Root growth and phosphorus uptake in wide- and narrow-row soybeans

Most of the research comparing the effect of different row spacing on seed yield in soybeans [Glycine max (L.) Merr.] has been focused on row spacing effects on aboveground crop characteristics such as leaf area, light interception, pod number, or biomass accumulation and their relationships with seed yield. Little work has been done on the effects of narrow-row spacing on root distribution. Plant distribution may also affect root distribution and interroot competition, and therefore, exploration and use of soil resources. A field experiment was carried out on the Pampas (Argentina) to determine the effect of narrow-row spacing on root distribution within the topsoil in soybean, and whether different root distributions affect phosphorus uptake. In December 1993, soybeans were planted at two row spacings, narrow rows (0.35 m) and wide rows (0.70 m). Root density was measured during seed filling (92 days after planting) at several points within the inter-row space down to a soil depth of 30 cm. Aboveground biomass was harvested at maturity and phosphorus (P) uptake was measured. Below the row line, narrow-row soybeans showed a greater root density than the wide row treatment at 5-10 cm depth, while roots of the wide-row soybeans had more lateral growth. Root density at both sides of the row down to a depth of 5 cm was greater for the wide-row treatment. Average root density for each depth for a section of 70 cm wide across the row line indicated there was no significant difference between treatments at any depth. The fewer number of rows for the wide-row spacing was compensated by a greater lateral extension of roots within the interrow space. This compensation resulted in a similar root density at each depth for both planting patterns, narrow and wide rows. Aboveground biomass and phosphorus concentration in plant tissue at maturity were not affected by row spacing. A similar phosphorus uptake for both treatments was consistent with the lack of effect of the different plant distribution on soil exploration by roots and on aboveground biomass accumulation.