Agriculture Reference
In-Depth Information
basal treatments of fertilizer at the full recommended rate. The supplementary N,
P and K was injected after emergence 150 mm to the side of the plants at a depth
of 60 mm. Thus, starter fertilizers combined with injection of further fertilizer
after emergence may make it possible to grow high-yielding onion crops at lower
levels of available soil NPK than is possible using conventional broadcast fertilizer.
Starter fertilizers are now widely used commercially in the UK on bulb and salad
onion and leek crops.
Micronutrients
As regards other nutrients, problems due to deficiencies of certain trace
elements have been periodically reported, particularly on peat (muck) soils.
Copper deficiency causes twisted, chlorotic leaf tips and bulbs with pale, thin
and brittle skins. Copper sulfate incorporated into the soil - or foliar sprays with
0.25% copper sulfate solutions - have corrected this problem. Ellerbrock
(1997) developed guidelines for copper requirements on peat soils based on the
quantity of HCl-extractable copper in the soil. Manganese deficiency, which
causes stunting and a striped chlorosis of outer leaves, occurs on peat soils of
pH > 6.5 and on chalky-clay soils high in phosphate. The problem can be cured
by foliar sprays applying 15 kg/ha of manganese sulfate or by soil application of
larger amounts. Zinc deficiency, curable by foliar sprays of zinc sulfate, and
molybdenum deficiency, curable by molybdenum seedcoatings, have been
reported on high- and low-pH peat soils, respectively.
Mineral nutrients and flavour
The effects of mineral nutrition on allium flavour have been reviewed by
Randle and Lancaster (2002). Sulfate availability probably has the greatest
effect on flavour intensity and quality of any environmental factor. Limiting
sulfur or making it abundant to the plant inevitably impacts on the quantity of
sulfur-based flavour-precursor compounds synthesized (see Chapter 8).
Freeman and Mossadeghi (1970, 1971) first showed that flavour intensity
could be varied from almost negligible levels to high levels by varying the
sulfate supply to onion and garlic plants growing in nutrient solutions. Their
work showed that a saturation point was reached, after which additional
sulfate resulted in little increase in pungency. However, in field experiments,
pungency has rarely responded to applied sulfur (Hamilton
et al.
, 1998), since
the element is already adequately supplied in most soils and sulfate is often
added through normal fertilizer practices.
For many types of onion high flavour intensity is desired. However, for the
production of very mild onions, the restriction of sulfate to the plant is
necessary. To produce mild onions, sulfate in the soil and water should not be
above 50 ppm (Randle and Lancaster, 2002). Sulfate is a readily leachable
anion, like nitrate, and is more easily leached from light, sandy soils than heavy
or highly organic soils, and hence light soils are most suitable for mild onion
production. Since sulfur is an essential element for normal plant growth,
