Nitrogen Fixation Associated with Sugarcane

Nitrogen fixation associated with sugarcane was monitored ushg acetylene reducti,on tests and bacteriological techniques. Soil cores and trash from cane fields showed acetylene reducing activity. This activity was usually low in association with sandy soils. Where high activity was detected in soil cores it correlat~d with the wet weight of roots in each core. Nitrogen-fixing bacteria resembling Azospirillum species were cultured from surface-sterilized roots of a number d cane varietes, the variety N7 being particularly well infected. Tenuous calculations based on acetylene reduction results and root weights suggest that sugarcane growing on Rydalvale series soil at the Experiment Station might derive about 25 kg of nitrogen per hectare per annum from nitrogen fixation. Introduction Nitrogen-fixing bacteria in the soils of the South African sugar indus'try were last studied in 1962 by Anderson1. Since then new methods have revealed that some nitrogen-fixing bacteria form intimate associations with the roots of tropical grasses and that these bacteria have a greater potential for nitrogen fixation than the free-living bacteria studied by Anderson. Ruschel and Vose2 have recently concluded that as much as 30% of the nitrogen in sugarcane growing in Brazil may be derived from bacteria. Such information, together with the recent dramatic increases in the price of fertilizer nitrogen, stirnulated the present study. The aim was to determine whether appreciable nitrogen fixation takes place in association with cane grown in South Africa and to define optimum conditions for this fixation.