Agriculture Reference
In-Depth Information
adapted to light interception, is the cause of this. Using this relationship it is
possible to predict growth rates in conditions of varying temperature and light,
such as may occur in the field or in a glasshouse used for plant raising,
providing growth is not restricted by lack of water or nutrients. As the plants
get larger they begin to shade each other, and competition for light will cause
the growth rates to diverge below the prediction of this equation. The higher
the plant density the sooner this will occur.
Crop growth can continue to a ceiling or potential yield that probably
varies with both temperature and daily light income and, therefore, with time
of year and region. Well-watered, well-fertilized, early-planted leek crops do
indeed reach a plateau of total shoot dry matter yield of around 1200g/m
2
(about 120 t/ha fresh weight) in high summer (see Fig. 4.50).
Later plantings plateau at a lower yield during autumn and winter,
probably because growth rate is slowed by low temperatures, and also because
the average daily light income is insufficient to sustain summer yield levels.
The fact that high-yielding, summer-grown crops tend to decline in yield when
left in the field over winter, due to the rotting of some leaves and the death of the
smaller plants, supports the latter view. As a consequence of these effects, leek
yields decrease the later in the season they are planted. The yields attained
correlate with both accumulated day-degrees and accumulated solar radiation
(light) during the growing period (see Table 4.8).
The diameter of the pseudostem or 'shaft' is often the important criterion
of marketability of leeks, and Wurr
et al.
(1999) took a diameter of 15 mm as
the minimum for marketable early leeks in the UK. From field trials they found
that shaft diameter in early leeks was related to effective day-degrees (EDD)
Fig. 4.50.
Shoot dry weight yields of leek cv. 'Autumn Mammoth Goliath' grown at
a density of 25 plants/m
2
with abundant irrigation and nutrients from a succession
of plantings at Wellesbourne, central England (note log scale for crop dry weight).
