The effects of plant density on the growth, development, seed yield and seed quality of soybean (Glycine max [L.] Merrill) were investigated in the 2001/2002 season on a Wakanui silt loam soil at Lincoln University, Canterbury. Seeds of two cultivars, Northern Conquest and March were hand sown on 1 November 2001 to produce five plant densities, 20, 35, 50, 65, and 80 plants m⁻². Each plant density for both cultivars was replicated four times in a randomised block design. Plot size was 4 x 1.8 m.
Seed yield was strongly related to total dry matter at the start of seed fill, which in turn was a function of the amount of photosynthetically active radiation (PAR) intercepted by the canopy. A green area index (GAI) of 3.4-3.6 was required to attain canopy closure interception of >95% incoming photosynthetically active radiation). Seed yield was also strongly related to PAR for both cultivars (r² = 0.87 and 0.98 for cv. Northern Conquest and March respectively). At the two lowest populations dry matter production per unit area was insufficient to allow the seed production of the higher densities. GAI and PAR increased as the plant population increased.
Seed yield increased as plant population increased to 50 plants m⁻², but there was no further increase after that. At 50 plants m⁻², cv. Northern Conquest had a yield of 300 g m⁻², while cv. March had a yield of 241g m⁻². The greater yield for the former occurred because it had more pods per plant. Yield components (pods per plant, seeds per pod and individual seed weight) all fell as plant population increased. Plant density had no influence on seed germination, but seed vigour was significantly reduced in both cultivars as plant density increased.
This effect became more marked the longer harvest was delayed. Time of harvest also had a significant effect on seed quality, particularly delaying harvest until 14 days after harvest maturity (i.e. 15% seed moisture content), which substantially reduced both germination and vigour. Seed quality also differed with pod position, as seeds produced from pods above the 8th node on the main stem had higher vigour than those from pods below the 8th node on the main stem or from pods on branches. Overall, the highest quality seed was produced from the 35 plants m⁻² density, although the seed yield from this density was lower than for 50 plants m⁻².[Show full abstract]
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