EFFECTS OF RATES AND TIMES OF APPLICATION OF STARTER DRESSINGS OF NITROGEN FERTILZER TO SURFACE SOWN PERENNIAL RYEGRASS WHITE CLOVER ON HILL PEAT

The effects on clover and grass growth of five levels of ammonium nitrogen (NH4‐N) applied before sowing, at sowing and after nodule initiation have been investigated. A pot experiment in which S184 white clover was grown in a peat soil showed that NH4‐N up to 688 mg N per pot (approx. equivalent to a field rate of 120 kg ha‐1 N) applied before sowing and at sowing did not affect clover growth. N‐fixing activity (C2H2‐reduction), on the other hand, was reduced progressively up to the highest level (688 mg N per pot). Application after nodule initiation increased growth relative to the zero‐N treatment at all levels of application. Maximum growth and N fixation occurred at 516 mg N per pot where the DM yield was 70% higher than in the absence of added N. A field trial in which S184 clover and S24 perennial ryegrass were surface sown on to a peat soil showed an increase in grass and clover growth in the first year in response to 120 kg ha‐1 N applied at sowing. Grass growth alone was increased at 120 kg ha ‐1 N applied 40 d before sowing. Lower rates of application before sowing and at sowing did not affect clover or grass growth. The effect of the delayed application of NH4‐N on legume growth was less marked than that in the pot experiment, 90 kg ha‐1 N stimulating clover growth by 40% in the first year. The effect was however different from that in the pot experiment, in that, whilst 30kg ha‐1 N increased N fixation relative to the zero‐N treatment, plants exposed to higher levels showed a depression in N‐fixing capacity. N‐fixation was correlated with nodule numbers in the delayed NH4‐N application, the closest correlation being with the number of multilobed nodules which was highest at 30 kg ha‐1 N and lowest at 120 kg ha‐1 N. It is suggested that circumstances exist when the use of a relatively low starter N dressing (20–60 kg ha‐1 N) at sowing would not increase clover or grass growth in the early stages of the establishment of a hill reseed. Under such circumstances higher rates of application (100 kg ha ‐1 N), preferably delayed until the seedlings are in a position to take up the nitrogen rapidly, would have the greatest effect. Copyright © 1979, Wiley Blackwell. All rights reserved