INTERACTIONS BETWEEN FIRST AND SECOND APPLICATIONS OF FERTILIZER NITROGEN AND IMPLICATIONS IN EFFICIENCY OF NITROGEN USE

DM yield and N uptake data are presented from primary growth and two successive regrowths of perennial ryegrass treated with combinations of three N treatments for the primary growth viz. 0, 50 and 100 kg N/ha (PN) and four N treatments applied for the second growths, viz. 0, 33, 66 and 100 kg N/ha (SN). Primary growth gave a response of 24·2 kg DM/kg applied N to PN50 with only a further l±0 kg DM/kg applied N from the second PN increment. A significant interaction between PN and SN treatments was shown in second growth. The residual DM response to PN was highest at SN0 and reduced as the level of SN was raised. Substantially greater residual responses to PN treatments were shown in the third growth. Third growth DM responses to SN treatments were high. High apparent recovery of fertilizer N reached 111% of primary N where SN66 followed PN50. High available soil N is partly responsible for both high apparent recovery of N and high DM response. The latter appears to be associated with inclusion of growths given no fertilizer N so that the full residual effects of fertilizer N and ‘priming’ of available soil N can be realised.     

Water potential and the respiration of microorganisms in the soil

The effect of soil water potential on the respiration of microorganisms in soil was studied. Bacterial respiration decreased rapidly below ?3 bar; at -20 bar it was very slight. The most rapid decline occurred above ? 6 bar. The respiration of a mixed population of microorganisms, however, was maintained at a fairly high level between ?8 and ?30 bar but subsequently declined until at ?50 bar it became negligible. At potentials below ?50 bar, additions of glucose caused no increase in respiration. The sensitivity of bacteria to relatively small negative potentials was probably due to their restricted movement as the soil pores drained.     

SWARD PRODUCTIVITY UNDER ALTERNATE OR REPEATED CUTTING OF ADJACENT SMALL AREAS

Cutting patterns producing high and low areas were imposed on swards of Dactylis glomerata and Lolium perenne to test whether output could be increased when compared with traditional uniform cutting methods. In two experiments, swards of both grasses were divided into parallel 9 cm strips which were cut alternately on different occasions to a height of 5 cm. These swards yielded a similar quantity of herbage DM to that from other swards cut uniformly at the same height and frequency. In a third experiment, swards of Lolium perenne were divided into alternate strips of unequal width (9 and 4.5 cm). The narrow strips were allowed to grow to a height of 15 cm and were occasionally trimmed at this height. The wide strips were cut to 5 cm at 2–or 4–weekly intervals. Areas cut to 5 cm every 2 weeks yielded significantly more DM when adjacent to high areas than when adjacent to areas cut to 5 cm every 2 weeks (i.e. under uniform cutting). The interaction between adjacent areas having different defoliation regimes is discussed in relation to the possibility of obtaining some additional output from a suitable no niform cutting system compared with uniform cutting. Sward productivity under no niform grazing patterns of livestock is also discussed.