238 PLANT SOCIOLOGY 



most active in the horizon of decomposition, in this respect agreeing 

 with the activity of mycorhizal fungi. It decreases with depth. 

 Rather inert nitrogen compounds slow up nitrification (Andre, 1921, p. 

 179). Olsen (1925) has determined that soils of fiat-moor communi- 

 ties with Carices, Molinia, and Deschampsia caespitosa, which did not 

 show a trace of nitric acid at the first examination, showed 22 to 25 mg. 

 per liter of soil after 25 days. From this it follows that the amount 

 of nitrate present at any given time gives no indication of the nitrate 

 supply of a plant community. Not only is nitric acid quickly assimi- 

 lated by plants, but it is also leached out by rain and is often barely 

 traceable. Thus the nitrate content of a soil is subject to considerable 

 fluctuations from time to time. 



These conclusions are of ecological importance because they lead us 

 to consider not the nitric acid present, nor the quantity of total nitro- 

 gen, as a measure of the nitrate available to plants, but rather the 

 capacity of the soil for nitrification. This is represented by the amount 

 of nitrates formed during a certain period of time. 



Investigations by C. Olsen and others show the dependence of many 

 species upon the inherent capacity of the soil for nitrification. Thus a 

 rich growth of Urtica dioica always coincides with an intensive capacity 

 for nitrification in the soil, and it would seem that the rich growth 

 depends directly upon nitrification. But the nitrifying Urtica soils 

 contain also comparatively large quantities of phosphoric acid, PO4, 

 which possibly have a part in the luxuriant development. 



Methods for the determination of nitrifying power are too compli- 

 cated for general use. For further information on the whole question 

 reference may be made to the investigations of C. Olsen (1927) and 

 Nemec and Kvapil (1927). 



Nitrate Test with Diphenylamine. — A simplified method which will 

 permit the ecologist to estimate the nitrate factor in his researches has 

 been introduced by Mohsch and later used by Hesselman, Raunkiaer, 

 and others in ecological studies. It is known as the diphenylamine 

 method. 



Hesselman (1917) and Raunkiaer (1926) started out with the 

 assumption that the nitrate content of the plants themselves would 

 give some information about the nitrifying power of a soil. No 

 nitrate can be produced in the plant tissues, therefore the nitric acid 

 present in the plant sap must have come from the soil. Negative 

 results carry less weight than positive ones. Not all plants store 

 nitrates. In some species young plants, capable of further develop- 

 ment may under certain circumstances, contain large (quantities of 

 nitrates, while older specimens have none at all. The nitrate test is 



