RHIZOSPHERE SOIL-WATER COLLECTION BY IMMISCIBLE DISPLACEMENT-CENTRIFUGATION TECHNIQUE

Progress in determining nutrient availability in the rhizosphere is re stricted by a lack of reliable and convenient methods for rhizosphere soil-water collection. A modified centrifugation method with a fluoroc arbon (Fluorinert(TM),FC-70) as an immiscible displacement liquid was developed. Our objectives were to: i) obtain an adequate soil-water vo lume from a small rhizosphere sample within a reasonable time; ii) col lect rhizosphere soil-water at container capacity (approximate to 90% of field capacity) to determine soluble soil ions; and iii) evaluate F C-70 as an extractant. The soil used was a Beadle clay loam (fine, mon tmorillonitic mesic Typic Argiustoll) with low and high levels of CaCO 3 (5 and 204g kg(-1)). Soil samples from the rhizosphere of 30-days-ol d sordan (sorghum (Sorghum bicolor L.), sudangrass (Sorghum sudanese L .) hybrid seedlings were thin-sectioned at 1-, 2- and 3-mm from the ro ot surface. The extraction parameters (sample size, volume of extracta nt, relative centrifugal force and centrifugation time) were varied to determine optimal values. We obtained adequate amounts of aqueous sol utions from moist soil (approximate to 6 g) when mixed with 2 mt of FC -70, packed into a filter unit, and centrifuged for 1 hour at 14,500 x g. The displaced soil-water was analyzed by inductively coupled plasm a spectrometry. The modified centrifugation technique with FC-70 offer s a reliable, rapid, safe, and contamination-free method for obtaining unaltered soil-water from the rhizosphere, at a moisture content norm ally found in soil.