Agriculture Reference
In-Depth Information
Soil and fertilizer requirements
Onion crops can be successfully produced on most fertile soils. Soil pH in the
range 6-7 is usually recommended, but on organic soils a lower pH is
satisfactory; for example, in Ontario, Canada onion crops grow satisfactorily
on peat (muck) soils with a pH of 4. Soils with large stones or clay clods make
mechanical harvesting difficult. The soil must be sufficiently well structured to
permit the preparation of a fine seedbed suitable for a small-seeded crop. Onion
yields are severely reduced by soil salinity, and they are among the most
sensitive crops in this respect (Allen
et al.
, 1998). Therefore saline soils are not
suitable. Onion crops are grown satisfactorily on sand, silt and peat soils in the
UK, and also on well-structured clays in The Netherlands.
Before sowing, the soil must be prepared. Normally, fertilizer and pre-
emergence herbicides will be incorporated. Phosphate (P) and potassium (K)
fertilizers, which are strongly adsorbed and held against leaching by soil solids, can
be incorporated long before sowing, when the land is first ploughed. Amounts of P
and K recommended in the UK range from 26-129 kg/ha of P and from 50-250
kg/ha of K, depending on the pre-existing P and K levels in the soil. Recom-
mendations for other countries are similar, and generally emphasize the
importance of soil analysis for P and K in deciding the appropriate amount to apply.
Fundamentals of nutrition
In gardening tradition onions are regarded as 'gross feeders', needing a highly
fertile soil to attain maximum yields. This was corroborated by comparative
fertilizer trials on 22 temperate vegetables species where bulb onions and leeks,
respectively, ranked fourth and fifth most responsive to P (Greenwood
et al.
,
1980b), fourth and ninth to K (Greenwood
et al.
, 1980a) and eighth and
seventh to nitrogen (N) (Greenwood
et al.
, 1980c). At first sight this is somewhat
paradoxical, because they are among the slower-growing vegetable species, and
bulb onions remove less P and K from the soil at harvest than many other
vegetables that achieve maximum yields with lower soil levels of P and K, e.g.
cabbages. However, because of the low root densities and lack of root hairs of
alliums (see Chapter 2), they need a high concentration of P and K in the soil
solution to drive diffusion to the root surface at a rate sufficient to satisfy their
requirements as seedlings.
Figure 6.1 shows the phosphorus inflow per unit root length of young onion
seedlings soon after emergence. At the earliest date in Fig. 6.1, the inflow rate is
nearly threefold that of later on. Hence a high soil solution concentration is needed
to meet nutrient requirements for maximum growth rates of seedlings, but later
inflows can be sustained by lower concentrations. The need for high soil fertility is
probably largely a reflection of this short but crucial stage in crop development.
Onions and other vegetable crops restricted by shortage of nutrients at this early
stage are likely to remain lower yielding than unrestricted crops until final harvest
(Costigan
et al.
, 1983).
