Uptake and Incorporation of14C-labelled Soil Organic Matter by Roots ofPisum sativumL.

14C-labelled soil organic matter (SOM) is taken up by pea roots under axenic conditions. After 18 h of culture, more fulvic acid or water-boiled-soluble soil components are taken up by the tissue than humic acid. The uptake is due to the SOM being adsorbed on to the roots surfaces and cell walls as well as to a process dependent on metabolism, probably protein synthesis. Studies using gel filtration show that humic acid comprises two discrete classes of brown substances based on mol. wt. separation, one >50 000 daltons and the other <2500 daltons. Fulvic acid and water-boiled-soluble material have components of mol. wt <25OO daltons only. Only the low mol. wt. materials are taken up by a mechanism dependent on metabolism irrespective of the soil fraction from which they are derived.