Response of Soybean Growth to Root and Canopy Competition

Interplant competition in soybean [ Glycine max (L.) Merr.] influences plant growth. Because of the difficulty of evaluating root competition, studies to date have concentrated on competition within the canopy. We used a below‐ground porous membrane to isolate and evaluate the effects of root and canopy competition on soybean growth. Field studies were conducted in 1985 and 1986 on a Captina silt loam (fine‐silty, siliceous, mesic Typic Fragiudult) with four competition treatments imposed on ‘Forrest‘ soybean: (i) no competition, (ii) root competition only, (iii) canopy competition only, and (iv) root and canopy (combined) competition. At growth stage R4, leaf area was reduced 29% by root competition and 48% by canopy competition. Root competition reduced net assimilation rate by 48% in 1985 and 10% in 1986, while canopy competition resulted in a 28% reduction both years. Combined competition reduced total plant dry weight an average of 78%, with intermediate dry weights for root or canopy competition. At R4 and R8, shoot dry weight was reduced 31 and 43% with root competition, and 57 and 62% with canopy competition, respectively. Canopy competition reduced root dry weight at R4 by 43% in 1985 and 70% in 1986. Root competition reduced root dry weights in both years by 24 and 42% at R4 and R8, respectively. Canopy and combined competition resulted in taller plants with smaller stem circumferences while root competition did not alter shoot structure. Results suggest that canopy competition may have a greater influence on plant development than root competition.