Growth and development of peas in response to different inoculation methods and sowing dates

Peas are the most valuable grain legume exported from New Zealand. The success of a pea crop depends on its capacity to form effective nitrogen-fixing symbioses with root nodule bacteria. Rhizobium spp. inoculation of peas has been demonstrated to enhance nodulation, biomass production, harvest index, and nitrogen in the seed. This study evaluated the effects of Rhizobium inoculation, nitrogen fertilizer application, and sowing date on the growth, development, final yield and nodulation of garden and field peas grown in Canterbury-New Zealand. The field experiment used the semi - leafless ‘Ashton’ garden pea and ‘Aragorn’ field peas sown on 15 October, 4 November and 30 November 2010 in a Wakanui silt loam soil. Peas were inoculated with either a peat based inoculum or ALOSCA® granules or received 50 kg N ha⁻¹ in the form of urea or were un-inoculated as the control. In most cases Rhizobium inoculation and nitrogen fertilizer application had no effect on total dry matter (TDM) accumulation. The accumulated DM of ‘Ashton’ in the field experiment increased with each delay in sowing date. The DM yield of ‘Ashton’ from the 15 October sowing was 6.34 t ha⁻¹ which was 1.37 t ha⁻¹ lower than from the 30 November sowing. The difference resulted from less solar radiation interception due to the green area index (GAI) being below the critical value for most of the season for the early sown crop. There was no difference in TDM accumulation of ‘Aragorn’ among sowing dates. In the field experiment, Rhizobium inoculation and nitrogen fertilizer had no effect on final seed yield. The highest yield of 4.08 t ha⁻¹ was obtained when field peas were sown on 15 October. The final seed yield of garden pea increased as sowing was delayed from 15 October (3.37 t ha⁻¹) to 30 November (3.91 t ha⁻¹). Both cultivars are determinate in their growth habit so their development occurred at a predetermined time with the cessation of leaf production at flowering. Therefore the yield potential in these pea crops depends on canopy growth rate, green area index (GAI) and consequently the quantity of solar radiation intercepted. Slow leaf appearance (long phyllochron) and therefore canopy development due to low temperatures during early vegetative development resulted in a GAI at flowering for ‘Ashton’ of only 2.4 when sown on 15 October. This was insufficient to intercept 95% of the photosynthetically active radiation and contributed to the lower seed yield of ‘Ashton’ in sowing 1 than in sowing 3. Under these field conditions, Rhizobium inoculation treatments were superior to the nitrogen treatment and control in nodulation induction. They gave higher nodulation scores compared with the nitrogen and the control treatment. However, the overall response of peas to Rhizobium inoculation was poor with a mean nodulation score ≤ 3 which is considered unsatisfactory. The November sowing, due to a more favourable soil temperature resulted in greater nodulation than in the October sowing. Overall the results suggest that commercial inoculation of Rhizobium to peas did not bring benefits for nodulation, growth or development and thus cannot be recommended. To achieve a final seed yield over 4 t ha⁻¹, to ensure economic viability in New Zealand, a higher sowing rate to achieve at least 100 plants m⁻² is recommended, particularly for early sown crops.