Field Method for Describing Root Activity Using Stable Strontium as a Tracer

Stable Sr was evaluated for use as a tracer for characterizing root growth or root activity of field-grown soybeans (Glycine max (L.) Merr.). Root activity is defined in this study as the absorption of Sr with subsequent accumulation of Sr in the trifoliates. Previous research had established that the amount of Sr absorbed was directly and positively correlated with total root length and root weight in the zone of tracer placement in the soil. A 0.138 mole ratio of Sr(OOCCh₃)₂^.0.5H₂O to SrCl₂^.6H₂O, containing 0.068g Sr ml^-1, was injected into the soil as an aqueous solution. A syringe and needle was used to inject Sr at various depths up to 0.45m. Needle size was reduced during the studies from 3.2mm to 1mm diameter in order to minimize soil disturbance. Strontium was injected on both sides of the soybean rows during the V2-V4 growth stage at 0.08-m intervals and at horizontal distances of 0.10 to 0.75m from the stem of the plant. The amount of Sr injected ranged from 0.138 to 0.276g per injection point. Newly developing primary trifoliates were tagged durring the growing season, as to their time of development. Most trifoliates, except for the terminal trifoliates, were harvested as they senesced, and analyzed for Sr. Terminal trifoliates were harvested at the end of the growing season when the entire plant was harvested. The stable tracer technique was used successfully to differentiate root growth 1) at different depths in the soil, 2) at different vegetative growth stages, 3) among different soybean breeding lines, and 4) between conventional till and no-till systems. The technique permitted a vertical and horizontal characterization of soybean root growth. The method was tested to determine the 1) amount of Sr to inject per injection point (0.136g Sr), 2) row length (0.45 to 0.90m), 3) number of injection points (10 to 20), 4) number of plants to sample (6 to 12), and 5) needle size (1mm).