Revegetation of alumina refinery wastes. 2. Glasshouse experiments

Abstract

In the absence of chemical limitations, the emergence and yield of Rhodes grass (Chloris gayana) on a 20 per cent fly ash : 80 per cent red sand mix were higher than those on fly ash or red sand alone or on the other mixes investigated. The poor emergence and yield in the red sand was attributed to the medium's low available water capacity and low unsaturated hydraulic conductivity. Mixtures containing from 40 per cent to 100 per cent fly ash were characterized by very low air contents at field capacity and were susceptible to water-logging. The extremely poor emergence and low yields observed in the mixtures containing 40 per cent and 60 per cent fly ash were considered to be due to high mechanical resistance to root ramification in addition to lack of aeration. In the absence of the limitations of excess salinity, alkalinity and sodicity, deficiencies of nitrogen and phosphorus were the major restrictions to the growth of Rhodes grass on fly ash, red sand and a 20 per cent fly ash : 80 per cent red sand mix. Manganese was also limiting in red sand, and boron was slightly deficient in fly ash, but these deficiencies were not present in the ash : sand mix. In a nitrogen (NH4NO3) by phosphorus (Ca(H2PO4)2.H2O) interaction trial on fly ash and red sand, maximum yields of Rhodes grass were obtained with 100 kg N ha-1 (red sand) to 200 kg N ha-1 (fly ash) and 400 kg P ha-1. The severe yield depression which occurred at nitrogen rates greater than 200 kg ha-1 on both wastes was attributed to NH4+ toxicity aggravated by the dearth of nitrifying bacteria and the wastes' low cation exchange capacities.