Agriculture Reference
In-Depth Information
Starter fertlizers
Improved recovery of mineral N from soil can be achieved using liquid
ammonium phosphate starter fertilizer injected 25 mm below the seed at
sowing time (Stone, 2000a). Using this system near-maximal yields (59 t/ha of
bulbs), which required 130 kg/ha of broadcast N, were attained using only 85
kg/ha N, of which 20 kg/ha was in the starter solution and the remaining 65
kg/ha was applied and irrigated in when the crop was about 1t ha (about salad
onion-size plants). The starter fertilizer gives a high P concentration just below
the seedling root, and this is able to supply the high inflow needed at this stage
(see Fig. 6.1), resulting in much enhanced growth, even in relatively P-rich
soils (see Figs 6.5a and 6.6).
The resulting larger plants with larger root systems could extract N from
the rest of the soil volume more efficiently than plants grown without starter
fertilizer. Ammonium phosphate fertilizer is adsorbed on soil solids and
therefore does not cause a damaging increase in soil solution concentration
(salinity) below seedlings. Furthermore, experiments indicate that onions grow
better when supplied with ammonium rather than N at the seedling stage
(Abbes et al. , 1995). The accelerated early growth can result in higher bulb
yields at lower N levels than with broadcast fertilizer N alone (see Fig. 6.5b). N
recovery was increased by a factor at of 1.6 in the responsive range, perhaps
eliminating the need for a residual 'N buffer' that remains in the soil after
harvest. The faster early growth using starter fertilizer results in the earlier
Fig. 6.5. The effects applying 20 kg/ha of N as ammonium phosphate 'starter'
fertilizer below the seeds at sowing on the response of onions to broadcast
ammonium nitrate mixed into the soil before sowing. (a) Green onions 9 weeks
after 50% seedling emergence. (b) Yields of marketable bulbs at maturity, also
showing the effects applying ammonium nitrate as a 'top-dressing' (T) and irrigating
it into the soil 9 weeks after emergence, both with and without 'starter' and basal
fertilizer. The levels of top-dressing tested were 50, 100 and 150 kg N/ha (from
Stone, 2000b. Courtesy of Soil Use and Management ).
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