Agriculture Reference
In-Depth Information
yields. Various nutrient application techniques have been developed to satisfy
the requirement for a high inflow per unit root length of young seedlings and a
high total demand during bulbing, while trying to minimize nutrient losses
and consequent pollution.
Simulation models for the responses of many crops, including onions, to N, P
and K fertilizer have been developed (Greenwood
et al.
, 1996, 2001; Greenwood
and Karpinets, 1997). These models use weather data to simulate crop growth,
nutrient uptake, water uptake and losses and gains of nutrients from different
depths of soil and give a prediction of nutrient responses with different climatic
conditions and applications of fertilizer. They incorporate the numerous inter-
acting processes influencing crop response to nutrients. As well as indicating
appropriate fertilizer applications, they are useful for studying how different
conditions may influence nutrient responses. The models are accessible on the
internet (
http://www.qpais.co.uk).
Nitrogen fertilizing
When it comes to nitrogen fertilizer requirements, onions present one of the
most difficult crops to satisfy in an efficient fashion. To attain maximum bulb
yields, rates of fertilizer must be applied that result in considerable residual N
being left in the soil at harvest. Typically, yields increase with increases in N
application in the range 0-150 kg/ha, and thereafter level off. However, even
in the range where yield is increasing sharply with additional N, the quantity
of N remaining in the soil after harvest increases (see Fig. 6.3c).
Consequently, the recovery of applied N by the crop decreases as appli-
cation increases, and is typically only about 37% at the application rates
needed for near maximum yields. This poor capacity to exploit soil N is again
probably a consequence of the shallow, sparse root system. As a further
complexity, onions are among the most sensitive crops to soil salinity, par-
ticularly at seedling emergence, and both plant population and initial plant size
are lowered by initial nitrate applications sufficient to achieve maximum final
yield (see Fig. 6.3b), the degree of population reduction being related to the
nitrate concentration in the soil.
To overcome this difficulty it is necessary to split N applications, typically
applying 60-80 kg/ha mixed into the soil as a base application just before
sowing, and a similar amount broadcast when the plants are about 10 cm tall.
The total quantity of N applied should be adjusted to account for the nitrate
already present in the soil before fertilizing. This can be determined by soil
analysis, and derives from nitrate left from fertilizing the previous crop and the
mineralization of organic matter. For spring-sown bulb onion crops in The
Netherlands it is recommended that total available N in the top 60 cm of soil
from both added fertilizer and pre-existing sources should equal 180 kg/ha.
In Germany and Scandinavia (Stone, 2000b; Gertsson and Bjorklund,
2002) target values for mineral N content in the top 30 cm of soil at successive
stages of onion crop development have been determined. These target values
