CHAP, iv.] The Food of Trees 87 



in question will soon explain the paradox. Take the case 

 of the Acacia (Robinia) for example. The analysis of its 

 foliage distinctly shows it to belong to the exacting class of 

 trees ; and yet it is to be seen in vigorous growth on soils 

 of inferior quality. It must, however, be borne in mind that 

 the total leaf-production of the Acacia is not heavy ; hence, 

 even on poor classes of soil, it has less difficulty in supplying 

 its higher requirements per unit of weight of dry leaf-sub- 

 stance than other genera having more moderate requirements 

 in this latter respect but endowed with a heavier total crop 

 of foliage. And that, in the case of this particular tree, the 

 symbiotic aid of the Bacteria which are to be found in the 

 nodules of all Leguminosae^ and which are capable of assimi- 

 lating nitrogen from the free nitrogen of the air may possibly 

 be of assistance in supplying nitrogenous food, to be used 

 for the easier assimilation of other mineral food, is not at all 

 difficult of comprehension. 



In the case of the Ash, again, which makes the greatest 

 demands of all the woodland trees as to mineral food-supplies 

 per unit of weight of dry leaf-substance, this really withdraws 

 only a moderate actual quantity of nutrient salts from the soil, 

 as its total amount of dry leafy substance is slight compared 

 with that of many other genera of trees, which require smaller 

 quantities of nutrients per unit of dry leaf-tissue, but yet with- 

 draw far larger total quantities from the soil annually. Subject, 

 however, to the Law of the Minimum, referred to at the open- 

 ing of this chapter, the productive capacity or fertility of any 

 soil is dependent on the quantity of nutrients available within 

 it in a form that can be absorbed by the rootlets, and on the 

 extent to which their absorption is favoured by the combination 

 of the various physical factors determining its tenacity, moisture, 

 warmth, and depth. But every soil is not,, as one of the earlier 

 sylvicultural chemists, Gustav Heyer, asserted \ adapted to the 



1 Forstliche Bodenkunde tmd Klimatologie, 1856, p. 3. He assumed 

 that almost any soil could produce any given kind of timber, if it only had 

 the requisite amount of moisture. 



