|dc.description.abstract||The Western Cape is one of the most important onion producing regions in South
Africa. It is well suited for the production of intermediate daylength onion cultivars.
Annually about 2500 ha are planted with a yield of about 120 000 tons. Only 9 000 tons
was exported in 1998 and of these 9 000 tons 4.9% was rejected due to poor quality. In
South Africa there are no guidelines for the production of intermediate daylength onions.
It is very important to have fertiliser guidelines, because optimal yields of good quality
can be achieved with a good fertiliser program. Produce with exceptional quality can
boost exports and at the same time stabilize local markets. However fertilisers are very
expensive and could be damaging to the environment ifused incorrectly.
To produce fertiliser guidelines N, P and K field trials were planted at three
differen~ localities (Koue Bokkeveld, Stellenbosch and Caledon). The localities were
chosen according to the difference in climate and soil texture, and these are important
onion growing areas. The minimum and maximum temperature for the Koue Bokkeveld,
Stellenbosch and Caledon was 11123, 12/25 and 13/27 °C respectively. At all the
localities 3 nitrogen-, 4 phosphorus- and 4 potassium levels were used. Treatments were
factorially arranged in a randomised block design, with two replicates. At Stellenbosch a
N fertiliser trial was planted with four N levels and four N application methods. The
treatments were factorially arranged in a completely randomised block design, with three
replicates. N, P and K was applied as limestone ammonium nitrate, single superphosphate
and potassium sulphate. Marketable and unmarketable bulbs were quantified at harvest.
Weight loss during storage was determined over a six month period and storage disorders
It was clear that high N levels, especially on soil with a high potential for
releasing N, had a negative effect on yield and keeping quality. At Caledon a significant
reduction in yield took place with the highest N level. High N levels also had a
significant effect on weight loss at Stellenbosch and Caledon. The same trend with high
N levels occurred in the N fertiliser trial. Weight loss was increased with the late
application ofN in the growing season. In spite of the fact that the P levels were high (58
- 66 mg.kg-I) in the different soils, the onions reacted very well to phosphorus fertiliser.
As a result of the higher P levels there was more foliage in the leaf canopy and the bulbs
were larger. This P reaction only occurred in the Koue Bokkeveld and at Caledon. At
Stellenbosch yield was not improved with P fertilisation and bulbs were of poor quality.
The highest P level resulted in the greatest weight loss in storage. K had an effect where
the K status and clay content of the soil was low. High potassium levels improved yield
and keeping quality remarkably in the Koue Bokkeveld. Some interactions also occurred.
The percentage unmarketable bulbs were influenced by a P and K interaction. An
increase in double bulbs was responsible for the increase in the percentage unmarketable
bulbs. At the lowest P level, higher K levels decreased the percentage unmarketable
bulbs. Alarming proportions were reached when high K levels were combined with the
highest P level. The P and K interaction also had an effect on the percentage sprouting. K
did not have any effect' on sprouting at low P levels. A combination of the highest P level
and the lowest K level produced the highest percentage sprouting. The interaction
between N and P, like the P and K interaction had an effect on the percentage sprouting.
The highest N level in combination with the highest P level increased the percentage
The N, P and K field trials showed that the optimum N level is less than 130 kg
N.ha-l on a soil with 16 % clay. On a sandy soil it can be as high as 160 kg N.ha•l The
optimum P level varied from 30 to 130 kg P.ha-l Where a poor P reaction was observed
the Ca and S status in the soil was low. It is possible that the drastic P reaction with
superphosphate (10.5 % P, 20.3 % Ca, 12.1 % S) in the Koue Bokkeveld can be a result
of the application of Ca and S. On the shale soil at Caledon, with a K status of 288
mg.kg-\ the optimum K level is less than 75 kg K.ha-l On the sandy soil of the Koue
Bokkeveld, with a K status of 43 mg.kg•l
, the optimum K level was 200 kg K.ha•l
. The N
fertilisation trial was done on a soil with an optimum N level of 115 kg N.ha-l (10 %
clay). It was demonstrated that the standard N application method (40 % N with planting
and the rest divided in three equal applications of 20 % N on 2, 4 and 7 weeks after
planting) is still an acceptable practice.
Follow-up trials are being done to determine the exact optimum level of