THE PRODUCTIVITY OF WOODLAND SOIL 505 



improve the physical properties of the soil generally, whilst 

 also protecting it mechanically. It was known, too, that humus 

 played an important part in supplying the nitrogen required 

 for forming the albuminoid substances necessary for plant-life ; 

 and it was believed that the small quantities of nitrogen usually 

 found in woodland soil became during the leaf-decomposition 

 transformed into ammonia, and subjected to a process of nitrifi- 

 cation producing nitric acid. 



About forty years ago the researches of Schloesing and 

 Miintz drew attention to the fact that the soil was a laboratory 

 in which myriads of living organisms prepared food for field 

 and woodland crops ; and since then the process of nitrification 

 has been rendered more intelligible than previously was the 

 case, though the discovery by Hellriegel and Wilfarth, in 1886, 

 of the Bacillus radicicola forming nodules on the roots of 

 leguminous plants, and rendering symbiotic aid in withdrawing 

 nitrogen from the air circulating in the soil and in storing it up 

 within these root-nodules in a form available for enriching the 

 soil and increasing the supply of plant-food, when a sufficiency 

 of nitrogen is not already present in the soil. Among forest 

 trees the production of root-nodules is not confined to legu- 

 minous kinds like Robinia and Laburnum, but is also very 

 highly characteristic of the Common and the White Alder, and 

 the Sand-buckthorn shrub. Since Hellriegel and Wilfarth's 

 discovery in 1886, the problem of the fixation of nitrogen in the 

 soil has been occupying the careful attention of a large number 

 of agricultural chemists in England, and amongst the most 

 thorough investigators have been those whose names are so 

 closely connected with the Rothamsted investigations — Lawes, 

 Gilbert, Warrington, and Hall. The investigations made in 

 Britain, however, only deal with purely agricultural and horti- 

 cultural conditions, and as yet there are no laboratories where 

 special investigations are being made with regard to woodland 

 soil. At present, therefore, we must look for information con- 

 cerning the productivity of woodland soil chiefly to Continental 

 countries in which the forests form a great and carefully pro- 

 tected national asset ; in particular to Germany, with its many 

 well-equipped chairs of forest soil-science at Universities, and 

 Forest Academies ranking as Universities. 



As long ago as 1863 it was estimated by Krutzsch of 

 Tharandt, that Beech, Spruce, and Pine required from 30 to 44 



