SOIL ANALYSIS AND ITS INTERPRETATION 343 



In the preceding discussion the soils have all belonged to 

 one type — the much-weathered soils of the South of England. 

 We have seen that in this case there is a close correlation be- 

 tween the potash, the alumina and the clay. For purposes 

 of a survey it seems superfluous to determine these two bases 

 in every sample taken. The iron oxide shows a general but 

 by no means a close correlation with the others ; but no con- 

 nection could be traced between iron oxide and fertility in 

 the soils examined by the author, the iron oxide being almost 

 always less than 5 per cent, in amount. Nor did it appear 

 that the ratio of lime to magnesia in these soils was significant. 

 The nitrogen is closely correlated with the organic matter, i.e. 

 the loss on ignition. The total phosphoric acid shows no great 

 variations in the different soils, but the available phosphoric 

 acid, like the available potash, varies greatly with the manage- 

 ment of the soil. Thus the figures obtained by chemical 

 analysis, apart from the loss on ignition and the calcium car- 

 bonate, fall into two groups : the nitrogen, potash and alumina, 

 which are so closely correlated with quantities already deter- 

 mined in the mechanical analysis that their separate deter- 

 mination is almost superfluous ; and the iron oxide, magnesia, 

 lime, etc., which do not give sufficiently useful indications to 

 be worth determining in every case. Since chemical analysis 

 fails to describe the soil with sufficient completeness for 

 agricultural purposes Hall and Russell recommend that for 

 purposes of a survey a large number of soils should be sub- 

 mitted to mechanical analysis, including the determination of 

 organic matter and of calcium carbonate, and then a carefully 

 chosen representative set should be analysed chemically so as 

 to characterise the type ; these can further serve as standards 

 with which farmers' samples can be compared by the citric acid 

 method. They agree with Whitney that mechanical analysis 

 should form the basis of the survey, because it alone takes 

 account of those physical functions — the regulation of the 

 water-supply and therefore of the temperature, of the air 

 supply, ease of cultivation, etc. — that play so large a part in 

 determining the value of a soil, 



