342 



SOILS. 



some difficultly-soluble compound formed on the surface of 

 the soil grains. The weakest acid had a stronger solvent 

 power; but the maximum effect was produced by the acid of 

 1.115 density. This being also the most readily obtainable, by 

 simple steam distillation of acid of any other strength, the 

 writer adopted it as best suited to the purposes of soil analysis. 

 To ascertain the time required for the desired action, viz., 

 the solution of the plant-food ingredients to the extent likely to 

 be of any avail to growing plants, digestions of the same soil 

 were made in the same manner for periods of i, 3, 4. 5 and 10 

 days, with the acid of 1.115 density. The results were as fol- 

 lows : 



ANALYSIS AFTER DIFFERENT TIMES OF DIGESTION. 



\Yhile these results pointed clearly t<> the five-day period as 

 being sufficiently effective so far as the plant-food ingredients 

 are concerned, it was not easy to understand why a ten-day 

 digestion should be less incisive than a five-day one. Instead 

 of repeating the ten-day experiment, it was thought preferable 

 to re-treat the residue from the five-day digestion for five days 

 more. The result was that only more silica and alumina went 

 into solution in other words, additional clay was alone being 

 decomposed. This being of no interest in the matter of plant 

 nutrition, the five-day period was definitely adopted by the 



