Bernard 



